Sail Life - Hauling out with a nasty surprise đŹ
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- Äas pĆidĂĄn 9. 09. 2024
- We go up on the hard an find a nasty surprise.
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Just got to the end. The stainless part corroded due to crevice corrosion. Crevice corrosion occurs when the stainless steel is exposed to anoxic water. Passivation relies on an oxidizing atmosphere (or solution) to form a thin passive layer on the stainless. If the atmosphere (or solution) becomes reducing instead, the stainless may lose its passive layer and get corrosion that looks just like that... like black rotting that penetrates the whole piece. It seems possible that a zinc could help de-passivate stainless. I am not sure. But the real fix is to make the shoe out of bronze (NOT brass... proper bronze alloy for below the waterline parts). You can protect the bronze with a zinc. Bronze is not subject to crevice corrosion. The zinc is just to protect in case there are stray currents in a marina. This is my opinion. I am not an expert either, but I have been around the block a few times. Stainless is actually a terrible material to install below the waterline. The other place you get severe crevice corrosion is on the prop shaft where the cutlass bearing is, or going through the stuffing box. Prop shafts are (or should be) made of special alloys that are less subject to crevice corrosion.
Maybe just a nerdy idea: What about 3D-printing a rudder shoe out of carbon fiber reinforced polymers? Just as a (temporary) replacement in case the stainless shoe gets too corroded for whatever reason when you cross the atlantic. They don't weigh much, and would be some spiffy new technology to master for Mads and Ava.
There might be some space left in the technical compartment for a 3D printer. Hint hint
I agree about bronze. There are plenty of places in the states that will cast these in bronze using your stainless part as a pattern. Or simply weld them up from bronze plate.
Titanium perhaps?
@@nutsandbolts432 I don't know a lot about titanium. Maybe it would work also. But bronze has been used for many 100's of years. We KNOW it works. A bronze shoe will last for the life of the boat as long as it is not subjected to severe galvanic corrosion somehow (such as can only really happen in a marina with large leakage currents).
Iâd change the angle of the clamshell. As the boat goes down in the water you will get pressured water forcing its way into the locker. Pointy bit should be forward and the hole back, to create a suction/low pressure as you move forwards and help to empty the locker of any water.
That's my thought, especially when an Atlantic crossing is on the schedule.
Agree 100 %
Yes, it's streamlined for a reason.
yes !!!!
Great minds think alike.
Your stainless shoe could be a real dog's breakfast of different alloys. The base metal weld filler, washers, bolts, and nuts could all be a different alloy. I only know to use 316 on a boat. Even then you can change the alloy by welding. My prop shaft seems fine with zinc protecting it but you have a copper coating to further complicate things. I would recommend changing to bronze if possible. If not that then make sure everything is 316 alloy and the fabricator knows how to compensate with proper filler metal and also properly pickle the welds. Best of luck !
I assume the corrosion was from mixing ferritic stainless and Austenitic stainless... It would explain the washers being basically gone. Probably acted like an anode
The problem is that if the box says A4 quality you have to trust it. There is no way you can tell the difference between a A2 and A4 quality Washer by looking at it. Probably the washers were A2 and maybe the bolds as well.
I would agree but I have through work seen so called 316 stainless sold with forged material certs. When it was checked using a PMI (positive material identification) tool it didnât have the required amount of Chromium. I suspect something isnât the right stuff probably the washers as you say
Looking at the surface that was under the washers, that seems quite possibly the explanation.
It would be helpful to see the washers as well.
Iâve seen similar issues when 304 and 316 have been mixed together in an assembly. When that situation occurred, a new set of nuts, bolts and washers were used on a stainless âPâ bracket fixture. Turned out that the bolts were the odd ones out, though the washers took the brunt of the corrosion damage. đ€·ââïž
@@PaulM-ls6qj Magnet.
