We overcame the envelope on our 36 foot sloop by adding quarter of a ton torpedo shape to fin keel, downwind 14 knots and submerging the bow in a few cases, we broached easily before, did help but expensive testing to make a torpedo
I sail O'Day day sailor twos and soon an O'Day 192. Both of these are not your heavier class sailboats. You can even plane out the day sailor 2 and I'm yet to find out about the 192. But the sale area to displacement ratio it's very good for speed however the ballast of displacement is very unstable. If you're sailing at the right angle to the wind, maybe somewhere around greater than 15 to 20° toward a dead run past a beam reach, the ballast to displacement ratio becomes almost insignificant to any dangerous heeling of the boat and you can maximize hull lift out of the water, probably also deploying a Spinnaker, reducing drag in the water and significantly overcoming hull speed dictatorship? I can see that as in aerodynamics of a car in motion there is a similar effect in hydrodynamics especially as you break whole speed as water is much heavier than air and the drag coefficient generated at the stern would seem to really go up exponentially pulling you back or rather exerting that vacuum or whatever you call it with water at a much greater rearward pull.
Very interesting and informative video. As an amateur designer I always wondered why it was always said that "hull speed" was the limits to design, but I always want to push that limit like in aircraft exceeding the speed of sound.
One of my most exciting-without-being-scary times was surfing a 30’ displacement hull on the aft stern wave of a harbor tug that passed my ketch in the gut between Hoboken and Staten Island. The light wind on the beam reach would not otherwise have allowed matching the tug’s speed. I only sheared off after noticing the wake was drawing me toward the tug’s stern. As to max hull speed, one theory has it that suddenly accelerating a sailing ship can literally drive it under the water when bow and stern wakes form only one wave trough between them. This sudden acceleration was posited when said ship was sailing under light winds aft and was hit by sudden squall and the crew was unable to reduce sail in time. Commercial sailing vessels were notoriously small-crewed and could carry acres of sail. Thoughts?
People don't mention, planning hulls in this. I had a sailing dinghy in the past, a laser. It definitely went above any hull speed based on length simply because on a reach or broad reach it would get up on the plane..
I have some questions: 1) In relation to hull speed, what would be the effect of creating small bubbles ahead of the bow and below the hull (by any suitable means)? Thoughts are that traveling through bubbles would reduce wetted area reducing friction. 2) If a swing keel were vertically split along the hull and hinged/slid/pivoted (take your mechanical method) so that the depth of the keel would be increased, you'd gain righting motion, thus your sails could catch more power. (Envisioning [rotate 90* in your mind] a beam over a fulcrum demonstrating foot-lbs.) Is keel design advancing or are efforts better spent elsewhere? My focus is with sailboats. 3) Instead of #2 going deeper, what if the bottom of the keel were winged and with a design that allowed the wing portion to be rotated starboard or port (and still be hydrodynamic)? This would move the weight off center and to the direction to better counter heel. 4) What if a sailboat had, for lack of a better term, canards molded above the waterline so that as the boat heeled, the canards would contact water impose a righting force?
Hi TronOfBorg. All excellent questions. 1.) For creating small bubbles, this is an area of active research. I have only seen it on powered ships though. You need a fairly large air compressor to introduce the bubble under the hull. That takes a lot of power. With the commercial engineered applications, I have seen claims of decreasing resistance by about 15%. That could increase speed by about 7%. 2.) If I understand your description correctly, yes keel design is advancing in this area. The Volvo racing yachts using swing keels with can pivot port/starboard to increase righting moment. We don't see these outside of racing yet, because they are expensive. 3.) Good idea to offset the wing. But you get more bang for your buck out of a swing keel instead. The swing keel does more to offset the center of gravity with less complex machinery. 4.) Yes, this could work in theory. I have not seen any direct applications yet. The main challenge is that you need forward speed to get large forces out of the canards (or any type of lifting foil). Except that lifting force also has its own sources of drag that it generates. You don't get it for free. So the idea of submerging foils could produce extra righting moment. But you will probably find better efficiency and simpler construction through changing the hull shape.
@@DatawaveMarineSolutions How about creating countering waves - something similar to bulb bow. But instead of creating them in a bow why don't create them: 1) either before the bow (maybe by hanging something from the bowsprit- so distance from the bow could be adjusted to speed) 2) sideways to the bow - maybe in a form of a proper formula for a distance between hulls in a multihull design. It would still be a fixed speed (like in a bulb bow design) but it might give some bonus on a light winds during long cruising. In case of proposed bubble-cavitation solution. How would such a solution affect buoyancy? In theory it should be reasonably easy to boil thin layer of water at the bow section, some solar panels could provide enough energy. My concern is not only danger to marine life in such design - filter of steam/bubles would have lower density than water. Wouldn't it limit application only to foiling or extremally lightweight craft?
@@gradowik Good idea, but the sense of scale just won't work in this case. When you hit hull speed, you have a wave that matches the length of the ship. To create a counter to that wave, you need a bulb about the same length of the ship. As far as the multihull concept, I haven't heard of anything like that. But more power to you if you can develop something that works. In the realm of bubble-cavitation, first a few clarifications. When we get cavitation, those bubbles collapse almost immediately after being generated. They aren't very good candidates. There are devices that straight up inject air bubbles under a hull. It doesn't affect buoyancy at all. The air bubbles are at the same pressure as the water, so the hull doesn't see any different in buoyancy. This also is not a concern for marine life. We don't normally use steam to create the bubbles. The easier route is actually compressed air, which requires less energy. The bubbles are gone after the ship passes. We don't use these bubble systems to reduce the ship draft or use it in light weight applications. In fact, large, heavy vessels are the only ones that have the space for the necessary air compressors. Running the air bubbles along the hull helps reduce the skin friction of water against the hull. We use it for fuel savings. Still questionable if the fuel savings are worth the cost of running the air compressor.
look up the newer generation vondee glope IMOCA 60 sailboats. theyre single handed offshore racing boats, and pretty much all of the new ones are using foils that extend out of the sides of the vessel to provide righting moment and even help lift the hull partially out of the water. the new americas’ cup boats are also monohulls using a similar idea, but they can lift themselves fully out of the water on hydrofoils
Higher RM, Lower L/D ratios, Shallower 1/4 beam buttock angles ....it is complicated....but propensity to surf, is primarily a comfort trade off; when you analyze it. or resistance vs. power ?.... yes, exactly.....and it's the bow wave that turns more power into more angle of heal ( eventually forcing a reef ? ...that is the question )
The formula for hull speed is not boat length, but water line length. You should have made this clear as correction #1. You also left out the terms for the broadest hull descriptions, which are planing hulls and displacement hulls. Sailboats have displacement hulls, which are designed to maximize water line length (being wide in the middle and wider still when heeled over). This helps them to achieve a better hull speed with small engines or via wind power. These hulls below hull speed are not sensitive to weight, so they tend to be heavy, which makes them handle better in waves and have mass to counter the heeling over of the wind. Once you understand these points of displacement hulls, you then realize that these hulls can't go faster than their hull speed - because no matter how hard the wind blows, they'll never get enough power to get over the hump. Beyond that, even with a motor, the power required for the heavy boat to start going up his bow wave is too great. Even if you COULD put a motor on to start to plane, it would be horribly inefficient, hence naval architects don't design boats like that. For planing hulls, they tend to be less heavy and pointy at the bow to help get over and on top of the bow wave. They of course require engines with power enough to go the desired speeds, but this is WAY less power than required to make a displacement hull plane.
"Ship design has many tricks to push faster than the hull speed". I'd love to see a video going over a few of those and the tradeoffs that come with them.
I enjoyed the Calculus, Geometry and Physics lecture. I have found most of the information on the internet is like getting a weather forecast. Everyone repeats what they have heard; no one verifies the facts or even knows what the facts are. Allow me to ask you a simple physics question; take two round objects the same length and size, both have the same nose cone similar to a jet aircraft. Object A; starts the gradual curve to the end (tail/stern) point at 50% of the length. Object B; starts the gradual curve to the end (tail/stern) point at 75% of the length. Simple Question: which object is faster given everything else is constant; Object A or Object B. Last question what is making Object A or B slower given everything else is the same (constant)?
