Why Is The Propeller At The Back?
ฝัง
- เผยแพร่เมื่อ 26 ก.ย. 2024
- Get your own Little Captain now: store.casualna...
✩ABOUT THIS VIDEO✩
In this video we investigate why the propeller of ships is found at the stern and explain why it just wouldn't work as well if it was at the bow.
✩JOIN OUR COMMUNITY✩
/ casualnavigation
The community is our space for anyone that wants to get even more from Casual Navigation. I am eternally grateful for all members of the community as you are the ones that help me and continue to drive this channel forward.
When you join, you’ll instantly become much closer to the channel, with a range of perks and rewards depending on the tier you select.
➼ Ad-Free
➼ Early Access
➼ Bonus Content
➼ Get Involved
✩WITH THANKS✩
➼ Images used under license from shutterstock.com
Propeller Airliner - Mechanik/Shutterstock.com
✩DISCLAIMER✩
All content on this channel is provided for entertainment purposes only. Although every effort has been made to ensure the content is accurate and up to date, it remains the responsibility of the viewer to determine its accuracy and validity. The content should never be used to substitute professional advice or education.
Older Finnish icebreakers have 4 propellers, 2 at the bow and 2 at the aft. New icebreakers have have 3 pods and one of those is located at the bow of the ship.
Do you know why they do this? Does it prevent the ship from pushing up on top of the ice?
@@johnathanclayton2887They want the ship to push on the ice because thats how ice breakers work, the ship pushes itself on the ice and breaks it with it weight, it doesnt plow trough the ice.
Reason will be that an icebreaker needs more manuvarabilty also in reverse and redundance in case a propeller gets damaged.
@@johnathanclayton2887 The 4 propeller configuration is intresting. Two bow propellers turn to inside to flush water against the hull, so that it would minimise friction of the broken ice against the hull. Two aft propellers turns to outside so that it will push the broken ice under the intact ice. With the 3 azipods configuration you get superior maneuverability in ice fields.
It is often used to wash the ice away from the hull, as the friction forces with the water are lesser than with the ice.
Orange hulls are also just totally weird, like stay away from polar rollers is you value your life and sanity...
Just a tip: having the two sides of the propeller blade be different shades makes the 3D effects a little easier to read.
As it is now the blade just disappears when the tip is facing you.
Edit: sooo many typos
Yes, that hurt my eyes so much
I love that your videos always start relatively simply with things that I might already know or are very intuitive. By the end you've always taught me something new or taken the discussion in a direction I hadn't anticipated! You're a great teacher.
Agree 100%. This 6.5 minute video is packed with the physics of the subject. These keep me wishing for even more.
@@chasbodaniels1744 Do you ever peruse BOATS & HARBORS'S YELLOW NEWS- bitd TRADE PAPER? they? "CAROLINA SKIFFS" MADE A CENTER MOUNTED OUT BOARD MOTORED COMMERCIAL FISHING BOATS!
I would have liked a little more contrast with the example of aircraft given in the beginning, which usually prefer tractor over pusher configurations. As it is, I think that case provides counterexamples to most of the reasons given here:
-Aircraft don't mount any control surfaces directly behind propellers, as they prefer "clean" (less turbulent) airflow over the control surfaces for predictable, consistent maneuvering. (Even rudders at the end of small aircraft are negatively affected by the rotating prop wash from a nose-mounted propeller)
-An aircraft's body does not block the thrust of the propeller mounted in front of it, as it is designed to be smooth and aerodynamic. In fact, the cleaner airflow at the front improves the propeller's overall efficiency (As far as I know, this is also the reason to commonly mount jet engines on the wings instead of at the back of the fuselage)
-Aircraft prefer tractor over pusher configurations, i.e. tension over compression in the axle/shaft and bearings. I find this intuitive, given the high tensile strength of steel (and most metals?) compared to its lower compressive strength combined with possible buckling limits.
This leaves two reasons: protection from damage, where the analogy to aircraft clearly does not apply (except for gear-up landings maybe), and easier sealing (where I'm surprised the additional pressure is significant compared to the head pressure, but my intuition may be wrong here), both of which do not apply to azipods mentioned at the end?
I don't intend to come across too negatively, I like this channel, and this is an interesting topic I had not come across, I just feel this video has not entirely answered the question in the title to me. Then again, I know much more about aircraft than about ships, so that might color my understanding here
Good points. Another thing to consider is weight distribution.