Thanks, also the use of Tefgel, when reassembling, to help prevent galling of the threads. I found that out when working on a mine water system.đ€Šââ
Have you been close to an steelboat for a longer period? Galvanic corrosion is always an electric problem. No matter if the source of the current is inside the boat or outside on another one. Even steel pillars in the water can cause such things. But only if you are close to it for long time. Seems you need to put some extra anodes on the rudder. For bigger ships there is an calculation process we need to deal with for class requirements. Just wanted to mention that it's not that of an easy deal.
Iâve seen that type of corrosion before at work on sailboats and motor yachts on drive and steer components and itâs typically caused by using an aluminum anode opposed to a zinc anode. Another common cause is mixing two different types of stainless together such as mixing ferritic and austenitic stainless together. Itâs very common for this to be overlooked. In your case I think those washers were a different type of stainless and the anode was NOT at fault. I would just keep an eye on it when you clean your hull.
Believe I had a post on AISI 316L and A4 to you a few years ago. Don't think you liked the ideas. But I am a corr. engineer. Consequently on my boat soake in SW you will not find A4 bolts and absolutely no AISI 316L not supported by catodic protection system. This is exactly what you expreience now. You may send me an mail, in Danish. Whish you all the best.
With pitting, its defenetly galvanic corrotion. Insulation from other metals is a must. Coppercoat, carbonfibre, may be the culprit.
If it happened ones, it will happen again if nothing has been changed. You should prepare / order the same shoe out of bronze with bronze fasteners next time you go out.
He has a stainless steel rudder structure...
Counterintuitively, stainless steel under the waterline in salt water is more susceptible to oxygen pitting and crevice corrosion specifically due to the low oxygen environment.
That is basically the smoothest and easiest-looking prop removal I've ever seen. Well done with whatever you lubricated it with at assembly; this is DEFINITELY the desired result.
Hi Mads,
I might call myself an expert in corrosion. đ
1. Zinc will never have a corrosive impact on stainless steel. Zinc will sacrifice itself in favor of the stainless steel.
2. 316 stainless steel covers are wide range of stainless steel alloys, not all are suitable for marine environments. Thatâs what I think is your problem here. Stainless steel suitable for marine environments are called duplex stainless steel. If your stainless steel guy will make the rudder shoe out of duplex stainless steel, I can guarantee you, that there will not be any issues.
Best regards from Denmark,
Philipp
It good to see all the thingies out of the water
Try painting under the shoe with epoxy to isolate it from the copper coat.
Or bedding it in butyl.
@@wallacegrommet9343 had already bedded with Sika-flex, thats the black residue you see under the shoe.
I missed that, obviously. Thought it strange that Mads hadnât bedded the piece! Turns out he had.
I wish Iâd paid attention in Spanish class too. I didnât know we had so many Hispanics in Texas (I lived in a small town surrounded by small towns)âŠlive and learn
The stainless shoe/strap isn't isolated as it's connected to the rudder stock which is then connected to the autopilot & steering gear etc
I would be carefully checking the bonding of the steering gear and making sure there is no possibility of a ground loop between those parts and the rest of the underwater hardware (except the saildrive which *is* an isolated system)
You should paint a pinstripe on the topsides both in that indent & above the waterline, would make her look a lot sleeker in the water as optically it will make her look less high!
I think the boat used to have those and Mads specifically took them off, as, in addition to his own opinion, the feedback by a majority of (commenting) viewers at the time supported that. Personally I wouldn't have taken them off, but now that they are gone, I probably wouldn't put them back on unless a big repainting was being done.
@@sleepyjesus7265 I think he should not put the same ones back on, maybe more thin/ modern line, as I agree that the original stripes are a bit outdated
Agree. A sheer stripe and boot stripe would definitely make it look sexy and less boxy. And probably faster toođ
@@garysouza2277 I've gotten the impression that Mads and Ava don't quite value speed that much. They probably couldn't handle the excess thrust the stripes would yield.
nooo the clean look is modern and makes her look like she has more freeboard
As far as I know you should never ever use stainless beneath the waterline. Stainless can corrode. It is a mixture of different metals and in seawater beneath the waterline you get electric corrosion because of the metal mix. Better use galvanised steel or cast iron. The very best would be bronze.