That is a bit of a trap question. Potentially, Object B has a higher pressure recovery than Object A. However, I would need to perform some calculations to see if the more gradual curve of Object B increases the wetted surface, which increases friction drag. My official answer: I can't say for certain, I would need to perform calculations.
I came across this video while searching for Hull speed formulas and how they relate to age of sail (1650's through 1820's) wooden ocean going ships. In theory, a longer 4th rate ship (Frigate) could go faster than an unrated Sloop. I had thought this was due to the longer hull AND the fact that it had more canvas that it could spread aloft. I was also aware that a ship that piles all canvas aloft could end up going slower than expected largely because the bow would bite deeper into the water depending on which sails were aloft. But, at least now I have a deeper appreciation of what went into sailing ship design based on this video - and that while Hull Speed as calculated is not the final word on things, it can at least give me a decent estimate on how the ratios are between the ships. Question? If you have a ship of a given configuration, where you know the length/beam ratio, and you have the keel length - is it possible to get a relatively decent estimate of its max speed give or take?
I have found from first hand experience that the hull sped only determines the stability and handling of the vessel. I could show you some great footage of going at least 5 times over the hull speed of one of my vessels. However the camera is on the bottom near the mast.
If the hull speed formula limit was a definite maxium limit that could not be exceeded in any circumstances then a small dinghy towed by a large vessel many times it's length would slow the speed of the longer vessel to the maximum hull speed of the dinghy which does not happen in that circumstance.
Hi Nick, this is a really interesting topic. The other descriptor of the hull speed as I remember reading is that it is the point at which the hull must climb its bow wave to go faster requiring more energy until the hull begins to plane. But what interests me is that the modern trend of wider transom boats create other not mentioned effects. Notably when a boat leans due to wind pressure the submerged hull shape goes from being symetrical to some form of asymetrical. In older narrow stern boats the sum of the submerged shape delivers a resulting lift to leeward whereas the sum of the submerged shape of wider transomed boats delivers hydrodynamic lift to windward and possible improves the hull’s, not pointing to the wind but, net movement into the wind. Does this make sense?
A little. I'm a little fuzzy if you are talking about the hull alone, or also including the hull, keel, and rudders. But I think the general idea is what we normally call sailing side force. More sailing side force means smaller lee angle and faster speed over ground to windward. That is another topic I would love to get into. But I always like to do my research first so that I can backup anything I say with real data and examples.
Evan Taaved Pert totally the wrong analogy...submarines, are more of a comparison to aeroplanes...the problem with boats/yachts is the surface wave effect
I've often wondered why no one has tried to design mechanism between the mast and keel which could compensate for the heeling force and keep the mast perpendicular to the water's surface no matter how hard the wind blows. I guess I'm a little jealous of the way wind surfer's boards don't heel over with that swivel joint at the base of their mast.
Part of it is a safety principle. We want to prevent the boat from capsizing. As you heel over more, the heeling moment of the sail decreases. It acts as a safety valve to reduce your chances of capsize. But the more advanced racing yachts do try to compensate and keep the ship upright with a canting keel: en.wikipedia.org/wiki/Canting_keel
there is (sort of) such a mechanism. a roller furling boom. in order to reduce the heeling moment you reduce the plane of the sail that's in the way of the wind. windsurfers do 'heel' by tilting the sail heavily to windward, thereby reducing the amount of sail area presented to the wind. however, simply reefing the sail lowers the amount of pressure applied to the mast, lessens the heeling and leaves the mast closer to perpendicular to the water. whether or not that is the faster option has a lot to do with the design of the hull and the modified shape it presents to the water.
We have a perfect real-life example of hull speed. In the early 20th Century many ships were re-engined with the new steam turbines to replace piston (expansion) steam engines. In some cases, the power almost doubled. But in almost every case the ship speed only went up a few knots. To the disappointment and dismay of the Admiralty.
This is a repost of a previous video about hull speed. Same video, but with a correct formula for the metric version of hull speed. Apologies to all the previous TH-camrs. But I had to take down the previous video and its incorrect formula.
My sailboat (Oswald Berckmeyer Miura 31) has a very pronounced tumblehome which theoretically should increase buoyancy as it heels. What are the drawbacks of this type of design and why is it not very common. I notice the odd modern solo race yacht for long distance has a bit of tumblehome.
One reason that you don't often see tumblehome is resulting difficulty to mass produce this. Mass produced hulls typically start with a mold that we lay the fiberglass in. The mold shapes the outside of the hull to guarantee a smooth exterior finish. But if you add tumblehome to the hull, you can't get the hull out of the mold. The only way would be a splittable mold (which adds to the cost). So production dominates that a lot. Another reason is simple convention and aesthetics. Tumblehome gets associated with older tall ships, which creates the impression that it is an old technology. As far as drawbacks: you get less righting moment per degree of heel with tumblehome. That can restrict vessel stability. Benefits: a little tumblehome allows a wave to ride up the hull, rather than force it down. That can potentially reduce your resistance. But it really depends on how that profile interacts with the rest of the hull shape.
A nice if somewhat very simplified explanation, of course the other ways to add righting moment such as to increase ballast, have a canting keel and increase the depth of the keel just to name a few. While adding greater righting moment is fine, the angle that the righting moment becomes effective a highly important factor as well. Another major speed limiting factor is weight and the dispersion of the weight eg. crew at the back causing the transom to drag highly increases the drag and turbulence the hull creates. Thats why you always see the rail meat amidships on racing yachts anytime the boat isn't on a run. In reality there are so many factors that limit the speed of a yacht that covering every aspect would take many hours of video. eg. balance of the sails etc. etc. etc.
With light boats, 25 ft or less sub 2500 lbs, I've often been able to push past hull speed or surf on a broad reach with the waves. There is a noticeable loss in directional stability after that point, you need to be gentle on the tiller to keep her on a heading.
Very interesting but I think for the boats I use, hull speed is the limiting factor. I use human powered racing kayaks which are also limited by the rules of the sport as well as human physiology. Winners are those with enough power to get to maximum speed first and can hold on longest. In close quarter marathon racing, we can use the wave generated by opponents to surf on and save energy for sprints at tactical moments.
Show me a sailboat with displacement hull exceeding hull speed. Its still a usefull formula. I dont think anybody claims its the limit. Like you said its the speed of diminishing returns. I heard of a 60 foot schooner with displacement hull going more than 5 knots faster than its hull speed in a gust while under engine power with all 7 sails locked into a perfect trim but in most conditions thats just not going to happen and would likely be more efficient to run at the hull speed.
When surfing a wave at sea, steering does NOT reverse because you exceed hull speed. Steering reverses because the wave behind you causes the water in the wave crest to move faster than your boat speed. Effectively, the bow of your boat has water flowing aft and for a short period of time the stern has water flowing forward.
I have some questions about your graph showing the 3 main components of resistance - viscous resistance is linear, wave making resistance has a hump (which is the hull speed hump that your presentation is about) and then you have another line with a hump which is labelled AIR - what is this and what determines the speed at which the hump occurs and the size of the hump? It seems to correlate with wave making. Also, your viscous line I assume is water viscousity. There is also viscous drag in air - is that subsumed in the air line I'm asking about? Is the air resistance just the drag on the exposed hull/deck/cabin or does it include the net drag on the sail plan? Also, switching back to the wave making hump - I'm assuming that the speed at which the hump occurs relates to LWL but the magnitude and shape of the hump are determined by shape and displacement - meaning that a light long skinny hull also has a hull speed bump but its much smaller so more easily overcome.
Yes, the viscous line represents water viscosity. Air resistance is the combination of viscous and potential flow resistance. The net resistance on anything above the design waterline. And yes, the speed at which the hump occurs relates to the waterline length, and magnitude is determined by the size and shape of the hull. With some hulls, you even see multiple humps.