Sealing isn't a problem for aircraft. Ships have to keep water out to preserve buoyancy. Aircraft can "leak" and it's fine, it's just air.
@@DavidGuild To be pedantic, many aircraft have pressurized cabins and do need to seal, but the engines are usually outside of the cabin. But yes, sealing is one of two points in the video that are truly specific to ships
Another reason tractor configuration is preferred on aircraft stems from those early days when a motor driven aircraft was a big lump of iron (the motor) inside a stick-and-rag frame. Pusher aircraft usually had that big iron lump right behind the pilot. Crashes in those low-speed days were often more sudden stops ... but that iron lump and its inertia wanted to keep on moving. The pilot, being comparatively squishy, wasn't much of an impediment to that. Pilots objected to that situation. So motor-up-front became preferred among pilots.
@@DonleecartoonsAnother factor is the weight distribution. An engine is often the heaviest part of an aircraft, so for light aircraft that could offset the center of mass greatly. Keeping the center of mass on top of or slightly in front of the center of lift helps prevent stalling.
I must say, The Little Captain is ridiculous adorable and reasonably priced. I want him! 🤔 Maybe a birthday gift to myself
Wow. This video makes great sense in every little detail!
Sometime ago I expressed my enthusiasm about the azipods to a friend of mine who was a mariner. He cooled me down abruptly, saying that they prefer the electric motors installed well deep into the ship, with magnetic couplers and big axles that they can actually see and service en route. If an azipod fails in the middle of the Pacific, they must limp at a quarter steam to their port of call; if a traditional power plant fails, they can always fix it - with the exception of a broken axle.
I like the compactness and manoeuvrability of the azipods nonetheless. When I was a young kid, I had a RC boat with two axles and two propellers, and a propelling pod wasn't even a concept at the time.
Thank you again for the great video.
Greetings,
Anthony
The Joint Marine Systems Ghost uses forward mounted propellers to cause supercavitation around the hulls to reduce drag
But that is neither a ship nor a boat. Could maybe call it a catamaran type vessel. The Juliet Marine Systems Ghost seems to function like the pods on Cruise ships.
@@Varangian_af_Scaniae a catamaran is a boat
@@Varangian_af_Scaniae No. Google supercavitation. It's a game changer concept that changes all what this video says. It's still at its infancy and it's not easy to develop though. But it's a possible future.
@@Varangian_af_Scaniae Yes, it is.
It's also worth noting that there ARE multiple airplanes with the propeller(s) at the rear, and the same with jets.
The Icon A5 being a beautiful modern example.
Also, planes don't use pods. We have nacelles.
The little captain is love, the little captain is life.
Stop lying.
@@wildgrizzlybear8920you have ruined my day, how will I ever live knowing that I’m the lyingest liar in the history of lying?
The only instance i can think of where the prop is at the front is trolling motors for small bass boats.
Considering how they are used and how they can be retracted to avoid damage, they are probably more analogous to maneuvering bow thrusters than bow mounted main screws.
Airplanes with front propeller reasons include better cooling . Prop planes with regular or diesel engines get intense airflow over the engine to cool it more than if the prop was behind. Its not always vital and turboprops can be different. Another poster also mentioned the lift benefit of the intese airflow over the wing behind the engine.
Pusher-props would also be difficult to implement on a taildragger (due to ground clearance), so you can have less complex landing gear. Additionally, with a single prop at the back, braking could become more difficult with more weight at the rear.
But front mounted props are also somewhat problematic when interacting asymmetrically with the control surfaces on the wings, so some pushers have been built.
I was already in the process of skipping the ad when i saw what it was ad for. Went back and watched it, cheers captain
And on the other side of things aircraft propellers are meant to pull and on multi engine aircraft the benefit of having front propellers that are wing mounted is that it provides artificial airflow over the wings making it more resistant to stalling
Brilliant question that I never considered and I am surprised by how much behind this question there actually is.
Will say, got my brother (who is fascinated by historial sea voyages and legends of old ships) the Little Captain, and despite him being a grown man in his late 30s, his face lit up like a child when it arrived. We briefly played a game of hiding it around the house waiting to see who would find it, which was cut short when one of the cats won a round. Now the captain is setup on a cat proofed shelf in my brother's room where he can safely watch over the chaos.