Canât believe it has been two years. WOW! Great job you two đ
Copper fouling is an electric conductor the stainless steel is not isolated
About corrosion: if there is no electrical contact, the problem boils down to the fact that two metals, separated by a dielectric material, inserted in an acid or saline medium, transfer of electrons occurs. Even though the screws are made of steel and the "lock" is also made of steel, they have different compositions, so there is a potential difference and, consequently, corrosion.
The first expert was right about the zinc being the problem. With no connection to an electrical system the zinc and stainless act like a small battery where both metals lose some material creating electricity that is discharged into the water. You can test this by connecting the leads of a volt meter one each to each type of metal while its in salt water. It should show 0.7 - 1.4 volts, maybe more. If the stainless is completely isolated from other types of metal and electrical it does not need a zinc. No dissimilar metals, no dissimilar metal corrosion. No current path, no electrolosys. Crevice corrosion may still be a problem.
Google Translate is your friend on foreign shores. Love your videos.
sailing dauntless had a similar issue with there sail drives and the cause was electrical current coming from the shore power connection leaching off current into the water
Would a cork or a rubber nub be good on that new cleat in the cockpit? It would be annoying to be in a storm and get thrown sideways and get a jab from the top half somehow.
Have you looked closely at the propeller hub rubber bushing , it looks like it has failed, these 3 blade Volvo props are renowned for premature failure. ?
I suspect that the type of stainless that you asked for and then paid more for wasn't what you got either for the shoe or the bolts and you had a differing metal corrosion problem
It's galvanic corrosion caused by a charge in the water. That it's not connected to the rest of the boat ground system is the problem. I had the same problem on a stainless valve that was entirely isolated from the rest of the ground bonding system. The charge travels through the water, and starts eating the metal. Check the whole rudder, and make sure it's bonded to your ground.
Good luck on the prop coating by the way. I was not impressed with mine. It wasn't completely useless, but from the sales pitch, I was expecting at least a solid year out of it.
Can't believe its two years since she went in the water, still looking good.
Thanks for mentioning the cost for the haul out and the anti fouling,. Very few youtubers do that and stuff like that interests me.
As others have suggested the issue with the stainless shoe corrosion is the coppercoat. In the same way that metal skin fittings should be isolated from coppercoat you need to ensure the stainless steel is totally isolated. I would correct ASAP.
The hull definately needs horizontal stripes which will make it go faster & make the sailboat look infinitely spiffyer.
I thought they had to be red to make it go faster. I just dont think any other color has the same performance credentials.
@@philgray1023 Gold is the way to go! Shows a certain flare or is that spiffyness? It is possible to buy sticky back coloured tape for that very use.
@@rogerdinnis3816 it should be navy blue, just like the rest, would look so much cleaner
Whatever color they are, it was going entirely too slow on that travel lift. Racing stripes required.
@@philgray1023
I noticed that some X-Yachts sailboats which I think are civilised performance sailboats have as many as 3 boot stripes that appear to be a shade of red?đ
Another YT channel âSailing Dauntlessâ had a corrosion problem where his sail drives were completely corroded to Swiss cheese. It had something to do with being connected to shore power and galvanic reactions.
it's great to see proof that your 'somewhat extensive refit' is doing very well indeed.
Could be an interaction between the coppercoat and the stainless. Interestingly had this happen on land! A fern frond created a bridge (when wet) between a steel roof designed for coastal use and a copper gutter, the corrosion on the steel was rapid!
You have done a wonderful job renovating Athena. I'd suggest that you get around to adding a boot stripe at the water line. It does two things. it gives you a more noticeable means of telling at a glance that your boat is riding at the proper level. If you add two or three 1" stripes an inch apart, it provides a visual ruler to determine just how far down in the water it has settled. I have walked marinas enough to have seen two boats who were slowly sinking and was able to notify the marina manager. It can help you manage your weight and balance as you begin loading it up for a long passage with extra ice cream. ;) Boot stripes and cove stripes can give the appearance of a shorter freeboard and longer hull. A rub rail can be partially camouflaged in a dark painted cove stripe.
hi Eva and Mat!