Very nice introduction to this topic. All engineering design involves some level of compromise. I feel this could become a series, with ever increasing complexity. Possible with links to text books and maybe web pages, so anyone interested could further their understanding.
The point of inflection where you encounter rapidly diminishing returns being at the hull speed is not simply coincidence. The diminishing returns have causal relationship with the hull speed, as in, that is where increasing wave forces are encountered. If you surpass the hull speed there is a period of rapid gain followed again by diminishing gains (increasing function of air/fluid/wave resistance) What will you say then, that total resistance is just coincidental?
I don't recall that the video ever implied total resistance was coincidental. Only that hull speed did not predict a hard limit on maximum boat speed. In general, I would argue that TH-cam is not the appropriate forum for a technical debate, which are the types of questions you are asking. Those belong in peer reviewed journals and scientific conferences. And when I present to conferences, I use a more technical and precise tone of presentation. These TH-cam videos intend to expose engineering concepts to a wider audience in a more accessible manner. That does require less precise terminology, which can lead to some misunderstanding. But you asked a technical question, and I will attempt to provide a brief technical answer. Based on current theories of resistance prediction supported by the ITTC, I argue that calm water resistance, expressed in non-dimensional coefficients, is a function of Froude Number and Reynolds number. The primary components are wave making, friction, form factor, and appendage resistance. Of these various components, wave making resistance is the one that demonstrates the change in resistance at hull speed. Before hull speed, it shows a characteristic hump / hollow curve. Beyond hull speed, the curve appears exponential, but the actual characteristic depends largely on the shape of the individual hull. Air resistance also gets added in as an additional component, purely as a function of Reynolds number. For calculating powering, we may also add in an additional powering margin based on wave resistance from weather.
@@DatawaveMarineSolutions my comment about total resistance was an argument ad absurdum. 8:19 "...having this point of diminishing returns next to hull speed is just a coincidence"
Climbing the bow wave seems to be spot on the theoretical hull speed, when being propelled by the in/outboardmotor of a sailboat. At least in my experience, but I'm not a scientist.
On the topic of sailboat hull shapes. Most sailboats have a reverse teardrop waterline: pointy at the bow, round at the stern. Most other foils, including sailboat rudders, have normal teardrop shapes (e.g., NACA0009 sections). What is the purpose of reversing the shape for hull design?
The common logic is that it focuses on creating a better ride in waves. A pointy bow with vertical sides cuts through waves. What is less obvious is that these hulls do have a teardrop shape, vertically. They start deep at the bow, and slowly rise up vertically towards the stern. Of course, there are always exceptions.
How does a bulbous bow affect the wave resistance? How about a variable length bulbous bow? How does a golf ball surface affect viscous resistance? Doesn't using underwater wings affect the righting moment?
What you did, int mention is the difference between displacement hulls and planning hulls or to put it another way moving a object in water means it is displacing that water or it must move the water out of the way as apposed to riding across the surface as in a John boat or flat bottom drag boat, mearly calling it resistance does not explain it.
So good channel:) Thank you. Could you tell us a little bit about water propellers design for ROVs - to have equal efficiency in both forward and reverse? Have you got something about such propellers?
INTERESTING... why not flood the bow? If the problem is the boat climbing the wake, instead of trying to plan over that or have enough power to drive your boat uphill why not have a ballast tank in the bow which you progressively flood with the increase of speed. (Within limits) this will force the bow to dig into the bow wave and keep the boat level, no? As you slow down, you can pump out the ballast tank.
Was it boatlength or waterline length in the formula? Good short video, but it could use some examples of say, planing hulls or classic hulls that use their lifting forces in a dynamic way (light winds, bow and aft overhangs out of water > less viscous resistance, high winds, bow and aft overhangs in the water > longer waterline, more hull speed). Both work fine but in different sea states (yet another dimension).
So, how about a catamaran with gaff rigged sails? More moment arm in the water, less surface area per righting force, less moment arm per area in the air.
Cool video, really makes me understand how I can get 9 knots out of a supposedly 7 knot sailboat! (Spencer 42) Subscribed! Looking forward to seeing the rest of what you post!
@Renegade Show Although probably achievable, the only way YOU'RE getting 9 knots out of that 42 foot, 19K pound, cutaway full keel POS of yours is in your dreams, that, or with a 2 knot tail current. Especially given your whopping two years of sailing experience and a canvas inventory that's probably as old as you. I would prefer you come back here and actually debate me about it, but you won't, because you know who I am, and you know I'm right.
What about adding side foils to the haul to lift it out of the water and reduce viscous drag which can be done simple on smaller day sailboats, allowing the haul rigidity to still be strong enough when dealing with 12-14ft hauls that wont buckle under it’s own wait once lifted out of the water... wouldn’t this be possible on say, a 14ft hobieCat or fj420.
I've never seen you make a video on how ( steel ships)(35 to 45 knots) underwater foils sooth, and increase speed, stability while lowering pitching motion, as you call green water on your front deck.
Ok, but there is another factor in sailing vessels that didn't get mentioned.....keel weight causing righting moment. if you want more power add sails AND keel weight? How do these interplay - or is just not enough to push beyond the point of diminishing returns?
Very correct. Keel weight factors heavily into the righting moment of your sailboat, which ultimately determines how much sail power the boat can carry. And yes, for racing boats, that is enough to get beyond hull speed. But I will admit that any speeds beyond hull speed, the sail power requirements go up extremely fast. So possible, but you really need to design the entire boat around going fast: hull shape, weight savings, sail design, etc. This is why we see racing sailboats as their own dedicated design.
Hi Nick. Great channel. Following on from this topic I would like to hear your take on XBOW and SWATH technologies. Are they as good at improving performance as claimed or are we being led up the proverbial garden path. Thanks, keep up the good work.
I have some videos planned in the next few months just for those. One of the hard problems is finding the time and data to do a true comparison of the X-bow. They are a private, patented hull design. And Ulstein does not want to give away their secrets.
When displacement hulls become planing hulls for very long and very often nothing good will come of it. You will eventually become a hull full of water.
Doesn't the catamaran or trimaran provide huge righting moment for sailboats? Also, we see competitive sailors pulling their rumps over to the side to add additional righting moment.
Yes, and no. Monohulls also have a huge righting moment, due to the deep keel and the weight at the bottom of that keel. Catamarans and trimarans provide that same righting moment using buoyancy of the amas. Those amas require far less weight than the deep keel. So it is more fair to say that catamarans and trimarans achieve the same righting moment, but with less weight.
I have been trying for a while now to understand and compare the amount of power in kilowatts required to move a monohull, a catamaran hull and a trimaran hull forward at different speeds. Why? I would like to understand how to match electric motors to different hulls. In addition, sails capture wind energy to propel hulls forward, but how much energy is actually captured per square feet of sail area. The difference between the two should be due to efficiency and resistance losses right? Do you have a video discussing these topics? These video helped in understanding a bit better the hull area. Thank you.
I do have several videos that discuss the relative merits of trimarans and catamarans. The best place to start will be my video on comparing between the different hullforms. th-cam.com/video/FYFB56Q8BTg/w-d-xo.html This doesn't directly talk about powering estimates though. That is far too complicated for one video.
You lost me at narrower bow. I’m looking at scow designs. I guess that the scow bow design includes the wider beam, which you mention as being good for righting moment (and, therefore, power being used for imparting velocity). But it seems to me that a scow bow makes planing easier. I’m still not understanding why a narrow bow is better.
You are trading wetted area or viscous drag for righting moment. The narrow hull designs will have less viscous drag and better sea keeping. In the world of racing scows the sea keeping is absolute garbage, but you're not racing in rough conditions so you're trading for substantially more righting moment. If you look at the more extreme examples the boats are designed to heel substantially more than a cruising yacht in an attempt to reduce that wetted surface area gaining benefits of both designs. A canting keel is accomplishing the exact same thing while keeping total heel in check.
