Saildrive Anodes and Galvanic Corrosion
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- เผยแพร่เมื่อ 23 ก.ค. 2024
- When I took our boat on land for some maintenance I found that the anodes on the saildrive were used up very unevenly. The anode on the saildrive looked almost new but the one on the propeller was completely gone. I suspected a fault of some kind here and started "troubleshooting". But after reading up on both parts I found out, that it is actually a feature of the Volvo saildrive and the Volvo folding propeller to be isolated against each other to prevent galvanic corrosion.
0:00 Intro
0:08 Our Setup & The "Problem"
1:33 Measuring the Anodes
3:06 The Solution
3:32 Explanation by Anodic Index
4:34 The Moral of the Story - ยานยนต์และพาหนะ
I have never (I mean NEVER) commented on a video before but I am so impressed by the clarity and focus of your video I simply must say how much I wish other people would do the same - there is so much time wasting rubbish on various forums about this subject but you have hit the nail right on the head and I wanted to applaud you for it. Fantastic job and thank you!
Very good video. I have owned my Dehler 36 for 6 years with a Flexofold prop and have changed both the prop and the sail drive anodes. Brilliant idea to check the saildrive leg by draining the oil to gain access to raw metal of the leg. Very concise discussion and easy to understand since I am a sailor...not an Electrical Engineer!!
Superb, clear video. The best seen on this topic.
The less conductive metal on show to the electrolyte (seawater) the less galvanic action can occur, the less your little prop anodes will suffer.
Use an enamel spray paint that protects the metal from water ingress, then antifoul on top of that. Stay away from the tight moving gaps of the folding prop mechanism with the paints ie blade gaps and pivot pin holes.
When you fit the anodes, tighten the bolts then ALWAYS tap the anode with a hammer a few times (not too hard) and then retighten the bolts, youll find the bolts were not as tight as they were before tapping the anode. ALWAYS do this on mechanical items that vibrate such as props, saildrives and prop shafts. TIGHTEN TAP TIGHTEN youll never have an anode come loose ever again.
Finally someone who got it rigt .. well documented too. This was a truly good explanatory video by far the best I've seen. Usually people seem to think this is some kind of voodoo or some other kind of magic.
If i may I'd like to refer to your video while talking aboat my anodes when im replacing my rubber diaphragm seal on my saildrive this winter.
Thanks for shearing.
Best regards from Jarle
Thanks for the compliments! Of course you can refer to this video. I'd be honoured :-)
Exactly correct. I see a lot of strange articles on cathodic protection on sailboats. Mostly wrong (I have worked with cathodic protection). Charley Horse is somewhat correct the surface area has a lot to say. Small anode to large Cathode surface area wise is critical. You can expect the propeller anode to be ‘used up’ same rate as last season, water temp dependent - so same sailing area. You can reduce by using epoxy paint on the areas you don’t mind painting of the bronze. This is of course an opinion issue what can be painted. I personally like to paint the propeller excluding hinges etc, epoxy paint and anti fouling. After 8 years no fouling/ small anode usage and very happy. But some believe in polishing the bronze. This is fine- but to stay fouling free you need copper ions released and the anode mostly will prevent this, although may be enough depending on where sailing and fouling activity. Have you experienced fouling prop?
Thanks for confirming my conclusion! I know what you mean with "strange articles". I read a lot about the topic online, and the conclusions from article to article were often not in agreement. The only plausible and coherent explanation for a non-professional on cathodic protection I found so far is from the book "Boatowner's Mechanical and Electrical Manual" by Nigel Calder.
I thought about painting the prop like you recommend but shied away from it since I was afraid that the folding mechanism/hinges could become blocked. That I could paint the prop only partially, i.e., the blades, didn't even cross my mind :-D. It was to cold outside for epoxy when I worked on it anyways ... But I do get very little prop fouling anyways. The prop in the video is untreated and has prior to the video been in the water for a bit more than a year.
Something Very late to the picnic that could prove useful - though I haven't got my boat with a Volvo Saildrive just yet, but am hoping to try for myself when I do. On forums, I have come across posters doing this for many years as antifoul from Boat propellers. They paint them with Zinc Cold Galvanising Paint, Marine version like with Pettit, is very expensive, but is identical to cheap land usage versions. I found for paint and roller application, tins of 96% Zinc, in a Silicone carrier. 3 coats of lesser concentration (75% ish from memory) Aerosol Spray, supposedly good for up to 18 months. To me, the 96% brush or roll on, seems the better option, and get a nice finish with one coat on top done with aerosol perhaps ? It is also providing extra Anode Protection ! So new Anodes, give a couple of coats ? Plus as on the boat I'm getting, the Rudder isn't far from the Sail Drive leg, I'll paint that too, plus after Epoxy Barrier coating the Cast Iron Keel, I think I'll slap say 5 coats on that, and see what effect it has. It has the advantage of Also being Non Toxic. Aerosol Sprays in UK are about 10 pounds a tin, The 1kg 96% Brush or Roll on, about 25 pounds a tin, both including VAT. I'm guessing a bit, but I think what gives the antifouling effect, is the Galvanic Surface erosion, keeps the surfaces clean. A kind of Self polishing effect ? 🤔 Best Wishes and Fair Winds. Bob.