If i understand the corrosion problem correctly, you have an "electrically isolated" metal object surrounded by a copper coat layer (conducting), fastened with with a different type of stainless metal and a peace off zink for good measure. All this is in a salty fluid (saltwater) with a possible current when in you are in a harbor. Basically a battery. The reason is the obviously the mix of the metals. craves-corrosion is one of the typical signs for electrical-corrosion. That is at least what my teachers used to say. Isolate that thing from the conducting hull, connect it to one other BIG stainless (ruddershaft) element on the boat to widen the surface area and do not mix different stainless metals, and you should be fine. You can cover the stainless with paint as well, but stainless is known for inter-metallic corrosion, hinds the craves-corrosion.
p.s. love your show, i watch it regularly and like it very much!! best reg. A.
I was surprised to see how you attached the clam shell cover. I always thought a clam shell cover pointed in the direction of the water flow and not at the sky. Since itâs purpose is to direct water past the hole. Not to mention the aesthetics⊠It looked extremely odd to my eye. You must do things differently in your part of the world. : )
You stayed at home! (Moaña) Unfortunately we coulnd't meet; I wrote you and email but I guessed you didn`t see it. Good luck and safe sailing!
Quite frankly I've worked pretty damn hard at solving this low oxygen crevicing problem. Sometimes I can find extremely small ( invisible) pin hole tunnels under a high pressure test.
I was losing the battle at every annual. Things like anti seize definitely increased the problem. Never use silver grey or copper. Use this: I finally got a grip by literally slobbing on, into, and all over with a thick sticky moly grease. It should be black and really messy staining in nature. Somehow it helps to slow the low oxygen or active passive dissimilarity within these noble alloys.
On this rudder I would also look for NAS bolts, washers, and nuts. National Aero Space hardware is the highest grade money can buy. Second choice would be a Mil Spec or AN grade fastener.
Good luck!
Some good opinion in the comment on SS below water line. BUT surely the shoe IS connected to the rudder shaft and that is connected to the steering that is all sorts of metal , cables etc etc
Cheers Warren
It might be worth checking the type of stainless steel: bolts/washers against the SS bearing holder (shoe). 304; 308; 316L or similar have different electrolyze corrosion resistance and might do a couple in the saltwater environment, as they are close to each other. The losing metals are bolts and washers, change them for 316L from a guaranteed manufacturer. Sail safe Rds
Relative motion between the various components would wear away the passivation layers of the stainless and accelerate the corrosion as well.
My understanding is that ALL conductive components below the waterline must be electrically bonded to establish a common potential. When all components are not bonded together, there will inevitably be dissimilar potentials, and therefore localised galvanic corrosion. Since you are already back in the water and effectively still have the same âcircuitsâ, next time you dive, use an extension wire and a DVM to measure the voltage between your problem component and the other anodes/bonded components. If you have a voltage difference, then thatâs the problem.
Could have been stray current in one of the marinas you spent time inâŠ.maybe in the UK where spent significant time sitting still????
Every metal below the water line should be bonded. Like all through hull fittings and valves inside your hull. And all inside chain plates.
Put on a sacrificial anode on the shoe and see if that helps.
I am surprised when you said Denmark was cheaper than Spain, for haul out that is. I spent a few holidays in Denmark and found it very expensive, but perhaps that was only the beer!
I have heard that isolated bits can have problems when some other boat has electrical issues nearby. The solution I have heard is to add that bit to the grounding system. Consult an expert, but this might give you a place to start.