@@otm646 I have no idea what a canting heel is (so I will read up on that), but this is the exact kind of succinct answer I was hoping for (but honestly not expecting) from someone on the internet. What do you know about surfskis? I’m really wondering about the dynamics of a surfski bow that digs into the next wave when someone is surfing. It’s almost like the bow is bracing against the wave, which means that it’s actually slowing it down. However, it seems stable and the boat will move at the wave speed. I would like to see some quantitative descriptions of wetted surface that compare a scow bow with a steep-stem pointy one. Any references for me?
Depleted uranium keel... much righting moment, much speed? But seriously this makes sense why catamarans are so fast. Wide stance -beam + higher righting moment, narrow long hulls, light weight for good transfer of energy rate. Is the point where the righting moment and wind energy transfer peak measured by sail spill as in you have more energy than you can use? Sorry obvi a sailing noob here.. can you make a sail boat plane out like a recreational boat?
Light one design class type boats can be designed to plane easily. I used to sail a windmill (a now pretty much dead class) it would plane, but the rudder got damn sensitive! Also the sailing surfboards planned well, seemingly most modern one design classes will plane, although the newest thing seems to be foils.
Hi. I want to bulld a boat for sitcom navigation.and I do not use sail.it will be powered by smal electric motor.now I am thinking wich type of hull to choose.semi v or modified v or flatbottom or displacement or deep v? Or mix of all of them or mix of some of them.what is you advise?
What is the minimum wetted surface area needed to affect an increase in calculated hull speed? For example, could I hang an 12' 4x4 post from the bow pulpit in the direction of travel, resting one end against the hull and thus increase my speed? Would a 12' 1"x1" 'stick' be just as good? Or, do I need an area that covers from the water surface to the depth of the keel to affect the calculated hull speed? hum...Worded yet another way, what is the ideal ratio of length vs depth to gain in hull speed?
Just adding small changes will not work. Any changes need to be a faired addition to the hull. So you basically need to add something that covers the entire hull.
Hull speed is purely a function of waterline length. Wetted surface may impact how easy or hard it is to achieve or exceed hull speed for a given condition, but it won't change the hull speed itself.
Although I first thought I get to hear my future and horoskope and what death I will die, he actually explains perfectly and understandibly. Profound knowledge and no bullshit. Just curious why I get the feeling of beeing tarot-carded
I think the parachute would perform about the same as a spinnaker. Up high has some potential. The major benefit to that is going higher gets you out of the atmospheric boundary layer. But you need to go fairly high to get any major benefit. Imagine doubling the height from 50 ft to 100 ft mast. That is only a 10% improvement on wind speeds, assuming a typical atmospheric boundary layer. Another option is a kite sail: www.skysails.info/
Getting the pliers ready to pull my foot out of my mouth just in case, but I'll try it anyway .... Round parachutes have a stability issue, and cannot be easily directed. Square/delta/wings ones maybe, but good luck with the launch and the retrieval on a sailboat. SkySails yatch: th-cam.com/video/4QJmD6m3das/w-d-xo.html Wingsails: th-cam.com/video/QGJ02kEDtXI/w-d-xo.html
it doesn't hve to b mono hull & not cat either & not even in water hull at speed. If sail is far bigger than boat blimp shaped than the right force & left forces can b balanced out & just all ends up being driving force, bc the big air catch in middle is greater than righting or left blow. Like a fat long foot ball.
It's not enough that imperial system is confusing and doesn't scale linearly, now you have to mix it with metric to make it even worse. Why use m/s² for gravity and not speed of squirrel with oversized testicles per average chin height of a lama. What kind of abomination is speed in knots for length in meters? It's like saying pressure is liters per square foot. Doesn't make sense. Other than that, great explanation.
I would add the “ winged keel “ acting to slightly lift the boat causing less hull surface to interact with the water ... just my experience sailing with different keel configurations
I will add it to the list of planned videos. I actually did a design similar to hull vane a few years back. Based on the problems I encountered with my design, the Hullvane concept has the right idea. I'm a little skeptical about their claim of savings in the range of 10-20% (www.hullvane.com/tech-talk/). But I have been surprised before. So it might be possible.
Very true. We can easily use engines to power the boat up to extremely fast speeds. At that point, you actually get limited by human endurance. At the high speeds, slamming into the waves creates shock loads that can damage humans. For the high performance military boats, they actually need to install special shock absorbing seats.
I am serious, for example, Classen's 127' Atalante or similar hull, can this be built without masts using two waterjets instead for a cruise speed of 22 - 24 knots?? here is a video of her: th-cam.com/video/X7cD6RMv07o/w-d-xo.html
@@DatawaveMarineSolutions are You going to cover the sails and rigging too? Maybe proper sail design could not only power the boat but also lift leeward side from the water? We already have genoas and spinakers lifting partially the bow, why not use aerodynamical part of sailing instead of fighting with physics of hydrodynamics?
bulbous bows only work at a narrow range of speeds. Sailboats don't travel at consistent speed (because the wind isn't always blowing with the same force) so a bulb isn't useful.
As a Mechanical Engineer, I have to say I very much enjoy your explanations in easy to understand ways. Perhaps in an upcoming video, you could comment on some of the new hull designs such as the Beneteau Air Step Hull (th-cam.com/video/v8NmQM_s8_E/w-d-xo.html) and the Shannon Silver Hull Design (th-cam.com/video/HVPEAU_gK0o/w-d-xo.html) and their importance or non-importance. Keep up good work. -johnny
same way air ship floats over water only u want to use air skitting to cut thru water better, at speed & switch off air skit when not out clubing, more button downed.
Talks a lot but doesn't actually tell you anything useful. Does not give any figures of what he thinks might be achieved. The fact remains that the well accepted formula will give you a pretty good estimates for your displacement hull speed.
We overcame the envelope on our 36 foot sloop by adding quarter of a ton torpedo shape to fin keel, downwind 14 knots and submerging the bow in a few cases, we broached easily before, did help but expensive testing to make a torpedo
Hull speed does not claim you can't exceed it, just that the energy to exceed it will be greatly increased.
love the picture of the Cal 40, campagned Whisper a 1968 Cal out of Coconut Grove, Florida for 6 yrs
I sail O'Day day sailor twos and soon an O'Day 192. Both of these are not your heavier class sailboats. You can even plane out the day sailor 2 and I'm yet to find out about the 192. But the sale area to displacement ratio it's very good for speed however the ballast of displacement is very unstable. If you're sailing at the right angle to the wind, maybe somewhere around greater than 15 to 20° toward a dead run past a beam reach, the ballast to displacement ratio becomes almost insignificant to any dangerous heeling of the boat and you can maximize hull lift out of the water, probably also deploying a Spinnaker, reducing drag in the water and significantly overcoming hull speed dictatorship? I can see that as in aerodynamics of a car in motion there is a similar effect in hydrodynamics especially as you break whole speed as water is much heavier than air and the drag coefficient generated at the stern would seem to really go up exponentially pulling you back or rather exerting that vacuum or whatever you call it with water at a much greater rearward pull.
Very interesting and informative video. As an amateur designer I always wondered why it was always said that "hull speed" was the limits to design, but I always want to push that limit like in aircraft exceeding the speed of sound.
One of my most exciting-without-being-scary times was surfing a 30’ displacement hull on the aft stern wave of a harbor tug that passed my ketch in the gut between Hoboken and Staten Island. The light wind on the beam reach would not otherwise have allowed matching the tug’s speed. I only sheared off after noticing the wake was drawing me toward the tug’s stern.
As to max hull speed, one theory has it that suddenly accelerating a sailing ship can literally drive it under the water when bow and stern wakes form only one wave trough between them. This sudden acceleration was posited when said ship was sailing under light winds aft and was hit by sudden squall and the crew was unable to reduce sail in time. Commercial sailing vessels were notoriously small-crewed and could carry acres of sail. Thoughts?