Video idea - talk about how sails were phased out. SS Archimedes had steam propulsion but was also fully rigged for sails. As time went on ships (specifically navy ships) had provisions for sails but they look less and less effective over time until they eventually disappear altogether. What were the last navy ships to have the option to go under sail? How did the maritime industry in general get comfortable moving away from sail and trusting in powered propulsion?
I now know more about boats and ships listening to your videos than I knew I could even know.
The complexity of captaining a ship is far more complex than I ever thought it was. Watching the captain s on deadliest catch made me believe it was mostly point tge ship that way and go. Never understanding the complexities of cargo, ship design, control. . .
By the way, ship propellers do come part way out of the water during severe pitching. My father had the misfortune to get a cabin in the stern of the ship he was on to come to the US in 1948 and had to experience this first-hand when the ship got into a storm.
Also should mention bidirectional ferries -- with these, you _can't_ get away from having some propellers up front, unless you're talking about paddle wheel ferries (which also existed well into the 20th Century).
The little captain looks like E.J. Smith. He ain’t getting on my boat… 😂🤣😛
Double upvote at 1:30 because that first part is such an awesome explanation. And a triple upvote by the end. Please use this as a benchmark for future shortish videos.
Having the propeller in the front would cause a challenge connecting the propeller to the engine, since the rudder is in the way. Thus needing a more complicated solution.
With electric drive that is irrelevant. Azipods just rotate the propellor and mount it below the hull.
One additional factor. With the propeller at the bow, the rudder is in the way of the driveshaft, making beveled gear necessary to wrap the shaft around the rudder, reducing efficiency for all types of ships.
You'd just have two surfaces on the same "rudder" as turned by the steering gear above, one below the prop shaft and one above the prop shaft, and connect them with a hoop, so its not really a major consideration like extra gearboxes to warp the driveshaft around the steering. Also that wouldn't apply to azipods as the pod itself can be both rudder and prop already, yet these are also mostly at the stern.
@@SheepInACart ? Ever seen a ducted Lake Erie propellor on a trawler?
what about having a chain drive attached to the propeller shaft the top shaft is powered by the engine itself
Another point is stability. With the rudder in the front any drifting of the vessel will cause it to turn further without corrections. It's like pushing a trailer instead of pulling it
You could have a rudder at the front and a large stabilizing fin at the rear, as long as the center of pressure is behind the center of mass it will be stable. This would be like a car steering with the front wheels, having non-steering rear wheels. Or even like a canard-type airplane, having the elevator at the front and main wing at the back. But a rudder at the front and less stabilizing at the rear would indeed be somewhat unstable, like a car with casters at the rear or when the rear tires are sliding (that Top Gear episode where they put pie-pans under the rear tires).
Makes sense
@@quillmaurer6563 "You could have a rudder at the front and a large stabilizing fin at the rear"
Sure, but any time you have a large thing to correct problems with some other thing, that tends to be rather inefficient.
Exactly this, its also why frontal canards on aircraft are much less common than tail mounted elevators, the negative feedback for angle of attack makes aircraft much safer and easier to operate, and you also don't have a critical threshold where rotation become irrecoverable (because the drag of the control surface causes simlar or more rotation in the direction your already turning than it creates in force to turn you back towards the center).
@@SheepInACart Aircraft with canards are just as safe and stable as those with rear tails. The larger, lower-loaded aft wing still puts the center of pressure aft of the center of gravity. The same could be achieved on a ship with a front rudder and large rear stabilizing fin, but it would probably be less maneuverable and more drag overall, so no reason to do so.
Beechcraft Starship, AASI Jetcruzer, Piaggo P-180 all pusher propeller aircraft for anyone who is interested. I see alot of comments about the complications with push props so didn't want people to think it can't work. The 180 is one of the fastest private aircraft.
Push props are more efficient but pull props are more convenient for aircraft ease of maintenance, design, visibility.
0:28 such a good portrait
As always, excellent video and I really like the fact that you seem to cover all bases, including details like water being pushed towards or pulled away from the seals.
love these videos. i live a pretty landlocked life, but giant container ships and gritty sailors have always fascinated me. Also your voice is pleasant. 👍
I love that all of the mechanics and engineers are having minor fits about the thought of extra gear boxes, prop shafts and everything else this would complicate.
Perhaps it would be so much cooler if there were some viewing windows to see all of this wonderful mechanical engineering artistry. Maybe have everything resting on a bed of sand to absorb vibrations.
big ships are direct drive from the engines. no clutch or gearbox . to reverse the engine reverses . so if there are 2 props there are 2 engines. 4 props could maybe run off the front and rear of engine or have 4 engines.