Thank you for the information!! Really useful
Thank you, great info. Had the same problem.
What a great video!!! thanks very much. Super helpful.
Thank you for the information and breakdown of conductivity between the metals. Super helpful stuff!
You're very welcome!
Sehr informatives Video. Allso immer schön die Bedienungsanleitung lesen. :-)
Very helpful video and nicely explained. Thank you.
Thank you! Such comments always motivate to make more videos :-)
Great information, well done!
Thank you!
Really useful - just lifted our boat and noticed the same. I was about to start checking/removing stuff until I found this. Many thanks
Glad it helped!
I went through the same head scratching on my Yanmar SD20 saildrive. Indeed I verified the prop is electrically isolated from the aluminum sail drive, and my prop has no zinc, I just have a zinc to protect the saildrive. With the prop electrically isolated, there is no place for charge to go, so galvanic reactions are very slow even without a specific zinc. Aluminum oxide has a way of covering and protecting the aluminum because it won't make easy electrical contact with a dissimilar metal, even if touching. When I look at how the stainless bearings inside the sail drive are mounted, they do make physical contact with the aluminum housing. Perhaps the prop has some kind of insulating sleeve. I always locktite the zinc mounting bolts, I had one fall off years ago.
very good and informative video.
Thank you!
Very Good !
very helpfull!
Thankyou for that explanation. I'm looking after the rubber bushing separator. I have the same setup, an MS25s and recently I installed a Volvo Penta 2 blade folding propeller. The original separator is metallic. I'm unable to find the part number of that rubber separator.
Thank you for this film. Since I only use original square parts from volvo penta, the zink anode on the sail drive makes a better job too. I would never buy the zink anode from any discounter any more. I believe, the chinese producer of the cheaper anodes use a different material as volvo penta use for their square parts. When I found corrosion on each sides of the big rubber seal between the gear box and the saildrive, I realized, that the anode on the sail drive near the propeller did not make a good job. (Sorry for my bad english)
Thanks for this! ^_^
It's not just the metal there is much more surface area on the propellor, effectively drastically increased interaction with the electrolyte (seawater) when the rotating under power. I would be curious to see the rate of galvavism over a time period of just sailing or at dock, versus under power (perhaps a long motoring trip or several)
I completely agree. I'll try to monitor it this year (gopro on a stick, too cold to dive) and see how it progresses.
Thank you for a very good info video. You seem to have knowledge of corrosion issues. So maybe you can answer me, a relay (Relay 872242 , Included in installation kit 872381 for changing from 2-pole to 1-pole starter motor., SN2300052118- ) is to do with corrosion pretention, or what it is for. If you have such a relay, on your boat motor. Will you get a different measurement result, when the motor is running?
I have a Volvo Penta 2002 with a sail drive. I have a issue with motor sometimes will not start, and I think it may be the relay because, when I run a cable from the ground on the battery, directly to the motor block (bypassing the relay), the motor starts, and continuous to start here after. This got me wondering what the function of the relay is, do you know?
I am actually not sure if I have the same relay somewhere and I am right now not on the boat.
I heard or read somewhere that there is no galvanic isolation between motor and saildrive unit in older Volvo systems like in your Volvo 2002. In the newer ones the saildrive is completely galvanically isolated from the motor. That prevents stray current from any other system on the boat to reach the saildrive and to form an electrical circuit through the water.
Therefore, I'd assume that the relay simply prevents the motor/saildrive from being permanently connected to battery negative/ground, and to answer your question more direct: yes, I think it is a corrosion protection.
In that case I would strongly advice against connecting ground/earth directly to the motor and to bypass the relay permanently. If you would then have any stray current in your system (or worse, stray current from another boat or even the floating bridge/dock through faulty wiring in the shore power, if you are connected) it could potentially form an electrical circuit and your saildrive leg might literally dissolve over some time. There are some pictures online of saildrive legs destroyed by this issue.
If starting works by connecting your motor to ground then I'd try to swap out the relay.
And please take everything I've written with a grain of salt. I am no marine mechanic and don't own the same motor :-)
PROTIP: Den Weißabgleich in der Kamera manuel einstellen. Dann ändert sich das Bild zwischen den Einstellungen nicht mehr von einem etwas "wärmeren" zu einem etwas "kälteren" Bild. Oder anderes herum. Wenn Du draußen filmst kannst Du einfach 5.600 Kelvin (Tageslicht) einstellen. Wenn Du indoor filmst, dann 3.200 K (Kunstlicht). (+- 200K, je nach Geschmack).
Jo, mache ich normalerweise auch meistens. Allerdings sind da 5-6 verschiedene Beleuchtungsquellen draussen und ich dachte mir ich lass die Kamera das mal schön selber rausfinden (weil die auch immer an und aus gehen). Ist mir aber auch aufgefallen ...