People don't mention, planning hulls in this. I had a sailing dinghy in the past, a laser. It definitely went above any hull speed based on length simply because on a reach or broad reach it would get up on the plane..
I have some questions:
1) In relation to hull speed, what would be the effect of creating small bubbles ahead of the bow and below the hull (by any suitable means)? Thoughts are that traveling through bubbles would reduce wetted area reducing friction.
2) If a swing keel were vertically split along the hull and hinged/slid/pivoted (take your mechanical method) so that the depth of the keel would be increased, you'd gain righting motion, thus your sails could catch more power. (Envisioning [rotate 90* in your mind] a beam over a fulcrum demonstrating foot-lbs.) Is keel design advancing or are efforts better spent elsewhere? My focus is with sailboats.
3) Instead of #2 going deeper, what if the bottom of the keel were winged and with a design that allowed the wing portion to be rotated starboard or port (and still be hydrodynamic)? This would move the weight off center and to the direction to better counter heel.
4) What if a sailboat had, for lack of a better term, canards molded above the waterline so that as the boat heeled, the canards would contact water impose a righting force?
Hi TronOfBorg. All excellent questions.
1.) For creating small bubbles, this is an area of active research. I have only seen it on powered ships though. You need a fairly large air compressor to introduce the bubble under the hull. That takes a lot of power. With the commercial engineered applications, I have seen claims of decreasing resistance by about 15%. That could increase speed by about 7%.
2.) If I understand your description correctly, yes keel design is advancing in this area. The Volvo racing yachts using swing keels with can pivot port/starboard to increase righting moment. We don't see these outside of racing yet, because they are expensive.
3.) Good idea to offset the wing. But you get more bang for your buck out of a swing keel instead. The swing keel does more to offset the center of gravity with less complex machinery.
4.) Yes, this could work in theory. I have not seen any direct applications yet. The main challenge is that you need forward speed to get large forces out of the canards (or any type of lifting foil). Except that lifting force also has its own sources of drag that it generates. You don't get it for free. So the idea of submerging foils could produce extra righting moment. But you will probably find better efficiency and simpler construction through changing the hull shape.
@@DatawaveMarineSolutions How about creating countering waves - something similar to bulb bow. But instead of creating them in a bow why don't create them:
1) either before the bow (maybe by hanging something from the bowsprit- so distance from the bow could be adjusted to speed)
2) sideways to the bow - maybe in a form of a proper formula for a distance between hulls in a multihull design. It would still be a fixed speed (like in a bulb bow design) but it might give some bonus on a light winds during long cruising.
In case of proposed bubble-cavitation solution. How would such a solution affect buoyancy? In theory it should be reasonably easy to boil thin layer of water at the bow section, some solar panels could provide enough energy. My concern is not only danger to marine life in such design - filter of steam/bubles would have lower density than water. Wouldn't it limit application only to foiling or extremally lightweight craft?
@@gradowik Good idea, but the sense of scale just won't work in this case. When you hit hull speed, you have a wave that matches the length of the ship. To create a counter to that wave, you need a bulb about the same length of the ship. As far as the multihull concept, I haven't heard of anything like that. But more power to you if you can develop something that works.
In the realm of bubble-cavitation, first a few clarifications. When we get cavitation, those bubbles collapse almost immediately after being generated. They aren't very good candidates. There are devices that straight up inject air bubbles under a hull. It doesn't affect buoyancy at all. The air bubbles are at the same pressure as the water, so the hull doesn't see any different in buoyancy. This also is not a concern for marine life. We don't normally use steam to create the bubbles. The easier route is actually compressed air, which requires less energy. The bubbles are gone after the ship passes. We don't use these bubble systems to reduce the ship draft or use it in light weight applications. In fact, large, heavy vessels are the only ones that have the space for the necessary air compressors. Running the air bubbles along the hull helps reduce the skin friction of water against the hull. We use it for fuel savings. Still questionable if the fuel savings are worth the cost of running the air compressor.
look up the newer generation vondee glope IMOCA 60 sailboats. theyre single handed offshore racing boats, and pretty much all of the new ones are using foils that extend out of the sides of the vessel to provide righting moment and even help lift the hull partially out of the water. the new americas’ cup boats are also monohulls using a similar idea, but they can lift themselves fully out of the water on hydrofoils
Higher RM, Lower L/D ratios, Shallower 1/4 beam buttock angles ....it is complicated....but propensity to surf, is primarily a comfort trade off; when you analyze it.
or resistance vs. power ?.... yes, exactly.....and it's the bow wave that turns more power into more angle of heal ( eventually forcing a reef ? ...that is the question )
The formula for hull speed is not boat length, but water line length. You should have made this clear as correction #1. You also left out the terms for the broadest hull descriptions, which are planing hulls and displacement hulls. Sailboats have displacement hulls, which are designed to maximize water line length (being wide in the middle and wider still when heeled over). This helps them to achieve a better hull speed with small engines or via wind power. These hulls below hull speed are not sensitive to weight, so they tend to be heavy, which makes them handle better in waves and have mass to counter the heeling over of the wind. Once you understand these points of displacement hulls, you then realize that these hulls can't go faster than their hull speed - because no matter how hard the wind blows, they'll never get enough power to get over the hump. Beyond that, even with a motor, the power required for the heavy boat to start going up his bow wave is too great. Even if you COULD put a motor on to start to plane, it would be horribly inefficient, hence naval architects don't design boats like that. For planing hulls, they tend to be less heavy and pointy at the bow to help get over and on top of the bow wave. They of course require engines with power enough to go the desired speeds, but this is WAY less power than required to make a displacement hull plane.
"Ship design has many tricks to push faster than the hull speed". I'd love to see a video going over a few of those and the tradeoffs that come with them.
Really good info from nick, it's a shame that he only has 2k subscribers there should be lots more
it's probably because most people can't understand it. But as he says, we're smarter than that!
I enjoyed the Calculus, Geometry and Physics lecture. I have found most of the information on the internet is like getting a weather forecast. Everyone repeats what they have heard; no one verifies the facts or even knows what the facts are.
Allow me to ask you a simple physics question; take two round objects the same length and size, both have the same nose cone similar to a jet aircraft.
Object A; starts the gradual curve to the end (tail/stern) point at 50% of the length.
Object B; starts the gradual curve to the end (tail/stern) point at 75% of the length.
Simple Question: which object is faster given everything else is constant; Object A or Object B.
Last question what is making Object A or B slower given everything else is the same (constant)?
That is a bit of a trap question. Potentially, Object B has a higher pressure recovery than Object A. However, I would need to perform some calculations to see if the more gradual curve of Object B increases the wetted surface, which increases friction drag. My official answer: I can't say for certain, I would need to perform calculations.
@@DatawaveMarineSolutions No trap, easy peasy.
I came across this video while searching for Hull speed formulas and how they relate to age of sail (1650's through 1820's) wooden ocean going ships. In theory, a longer 4th rate ship (Frigate) could go faster than an unrated Sloop. I had thought this was due to the longer hull AND the fact that it had more canvas that it could spread aloft. I was also aware that a ship that piles all canvas aloft could end up going slower than expected largely because the bow would bite deeper into the water depending on which sails were aloft. But, at least now I have a deeper appreciation of what went into sailing ship design based on this video - and that while Hull Speed as calculated is not the final word on things, it can at least give me a decent estimate on how the ratios are between the ships.
Question? If you have a ship of a given configuration, where you know the length/beam ratio, and you have the keel length - is it possible to get a relatively decent estimate of its max speed give or take?
Yes. We generally need to know a few more dimensions and the sail plan. But yes, with that basic information, we can get a decent estimate.
I have found from first hand experience that the hull sped only determines the stability and handling of the vessel. I could show you some great footage of going at least 5 times over the hull speed of one of my vessels. However the camera is on the bottom near the mast.
If the hull speed formula limit was a definite maxium limit that could not be exceeded in any circumstances then a small dinghy towed by a large vessel many times it's length would slow the speed of the longer vessel to the maximum hull speed of the dinghy which does not happen in that circumstance.