@@ronblack7870 This is true for many of them, but I don't think it's true for all of them -- cruise ships, for starters. Also, are you sure that something like a roll-on/roll-off carrier would use a low-speed direct-drive engine? Seems that the long stroke length cylinders might stick up inconveniently high for optimal layout of the vehicle decks.
@@Lucius_ChiaraviglioMost ro-ros are direct drive diesel, if you look at pictures of them you can see the exhausts are offset. The engine is still centered
CPP. Controllable Pitch Propellor. No need to run the engine astern.
Inspired by another comment here, I just got a really fun idea: The host of this channel should go to Port Revel Shiphandling Training Center. It's a lake in France with manned 1:25 scale model ships designed to handle similar to their full-scale counterparts, used for training captains and pilots. Let him try his hand at operating a miniature cargo ship, put his knowledge to the test. Maybe send Mike Brady from Oceanliner Designs too, he would certainly have a good time there even if it isn't as closely related to his old oceanliner content.
I have read that having the prop at the rear also helps energize the boundary layer on the hull, making it more efficient. This is less important for an aircraft where the air has less density. For propeller planes, the issue is for cooling of the engine.
Kudos to your topics, graphics, history and overall approach.
This is a super cool video!! It actually ended up answering a question i had from a completely unrelated video i saw, where a gentleman was interviewing an engineer of a brand new cruise ship and they talked about the pods!! I was wondering why choose those, but the other video never gave an explanation other than "efficiency". This is a fantastic video!!
At 3:37 the propeller and rudder under the hull reminds me of the inboard (as opposed to outboard and inboard-outboard) motor boats / yachts from when I was young
And I remember how careful you had to be to not run them aground
Even as a youth (about a million years ago) I thought the traditional inboard motor scheme was less than optimal for shallow waters (like the Gulf Coast)
Another great explainer!
It's weird, I don't have a boat of any kind and I'm not even keen on being on the water but I look forward to your videos and always find them fascinating. Thank you.
My initial guesses were;
1. Protection, the stern is the least likely place to get smacked.
2a. Access.
2b. Hydrodynamics. Why is this paired with access? Well, a bow screw would require a choice, exposed and easy to work on in dry dock or with added Hydrodynamic shielding, a complex prospect as such a structure would have to try to minimise obstruction to the screw while reducing drag.
3. Convention. Like bicycle drivetrains being on the right or the best chocolate coming in purple wrappers, it's a well established convention to whack a screw aft since so many early ships did so therefore giving early experience for such designs.
4. Mass management, no doubt keeping extra mass aft helped earlier ships open up hull space be it for goods or passengers, this goes double with motor vessels which can wedge the engine anywhere along the hull if need be.
Props to you for an excellent video!
Really fascinating! It's obvious but only in retrospect.
My old main ship had tow azimuths under the mid part of the vessel. Was working quite good when pacing on the ship, due to the sucking from the props. Also been on a ship with a retractable in the front, which when laying standby could work like a anchor and keeping the heading quite good
I love that you modeled a king air, that was my favorite part.
Hey, being a sailor, I'm quite sure you'll have your own boat. Don't have to be nothing fancy, could just be a small launch with a pair of motors, but maybe you could do a video on her?
That's a good point - a lot of TH-camrs who focus on car content talk about their own cars, even if that's not the bulk of their content. Would be interesting to see his boat, if he has one. I could even imagine a home-built boat of his own design given his interest in naval architecture. But he's not talked much about himself or his background, only recently showed his face for the first time, I think he wants to keep the content purely on the subject and not about himself. Though that could change over time, a lot of channels started out very non-personal and the host gradually revealed more about themselves, and showing his face is a step in that direction.
Now that I think of it, the perfect boat for him would be a manned ship model - these are scale models (commonly 1:25, meaning still a fairly large boat) of large ships that are used for training in shiphandling, designed to behave similar to their larger counterparts. I've always thought a chance to skipper one of those would be incredibly fun. There's a place called Port Revel in France that has a lake to operate these as a ship captain and harbor pilot school, but I'd love to see someone make something similar as a theme park.
@@quillmaurer6563 That's what I was going for. If he wants to do more things about himself, then this was meant to be a suggestion for a possible method of proceeding.
I bet he has a dueling scar from defending his schooner.