Hi Athena, we have used pellerclean in the Med before and we got good results with it. Once every two or three years a new coat. It lasts. The rudder looks very much like elektrolyse due to different potential of the metals used. Very difficult to asses what did the trick. The bolts or the rings or something else. Try to use best quality 316 stainless. Lucky it did not pitted the rudder shaft itself. You also showed you use normal anti fouling over the sail drive. If there is any copper in that anti fouling that is also tricky and could pit the aluminium casing of the sail drive. It is not recommended, even with a primer in betweenâŠ.. Greetings Ruben
I was always told to not use stainless below the water line because the lack of oxygen causes it to corrode and to use bronze instead. Stainless for above the waterline.
i refuse to believe that Athena has been in the water for TWO YEARS already ... was it not just yesterday that you put her in? OO
One source of corrosion is when you are tied up in a marina and sharing shore power with other boats. Not that you actually "share" the power, but you are connected to the same electrical system. Even if your boat is perfectly fine and insulated you might act as a suffer anode due some other boats inferior insulation. I experienced this on my prop and zink anode after a season doing the Göta Canal.
Any chance the copper coats has set up a circuit
That was my thought too.
Definitely a strong possibility. The entire hull is cathodic, if I have that right.
No, the copper particles are all embedded into epoxy. No electrical connection to any other metal.
@@NixToDo And there is no penetration of metal from the hinge into the copper layer, like bolts/screws?
If the copper coat is suspected of conducting electricity that deteriorates the rudder shoe, would Mads be able to use his meter in some way to determine if there is or is not a circuit that includes the rudder shoe? :ÂŹ) Webhead USA
The conclusion therefore reported to the Admiralty in 1763 was that iron should not be allowed direct contact with copper in sea water.
knowing how you love to tinker, could you put a multimeter on that rudder shoe, log it to an arduino/pi and see if there is/are currents/potential somehow being 'created' within the rudders 'electrically isolated' area while under sale. Also, you mentioned that you heard something strange from the rudder, and as you didn't mention anything in the video solving that mystery, maybe a 'vibration' sensor (is there such a thing?) add a few temp sensors, current sensor (?), mini camera. what else? lots of pins on the arduino/pi. Great videos! cheers.
Great vid. Thank you. Greetings from Spring Green, Wisconsin USA
You need a folding propeller!!! Being Danish a Flex-o-Fold is the obvious choice - I had one on my previous boat and it was a thing of beauty!!! It completely transformed my boat after replacing the fixed three-bladed prop with no negative side-effects!!!
Great video Mads, I had a great moment of nostalgia when you got up on the hard and started some DIY!
I'm surprised you don't haul the boat out at the very least once a year. It is important to know how what's below the water line is going. Thru hulls, fittings, sensors, engine shaft and propeller can bring surprises very suddenly. For instance had you lift the boat before you would have noticed the corrosion on the rudder's shoe fitting, an anode would have done the job. Personally I wary of all metal fittings under the water line and they are all equipped with their own anode or if to small linked by a heavy copper wire to an anode.
Hope you will not stumble on other easy surprises on your boat and I wish happy and long sailing voyages !âŠ
Mads Hi. The corrosion looks like crevis corrosion. The plate looks like it and the washers had water trapped underneath in a low oxygen situation. The black staining that is around the shoes "holes" or damage is typical of crevis corrosion. The washers also look like they have a similar issue. A4-70 grade bolts and washers can be 304 or 316 Stainless Steel. In this instance youu would want to be sure that you have 316L fixings and that no water become trapped or not moving lowering the oxygen content to cause the corrosion. There is an artical on our website that includes this kind of failure issue. Sail Safe Guys, Ant & Cid.
Not all stainless is equivalent, even when it says it is... We've have 'guaranteed' 316 hose clamps fail at the screw, because the screw was from a different supplier. We've had 1000m pressure rated wet connects fail at the 316 fitting because the male end was isolated from the case (which had zinc placed far more regular than necessary) and the two parts, though both very expensive 'guaranteed' 316, were just "ever so slightly" not... If there is any dissimilar metals, the ocean will make a dielectric of some potential. Use teflon spacers and coated fasteners, the cheap route, or source all metal from a supplier that guarantees the steel properties. That used to be easy, but now you have to verify (going back to the hose clamps, same US vendor for years, then all the sudden). Or, just bite the really big bullet and use titanium everywhere!