Hi Nick, this is a really interesting topic. The other descriptor of the hull speed as I remember reading is that it is the point at which the hull must climb its bow wave to go faster requiring more energy until the hull begins to plane. But what interests me is that the modern trend of wider transom boats create other not mentioned effects. Notably when a boat leans due to wind pressure the submerged hull shape goes from being symetrical to some form of asymetrical. In older narrow stern boats the sum of the submerged shape delivers a resulting lift to leeward whereas the sum of the submerged shape of wider transomed boats delivers hydrodynamic lift to windward and possible improves the hull’s, not pointing to the wind but, net movement into the wind. Does this make sense?
A little. I'm a little fuzzy if you are talking about the hull alone, or also including the hull, keel, and rudders. But I think the general idea is what we normally call sailing side force. More sailing side force means smaller lee angle and faster speed over ground to windward. That is another topic I would love to get into. But I always like to do my research first so that I can backup anything I say with real data and examples.
I have heard of towed sailing vessels being very unstable above the hull speed. Like being towed upstream by a riverboat.
Hull speed is the nautical equivalent to the sound barrier for airplanes... not a wall, but an exponential resistance to going faster
Evan Taaved Pert totally the wrong analogy...submarines, are more of a comparison to aeroplanes...the problem with boats/yachts is the surface wave effect
Good analogy
Only the noise of a two stroke outboard breaks the sound barrier, not the boat itself.
I've often wondered why no one has tried to design mechanism between the mast and keel which could compensate for the heeling force and keep the mast perpendicular to the water's surface no matter how hard the wind blows. I guess I'm a little jealous of the way wind surfer's boards don't heel over with that swivel joint at the base of their mast.
Part of it is a safety principle. We want to prevent the boat from capsizing. As you heel over more, the heeling moment of the sail decreases. It acts as a safety valve to reduce your chances of capsize. But the more advanced racing yachts do try to compensate and keep the ship upright with a canting keel: en.wikipedia.org/wiki/Canting_keel
there is (sort of) such a mechanism. a roller furling boom. in order to reduce the heeling moment you reduce the plane of the sail that's in the way of the wind. windsurfers do 'heel' by tilting the sail heavily to windward, thereby reducing the amount of sail area presented to the wind.
however, simply reefing the sail lowers the amount of pressure applied to the mast, lessens the heeling and leaves the mast closer to perpendicular to the water. whether or not that is the faster option has a lot to do with the design of the hull and the modified shape it presents to the water.
We have a perfect real-life example of hull speed. In the early 20th Century many ships were re-engined with the new steam turbines to replace piston (expansion) steam engines. In some cases, the power almost doubled. But in almost every case the ship speed only went up a few knots. To the disappointment and dismay of the Admiralty.
This is a repost of a previous video about hull speed. Same video, but with a correct formula for the metric version of hull speed.
Apologies to all the previous TH-camrs. But I had to take down the previous video and its incorrect formula.
My sailboat (Oswald Berckmeyer Miura 31) has a very pronounced tumblehome which theoretically should increase buoyancy as it heels. What are the drawbacks of this type of design and why is it not very common. I notice the odd modern solo race yacht for long distance has a bit of tumblehome.
One reason that you don't often see tumblehome is resulting difficulty to mass produce this. Mass produced hulls typically start with a mold that we lay the fiberglass in. The mold shapes the outside of the hull to guarantee a smooth exterior finish. But if you add tumblehome to the hull, you can't get the hull out of the mold. The only way would be a splittable mold (which adds to the cost). So production dominates that a lot.
Another reason is simple convention and aesthetics. Tumblehome gets associated with older tall ships, which creates the impression that it is an old technology.
As far as drawbacks: you get less righting moment per degree of heel with tumblehome. That can restrict vessel stability.
Benefits: a little tumblehome allows a wave to ride up the hull, rather than force it down. That can potentially reduce your resistance. But it really depends on how that profile interacts with the rest of the hull shape.
A nice if somewhat very simplified explanation, of course the other ways to add righting moment such as to increase ballast, have a canting keel and increase the depth of the keel just to name a few. While adding greater righting moment is fine, the angle that the righting moment becomes effective a highly important factor as well. Another major speed limiting factor is weight and the dispersion of the weight eg. crew at the back causing the transom to drag highly increases the drag and turbulence the hull creates. Thats why you always see the rail meat amidships on racing yachts anytime the boat isn't on a run. In reality there are so many factors that limit the speed of a yacht that covering every aspect would take many hours of video. eg. balance of the sails etc. etc. etc.
With light boats, 25 ft or less sub 2500 lbs, I've often been able to push past hull speed or surf on a broad reach with the waves. There is a noticeable loss in directional stability after that point, you need to be gentle on the tiller to keep her on a heading.
Very interesting but I think for the boats I use, hull speed is the limiting factor. I use human powered racing kayaks which are also limited by the rules of the sport as well as human physiology. Winners are those with enough power to get to maximum speed first and can hold on longest. In close quarter marathon racing, we can use the wave generated by opponents to surf on and save energy for sprints at tactical moments.
Show me a sailboat with displacement hull exceeding hull speed. Its still a usefull formula. I dont think anybody claims its the limit. Like you said its the speed of diminishing returns.
I heard of a 60 foot schooner with displacement hull going more than 5 knots faster than its hull speed in a gust while under engine power with all 7 sails locked into a perfect trim but in most conditions thats just not going to happen and would likely be more efficient to run at the hull speed.
I’ve had sailboats surfing down waves at sea , your steering reverses if you do a higher speed then your hull speed !
AGHHHHHHHHHHHHHHH THAN, THAN. THEN is a moment in time. THAN is a comparison
When surfing a wave at sea, steering does NOT reverse because you exceed hull speed. Steering reverses because the wave behind you causes the water in the wave crest to move faster than your boat speed. Effectively, the bow of your boat has water flowing aft and for a short period of time the stern has water flowing forward.
I have some questions about your graph showing the 3 main components of resistance - viscous resistance is linear, wave making resistance has a hump (which is the hull speed hump that your presentation is about) and then you have another line with a hump which is labelled AIR - what is this and what determines the speed at which the hump occurs and the size of the hump? It seems to correlate with wave making. Also, your viscous line I assume is water viscousity. There is also viscous drag in air - is that subsumed in the air line I'm asking about? Is the air resistance just the drag on the exposed hull/deck/cabin or does it include the net drag on the sail plan? Also, switching back to the wave making hump - I'm assuming that the speed at which the hump occurs relates to LWL but the magnitude and shape of the hump are determined by shape and displacement - meaning that a light long skinny hull also has a hull speed bump but its much smaller so more easily overcome.
Yes, the viscous line represents water viscosity.
Air resistance is the combination of viscous and potential flow resistance. The net resistance on anything above the design waterline.
And yes, the speed at which the hump occurs relates to the waterline length, and magnitude is determined by the size and shape of the hull. With some hulls, you even see multiple humps.
Very nice introduction to this topic. All engineering design involves some level of compromise. I feel this could become a series, with ever increasing complexity. Possible with links to text books and maybe web pages, so anyone interested could further their understanding.
I wonder, if I could just use the engine to break the wave barrier, then maybe I could plane on the sails alone?
Interesting info that applies to lots of other disciplines.
The point of inflection where you encounter rapidly diminishing returns being at the hull speed is not simply coincidence.
The diminishing returns have causal relationship with the hull speed, as in, that is where increasing wave forces are encountered.
If you surpass the hull speed there is a period of rapid gain followed again by diminishing gains (increasing function of air/fluid/wave resistance) What will you say then, that total resistance is just coincidental?
I don't recall that the video ever implied total resistance was coincidental. Only that hull speed did not predict a hard limit on maximum boat speed. In general, I would argue that TH-cam is not the appropriate forum for a technical debate, which are the types of questions you are asking. Those belong in peer reviewed journals and scientific conferences. And when I present to conferences, I use a more technical and precise tone of presentation.