@@darealsherlock8026 Yeah, that would be good. I think this channel has enough loyal fans that we're eager to see more of the man behind the curtain. Thinking of all this, a video that would be really fun would be for him to go to the Port Revel Shiphandling Training Center in France. It's a lake with a bunch of 1:25 scale ships designed to behave similarly to their full-scale counterparts, used for training captains and pilots. Would be great for him to try his hand at operating them, put his knowledge to the test.
An obvious exception are tugs. Wether using pods or Voith Schneider, it's generally in the center under the hull. But that's for a good reason as tugs need to be able to provide incredible amounts of thrust in any direction, and have no need for efficiency at speed.
Your animations always are a joy to watch!
I think the most important reason, is to protect the propeller from possible damage if mounted at the front @4:00 . 😊
Imagine the dolphins getting chewed up in the propeller at the front 🏄🐬🦈🐋🐳🔥🔥🔥👻
Hitting floating debris was my first thought.
I have heard once that for aircraft, propellors at the back improve efficiency, however, at the front they provide extra lift as they move more air over the wings.
Also, when moving ahead the pivot point is aprox 1/3 of the ship's length from the bow, so having the rudder and the prop at the bow would make the ship less maneubravle bc the moment arm would be much smaller.
A very stern lesson, indeed!
If I might offer a suggestion…
I should be very interested to see you do a video on the evolution of the propeller itself. I seem to remember you doing one on the number of blades (or maybe that was someone else; I do consume a lot of nautical and maritime related content on TH-cam), but nothing covering, or even touching upon the quite radical difference in blade characteristics and design, from their beginnings to now, and their several permutations over the years in between.
The primary reason for having the airscrew at the front of aircraft is space availability, and the secondary reason is engine cooling, which is markedly worse in pusher designs
Aircraft need to pitch up for both takeoff and landing, so front mounted propellers need less landing gear height to avoid a prop-strike.
On the other hand, no, pusher designs neither have worse cooling in flight (propellers do not increase mass flow rate of air for a radiator, only its speed), and almost all turboprops are still tractors, despite no requirement for separate cooling air and their very small engine size.
Nobody has really mentioned the effect of the boats wake on steering. When a displacement hull is moving forward, it creates a bow wake. The closer the hull gets to hull speed, the more steep this wake becomes. Over simplified, it pins the front of the boat in place, moving forward takes energy (either from the prop, or from momentum), moving sideways (over the wake) can take a lot of energy. With a kayak, we see this very well. It is easy to slide the rear of the boat around within the widened bow wake (making a gentile arced turn and maintaining momentum), but with considerably more effort we can raise the bow over the bow wake, creating a faster dynamic turn - burning off almost all forward momentum and often causing beginners to flip. A rear rudder is efficient at controlling the slide of the stern - it prevents the rear of the boat from sliding side to side, while allowing controlled arching turns. Front mounted propellers do have a place at low speeds - bow thrusters and trolling motors. Most boats are not designed to lift the bow over the bow wake at higher speeds - but when they do it can be very dynamic-think speed boat swapping ends.
Honestly the propeller in the back is very intuitive
Bought a little captain can’t wait to get it!
The Frisia 3 ferry uses the Voith Schneider drive and therefore has 4 propellers. 2 in front and two in the back. The ferry is built to be symmetrical at the front and back and rides equally well in either direction. Its a German ferry.
Thanks!
Had no idea there were so many reasons for this. Great engineering content
The USCGC Mackinaw has 3 propellers. 2 x 24 foot diameter screws on the stern, and a single 12 foot diameter screw on her bow. This was actually to churn Lake water under her and she crushed ice with her 5,252 ton bulk.
Worth saying that a number of aircraft have used rear propellers (most famously the first plane from the Wrights, but there have been others).
Arguably these days it's most popular to mount the engine in the middle of the plane, on the wings.
Of course when you're thinking of "these days," you're probably thinking of jets. Propeller aircraft most often - but not always - have tractor propellers, even when mounted on the wings. Though wing mounted propeller engines could be seen as sort of similar to the Azipods mentioned in the videos, mounted separately from the main hull/fuselage rather than at one end of it.
Of course there are other factors but with aircraft the major consideration of bailing out tends to favor Tractor arrangement, pusher planes being notorious people blenders.
@@MediumRareOpinions That's applicable to military aircraft, and the few pusher-engine fighters often had systems to explosively jettison the propeller if bailing out, or some of the first ejection seats were developed for this reason. Not a concern for civil aircraft though, most civil aircraft, even small propeller planes, the occupants don't have parachutes, so you'd have bigger problems than being chopped by a rear propeller if you jumped out.