These TH-cam videos intend to expose engineering concepts to a wider audience in a more accessible manner. That does require less precise terminology, which can lead to some misunderstanding. But you asked a technical question, and I will attempt to provide a brief technical answer.
Based on current theories of resistance prediction supported by the ITTC, I argue that calm water resistance, expressed in non-dimensional coefficients, is a function of Froude Number and Reynolds number. The primary components are wave making, friction, form factor, and appendage resistance. Of these various components, wave making resistance is the one that demonstrates the change in resistance at hull speed. Before hull speed, it shows a characteristic hump / hollow curve. Beyond hull speed, the curve appears exponential, but the actual characteristic depends largely on the shape of the individual hull.
Air resistance also gets added in as an additional component, purely as a function of Reynolds number. For calculating powering, we may also add in an additional powering margin based on wave resistance from weather.
@@DatawaveMarineSolutions my comment about total resistance was an argument ad absurdum.
8:19 "...having this point of diminishing returns next to hull speed is just a coincidence"
Climbing the bow wave seems to be spot on the theoretical hull speed, when being propelled by the in/outboardmotor of a sailboat. At least in my experience, but I'm not a scientist.
On the topic of sailboat hull shapes. Most sailboats have a reverse teardrop waterline: pointy at the bow, round at the stern. Most other foils, including sailboat rudders, have normal teardrop shapes (e.g., NACA0009 sections). What is the purpose of reversing the shape for hull design?
The common logic is that it focuses on creating a better ride in waves. A pointy bow with vertical sides cuts through waves. What is less obvious is that these hulls do have a teardrop shape, vertically. They start deep at the bow, and slowly rise up vertically towards the stern. Of course, there are always exceptions.
@@DatawaveMarineSolutions Thank you for pointing out the vertical profile--never gave it any thought, but it's obvious now that you mention it.
How does a bulbous bow affect the wave resistance? How about a variable length bulbous bow? How does a golf ball surface affect viscous resistance? Doesn't using underwater wings affect the righting moment?
What you did, int mention is the difference between displacement hulls and planning hulls or to put it another way moving a object in water means it is displacing that water or it must move the water out of the way as apposed to riding across the surface as in a John boat or flat bottom drag boat, mearly calling it resistance does not explain it.
So good channel:) Thank you. Could you tell us a little bit about water propellers design for ROVs - to have equal efficiency in both forward and reverse? Have you got something about such propellers?
INTERESTING... why not flood the bow? If the problem is the boat climbing the wake, instead of trying to plan over that or have enough power to drive your boat uphill why not have a ballast tank in the bow which you progressively flood with the increase of speed. (Within limits) this will force the bow to dig into the bow wave and keep the boat level, no? As you slow down, you can pump out the ballast tank.
What stops me from getting a trimaran and mounting as many sails as humanly possible in it?
Was it boatlength or waterline length in the formula? Good short video, but it could use some examples of say, planing hulls or classic hulls that use their lifting forces in a dynamic way (light winds, bow and aft overhangs out of water > less viscous resistance, high winds, bow and aft overhangs in the water > longer waterline, more hull speed). Both work fine but in different sea states (yet another dimension).
Waterline length.
So, how about a catamaran with gaff rigged sails? More moment arm in the water, less surface area per righting force, less moment arm per area in the air.
the other thing is longer sails are more efficient, a better lift to drag ratio.
When hulls get skinny enough, around 8:1 or greater LWL:BWL, bow wave generation gets to be so small hull speed theory no longer applies.
we know it does not apply to light catamarans. wish you did a video on the prismatic coefficient.
Cool video, really makes me understand how I can get 9 knots out of a supposedly 7 knot sailboat! (Spencer 42)
Subscribed! Looking forward to seeing the rest of what you post!
Renegade Show Spencer 53 here!
@Renegade Show Although probably achievable, the only way YOU'RE getting 9 knots out of that 42 foot, 19K pound, cutaway full keel POS of yours is in your dreams, that, or with a 2 knot tail current. Especially given your whopping two years of sailing experience and a canvas inventory that's probably as old as you. I would prefer you come back here and actually debate me about it, but you won't, because you know who I am, and you know I'm right.
@@Esschert who are you??
@@frankchristo Why do you need to know??
What about adding side foils to the haul to lift it out of the water and reduce viscous drag which can be done simple on smaller day sailboats, allowing the haul rigidity to still be strong enough when dealing with 12-14ft hauls that wont buckle under it’s own wait once lifted out of the water... wouldn’t this be possible on say, a 14ft hobieCat or fj420.
I've never seen you make a video on how ( steel ships)(35 to 45 knots) underwater foils sooth, and increase speed, stability while lowering pitching motion, as you call green water on your front deck.
Ok, but there is another factor in sailing vessels that didn't get mentioned.....keel weight causing righting moment. if you want more power add sails AND keel weight? How do these interplay - or is just not enough to push beyond the point of diminishing returns?
Very correct. Keel weight factors heavily into the righting moment of your sailboat, which ultimately determines how much sail power the boat can carry. And yes, for racing boats, that is enough to get beyond hull speed. But I will admit that any speeds beyond hull speed, the sail power requirements go up extremely fast. So possible, but you really need to design the entire boat around going fast: hull shape, weight savings, sail design, etc. This is why we see racing sailboats as their own dedicated design.
@@DatawaveMarineSolutions Thanks for that...appreciated.
So if you want speed it's tri>cat>mono
Hi Nick. Great channel. Following on from this topic I would like to hear your take on XBOW and SWATH technologies. Are they as good at improving performance as claimed or are we being led up the proverbial garden path. Thanks, keep up the good work.
I have some videos planned in the next few months just for those. One of the hard problems is finding the time and data to do a true comparison of the X-bow. They are a private, patented hull design. And Ulstein does not want to give away their secrets.
Cheers
When displacement hulls become planing hulls for very long and very often nothing good will come of it. You will eventually become a hull full of water.
Doesn't the catamaran or trimaran provide huge righting moment for sailboats? Also, we see competitive sailors pulling their rumps over to the side to add additional righting moment.
Yes, and no. Monohulls also have a huge righting moment, due to the deep keel and the weight at the bottom of that keel. Catamarans and trimarans provide that same righting moment using buoyancy of the amas. Those amas require far less weight than the deep keel. So it is more fair to say that catamarans and trimarans achieve the same righting moment, but with less weight.
I love your lectures.
Hello, I am enjoying watching your videos. Do you plan do do anything on hydrofoils?
Eventually, yes. I get lots of great video ideas from the TH-cam comments. But no real logic as to when I will do a video.
Cool. I am about as excited by them as you seem to be by catamarans, lol
I have been trying for a while now to understand and compare the amount of power in kilowatts required to move a monohull, a catamaran hull and a trimaran hull forward at different speeds. Why? I would like to understand how to match electric motors to different hulls.
In addition, sails capture wind energy to propel hulls forward, but how much energy is actually captured per square feet of sail area.
The difference between the two should be due to efficiency and resistance losses right?
Do you have a video discussing these topics?
These video helped in understanding a bit better the hull area. Thank you.
I do have several videos that discuss the relative merits of trimarans and catamarans. The best place to start will be my video on comparing between the different hullforms. th-cam.com/video/FYFB56Q8BTg/w-d-xo.html This doesn't directly talk about powering estimates though. That is far too complicated for one video.
Is viscous resistance determined at the surface of the waterline, or is it the entire surface area of the submerged boat?
Viscous resistance depends on the entire surface area of the submerged hull.
You lost me at narrower bow. I’m looking at scow designs. I guess that the scow bow design includes the wider beam, which you mention as being good for righting moment (and, therefore, power being used for imparting velocity). But it seems to me that a scow bow makes planing easier. I’m still not understanding why a narrow bow is better.
You are trading wetted area or viscous drag for righting moment. The narrow hull designs will have less viscous drag and better sea keeping.
In the world of racing scows the sea keeping is absolute garbage, but you're not racing in rough conditions so you're trading for substantially more righting moment. If you look at the more extreme examples the boats are designed to heel substantially more than a cruising yacht in an attempt to reduce that wetted surface area gaining benefits of both designs. A canting keel is accomplishing the exact same thing while keeping total heel in check.
@@otm646 I have no idea what a canting heel is (so I will read up on that), but this is the exact kind of succinct answer I was hoping for (but honestly not expecting) from someone on the internet.
What do you know about surfskis? I’m really wondering about the dynamics of a surfski bow that digs into the next wave when someone is surfing. It’s almost like the bow is bracing against the wave, which means that it’s actually slowing it down. However, it seems stable and the boat will move at the wave speed.
I would like to see some quantitative descriptions of wetted surface that compare a scow bow with a steep-stem pointy one. Any references for me?
Depleted uranium keel... much righting moment, much speed? But seriously this makes sense why catamarans are so fast. Wide stance -beam + higher righting moment, narrow long hulls, light weight for good transfer of energy rate. Is the point where the righting moment and wind energy transfer peak measured by sail spill as in you have more energy than you can use? Sorry obvi a sailing noob here.. can you make a sail boat plane out like a recreational boat?
Light one design class type boats can be designed to plane easily. I used to sail a windmill (a now pretty much dead class) it would plane, but the rudder got damn sensitive! Also the sailing surfboards planned well, seemingly most modern one design classes will plane, although the newest thing seems to be foils.
Hi.
I want to bulld a boat for sitcom navigation.and I do not use sail.it will be powered by smal electric motor.now I am thinking wich type of hull to choose.semi v or modified v or flatbottom or displacement or deep v?
Or mix of all of them or mix of some of them.what is you advise?
Americas cup is flown by multihulls on foils. What happened to the monohulls with spinnakers? Did monohulls point better?
button down then sub train blowing your siding plastic overlapping skirting up & finally u r up on foil damn near completely out of the water.
Hull speed : The max speed of a displace hull boat. Any speed greater than the hull speed of a displacement hull boat, the boat is planing .
if heeling is the problem , then use a trimaran or a catamaran
Please, comment on a and d of Lapstake hails. Thankyou
What is the minimum wetted surface area needed to affect an increase in calculated hull speed? For example, could I hang an 12' 4x4 post from the bow pulpit in the direction of travel, resting one end against the hull and thus increase my speed? Would a 12' 1"x1" 'stick' be just as good? Or, do I need an area that covers from the water surface to the depth of the keel to affect the calculated hull speed? hum...Worded yet another way, what is the ideal ratio of length vs depth to gain in hull speed?
Just adding small changes will not work. Any changes need to be a faired addition to the hull. So you basically need to add something that covers the entire hull.
Hull speed is purely a function of waterline length. Wetted surface may impact how easy or hard it is to achieve or exceed hull speed for a given condition, but it won't change the hull speed itself.
Although I first thought I get to hear my future and horoskope and what death I will die, he actually explains perfectly and understandibly. Profound knowledge and no bullshit.
Just curious why I get the feeling of beeing tarot-carded
Hahaha bruh!
What do you think about using a round parachute high above the bow to get better wind downwind ?
I think the parachute would perform about the same as a spinnaker. Up high has some potential. The major benefit to that is going higher gets you out of the atmospheric boundary layer. But you need to go fairly high to get any major benefit. Imagine doubling the height from 50 ft to 100 ft mast. That is only a 10% improvement on wind speeds, assuming a typical atmospheric boundary layer. Another option is a kite sail: www.skysails.info/
Getting the pliers ready to pull my foot out of my mouth just in case, but I'll try it anyway ....
Round parachutes have a stability issue, and cannot be easily directed. Square/delta/wings ones maybe, but good luck with the launch and the retrieval on a sailboat.
SkySails yatch: th-cam.com/video/4QJmD6m3das/w-d-xo.html
Wingsails: th-cam.com/video/QGJ02kEDtXI/w-d-xo.html
Bring down the cost of anti roll gyros and righting moment will be relevant rather than theoretical.
it doesn't hve to b mono hull & not cat either & not even in water hull at speed. If sail is far bigger than boat blimp shaped than the right force & left forces can b balanced out & just all ends up being driving force, bc the big air catch in middle is greater than righting or left blow. Like a fat long foot ball.
It's not enough that imperial system is confusing and doesn't scale linearly, now you have to mix it with metric to make it even worse. Why use m/s² for gravity and not speed of squirrel with oversized testicles per average chin height of a lama. What kind of abomination is speed in knots for length in meters? It's like saying pressure is liters per square foot. Doesn't make sense. Other than that, great explanation.
Metric measurement system = unstoppable force.
Shipping tradition = immovable object.
Horrible units of measurement result from combining the two. :-)
@@DatawaveMarineSolutions right you are. Same with units used in flight.
I would add the “ winged keel “ acting to slightly lift the boat causing less hull surface to interact with the water ... just my experience sailing with different keel configurations
Damn interesting!
Hello, Have you heard about "HULLVANE" ??? Can you please look at it and make a video. Thanks
I will add it to the list of planned videos. I actually did a design similar to hull vane a few years back. Based on the problems I encountered with my design, the Hullvane concept has the right idea. I'm a little skeptical about their claim of savings in the range of 10-20% (www.hullvane.com/tech-talk/). But I have been surprised before. So it might be possible.
What about replacing the mast(s) and sails with water jets?? No righting issues then.
Very true. We can easily use engines to power the boat up to extremely fast speeds. At that point, you actually get limited by human endurance. At the high speeds, slamming into the waves creates shock loads that can damage humans. For the high performance military boats, they actually need to install special shock absorbing seats.
I am serious, for example, Classen's 127' Atalante or similar hull, can this be built without masts using two waterjets instead for a cruise speed of 22 - 24 knots??
here is a video of her: th-cam.com/video/X7cD6RMv07o/w-d-xo.html
@@DatawaveMarineSolutions are You going to cover the sails and rigging too?
Maybe proper sail design could not only power the boat but also lift leeward side from the water? We already have genoas and spinakers lifting partially the bow, why not use aerodynamical part of sailing instead of fighting with physics of hydrodynamics?
why don't use a bolbous bow to cacle this wave?
bulbous bows only work at a narrow range of speeds. Sailboats don't travel at consistent speed (because the wind isn't always blowing with the same force) so a bulb isn't useful.
.18 miss- pause cap 101x =. 2754 vain glory 1x NT; money purse 2x OT
1:51 paused & 1:53
Is this the same as speed over ground?
No. This measures speed through the water and related to your hull length.
Hydrofoils still confuses me :D
As a Mechanical Engineer, I have to say I very much enjoy your explanations in easy to understand ways. Perhaps in an upcoming video, you could comment on some of the new hull designs such as the Beneteau Air Step Hull (th-cam.com/video/v8NmQM_s8_E/w-d-xo.html) and the Shannon Silver Hull Design (th-cam.com/video/HVPEAU_gK0o/w-d-xo.html) and their importance or non-importance. Keep up good work.
-johnny
air hockey sailing skin on sail boat so less drag.
same way air ship floats over water only u want to use air skitting to cut thru water better, at speed & switch off air skit when not out clubing, more button downed.
1.35 x Sq root of WL
u can cheat & on less windy day after blowing skit u can add just a bit of mechanical speed to help get up on foils.
Talks a lot but doesn't actually tell you anything useful. Does not give any figures of what he thinks might be achieved. The fact remains that the well accepted formula will give you a pretty good estimates for your displacement hull speed.
So hull speed isn't real, but it is accurate. Got it.
w/only keel or keels short but 2 or 3 in water & few perfectly placed foils u could break lots of speed records fat foot ball sail.
Interesting
'But you're smarter than that' lol, I have no idea what you're talking about.
How to cancel out hull speed is simple get a catamaran done don't need this.
Cats have their own design issues. I'm old enough to remember when people who took cats into blue water were thought to be crazy or suicidal.
EKRANOPLANE