Good presentation. 👍💯😀
Can you do a video on who owns cargo containers and how their logistics works?
Another point on running aground…. If the propeller, rudder, etc is at the rear of the ship it’s more likely to be available to help you back off of whatever you just ran into. Whereas up front, there’s a good chance it’s either damaged, or literally stuck from being driven into the ground.
Algorithm support comment
sui
For “engagement”?
@@samuelhulme8347 ya, just something free I can do to help channels I like
@@andreaskole958My reply will benefit Casual Navigation because that’s how the TH-cam system works
The main point is missing here. The efficency of the propeller in the wakefiled of the vessel is much higher becaus the water getting into the propeller streams slower and thus the propeller can better excellerate the water.
Video idea: how did the big sailing ships manouvered without an engine back in the days for docking
S.S. Archimedes; what a great name for the first propeller-driven ship!
Now I can put a little sea man on my desk!
Azipods are not used in cargo ships because of the cost. They are more expensive. Also cargo ships sail long distances at the same low speed. Simple salutation works well. Nowadays Finnish icebreakers have Azibods too.
On an outboard motor, then, does the turning moment increase with speed, as the propeller mount (or whatever that sticky-outy bit is called) acts as a rudder?
The very first intuitive idea on teh question for me has been: Pushing the water against the frontal hull would create a lot of pushback, deminishing the pulling.
Everything else has been cream on top in my head. ut for a split-second intuition, i could imagen the people of the past are well aware of a number of problems with a frontal mounted propeller.
Another reason is that a ship's hull is relatively large and the propeller relatively small compared to a plane's propeller and fuselage. The airplane's propeller must be large because the density of air is much lower than water. And aircraft have to be light. The bulk of the wash from an airplane propeller is near the tips so most of it can get around the fuselage. Jet and turbojet powered aircraft do put engines in different locations, because the front wouldn't work well.
I would assume at the stern you are also less likely to run into plants, ropes or other similar objects that will wrap themselves around the propeller?
Next time on Casual Navigation: Why is the deck on top and the hull below 😅
If you've ever tried backing a motor boat out of a slip you'll understand how much easier it is to control it the other way. Its due to the way the hull of the ship moves through the water and how the water moves over the hull. If you lay a piece of paper on a flat tabletop then blow across the paper you may understand that last part. The forces acting on the hull become very unpredictable when water is being pushed between it and the main body of water rather than when it is being pushed through the water only. Turbulence is created that can make the boat go left when you think it should go right, or shimmy back and forth with increasing intensity.
The only ships I believe that do have in a certain mode with props at the front are either icebreakers or icebreakering tankers.
These are designed to run bow first for normal running, and stern first to break thick ice. But have similar efficiency at lower speeds. Lastly these use a pair or quad of azipod thrusters as well as rudder. This increases manoeuvrability.
The purpose of an icebreaker's bow propeller was to suck water out from under the ice in front of the ship, making it easier to collapse and break. That turned out to be not as true as was thought, so bow props were left off new ships or removed from old ones.
Double ended ferries do too (to go with the matching bridge at each end).
Makes a lot of sense
The reason airplanes usually don't use pusher props is because of how the propeller interacts with the airflow coming off the fuselage or wing ahead of it. It creates vibrations and other issues that negate the efficiency benefits. Cooling can also be an issue for air-cooled engines, and the prop diameter is limited by the landing gear (more so than traditional airplane propellers)
me at any given time: "I have no questions about boats, ships, or nautical vessels of any kind"
*casual navigation posts a video
"...hey wait yeah why IS the propeller at the back?"
Have you done a video about cylindrical sails yet?
Push/pull done
Thinking of the comparison between airplanes and ships, I realized another major factor the video missed. Having the propeller at the rear - be it of an airplane or ship - is more efficient overall as it re-energizes the boundary layer, though the propeller itself suffers in efficiency due to turbulence entering it. This improvement is more pronounced on ships than on aircraft for a couple reasons. Ships have skin friction - water dragged along the side of the ship as the ship moves through it - as a greater portion, I've heard even the vast majority, of their drag. The propeller re-energizing this helps a lot, the ideal thing would be to draw in all this water that's been slowed down and speed it back up, resulting in nearly zero water movement in either direction behind the ship. Not to mention that the size of the boundary layer is probably larger relative to the propeller than it is for an aircraft, which has a much larger propeller due to less dense air. Airplanes have skin drag as well, but it's a smaller portion of their overall drag, as they're in contact with a lot less mass of fluid, and much of the drag is induced and wingtip-vortex drag from the wings, a lot of drag being a by-product of the need to generate lift that doesn't apply to ships. Meanwhile the water flow behind a ship's hull is probably a lot less turbulent than behind an airplane, as ship hulls are designed only to be as streamlined as possible, disturb the water as little as they can, while aircraft have to redirect air downward in order to generate lift - this is very visible in videos of airplanes flying low over dusty ground, punching in and out of clouds, or the patterns of smoke and flames behind military aircraft dispensing flares. This "wake" of downward-moving air behind an airplane is such a big factor that aircraft must be carefully directed to avoid being too close behind one another, especially a smaller aircraft behind a larger one, and there have been several crashes caused by this. Having the propeller operating in this much more disturbed air would reduce the propeller's efficiency, while likely also causing vibrations and noise as the propeller moves through the wake - certainly a lot of noise on some pusher-propeller aircraft (most famously the Piaggio P.180) from engine exhaust passing through the blades.
Was following the compression theory except for the glaring fact that ships have reverse, which then makes the stern the bow so to say, but the shaft doesn't pull out even at back full while under way.
I have seen that some ferry has a propeller at the bow and the stern, and they are not double ended like those Staten Island ferry. Is it so that they could run in both directions without having to turn? Or are they helping in maneuvering of some sort?
BTW, of all the propellers, I think the Voith Schneider is quite a unique one.
For anyone wondering about why plane propellers are usually at the front:
Planes need to point a bit up into the oncoming airflow to take off. If the propeller were at the rear it would either crash into the ground, or the plane's gear would need to be very long especially at the nose. It's the same thing for landing as usually planes need to keep their nose fairly high when landing.
Also, when a vessel is making way ahead the pivot point is approximately 1/3 of de LOA from the bow, meaning that if you have the rudder in the bow, the lever would be 2 times smaller making the maneuver much difficult
I think that if you had the rudder exerting the sideways turning force at the bow, the pivot point would be further aft, probably 1/3 of the way from the stern. All to do with the balance of forces, equilibrium and stuff.
nice one!
Those old time sailers knew their stuff! 🎉
Could you do a video on catamaran boat and why only fast passenger ferrys use it? And not cargo ships?
3:45 That is exactly what many very small boat with outboard motor does, they can retract it back when in shallow water.
I say it is just that most aircraft engineers that get it wrong. Push props are so much cooler than pull props!
(though there are several reasons why pull props are more common than push props. But push prop aircraft are not unheard off. Wright Flyer I even used propellers in push configuration. There are more pros and cons to take into account when it comes to aircraft designs.)
I agree with some of the other comments - You never came back to aircraft to complete the comparison.
For example: WHY are props in front on airplanes? Then, there are the anomalies that are XB-35, B-36 Peacemaker, Beechcraft Starship, Reaper UCAV, etc, etc…that DO have the props on the back.
Can you talk about the forward propellers (bow thrusters) on ships with a bulbous bow?
the propeller will also be in the water most of the time at the back
👍 Great stuff (as always) !!
Love these videos. I was traveling back from the Channel Islands on a tri-maran fast ferry would like to see a video of the characteristics on one of these.
I have a question; why are the sterns of most ships flat, or very blunt?
The bows are relatively sharp, but the aft end of the hulls are almost flat.
Now could you tell me why ship main decks curve longitudally.
Propellers on planes generate extra lift because the propwash is so much faster than the general airspeed, and lift varies by the square of the speed. Pusher prop planes and jet planes don't have this benefit, which greatly helps shorten takeoff runs.
The video I never knew I needed. Why don't ships that need lots of maneuverability, like warships, have a second rudder at the front to work opposite the one at the stern?
I would have thought the most important factor is something you didn't mention at all: Steering stability. With the centre of pressure behind the centre of mass of the ship, the thrust acts opposite to the perturbation, returning the vessel to its initial course. If the centre of pressure is ahead of the centre of mass, this is not the case and the vessel tends to turn off course.
The same is true of rockets.
Hi!
You have mentioned that pilots don stay on the ship over the whole passage. I find it interesting and would like to know more. But i could not find any thing on the subject on you tube. Maybe it could be a video topic?
Well I'm convinced.