I have an Island Packet with a ss shoe from the end of the keel to the bottom of the rudder. It is essentially isolated electrically from the rest of the boat and has its own anode. So a very similar setup. And I had almost identical erosion to you with the threads on the fasteners being almost gone, parts of the heads of the fasteners eroded away and what looked like burnt away tracks in the shoe itself. Iwas told it was electrical erosion and not galvanic action.
Although I've changed nothing it is not getting worse so it's nothing to do with disimilar metals or an electrical problem on the boat. It was almost certainly environmental, probably some dodgy electrical system in one of the marinas between Brighton and Weymouth.
Good time for the boot and cove stripes, just saying... great video.
Hi if all else fails try looking to see if the rudder post etc.... is connected electrically via the steering cables and quadrant if nothing else.... and of course at the wheel end you have electrical gear....you may find moisture ingress in the electrics has caused a current path causing the stainless steel degridation.... an insulation resistance test could show that up, and help resolve the problem especially if it is your rudder getting an imposed current in relation to your zinc and propeller.
You should not need to haul out to test the insulation resistance....just test the electrical circuits feed to anything attached to the steering assembly such as nav system, compass light etc by isolating both positive and negative of each of those circuits......in fact as I'm writing this I'm realising the video will have screened coax which will be a connection.... good luck
Good luck with the prop antifouling . I have never found anything that works yet.
Mads, every international sailor Iâve talked to states that the quality of workmanship and the prices are better in European boatyards than in the US. My sample set is small
(about 10), but they were all emphatic.
Should have given the rudder shoe a coat of the prop seajet coating too. Wouldnât hurt.
The corrosion on the rudder skeg bearing looks to be caused by dissimilar metals washers~bolts~bearing all just a little bit different the fact the washers are gone would indicate this. Hallberg Rassy sell a bronze bearing for there 40/42 which perhaps you could alter your skeg etc. to match. Best to go with bronze if you get a new one made. I take it that no one is making this stuff for the Warrior any more? Is there an owners association for the Warrior perhaps they could help. Great to see the copper coat looks good. Good luck. đ
Make one of the bolts on the rudder hinge longer and fit a anode to it if the problem persists. Orherwise a zink coating might last a little while too.
Mads. I was a bit surprised that you pointed you anchor drain cover downwards and to the stern. On my HR34 the cover just points sternwards. You might find that when you pitch up and down that you might force water into the locker even though you are moving forwards. Bw John
Copper coat is setting up circuit, ground rudder bolts, or copper coat
The rudder shoe just needs a zinc added. I have the same things on my boat made of quarter inch stainless plate. I've been in the water for 20 years but with zincs on it with no corrosion.
It's Sundsy, Nov 13th, 4pm EST. Normally you have posted by now. I hope that all is well. Cheers
Beautiful yard. The boat looks very lovely
Yes... sanding
Crevice corrosion there are loads of articles on it the fix you received is most likely only gonna last a short while.
Hi @Sail Life! We had a similar issue with the heel strap on our Island Packet 35, only ours was more severe. Our electrician noted that the pitting(or cratering) we had on 1 seacock and our heel strap, was a sure sign DC stray current corrosion. We had similar corrosive issues on our stainless steel bolts and washer which evaporated as your had. What we found was that one of our 2-0 ground wires started to fail which was putting a bit of resistance in our DC circuit. This caused stray DC Curren to flow out through our engine and prop, which became a giant cathode. Since our rudder post was bonded, as was 1 of seacocks (weirdly, the other two seacocks were left off of the bonding loop), they became the anode as the DC current used them as the path back to the source, our more specifically, our batteries.
Is your rudder post bonded? If so, you might want to poke around with a multimeter and check for the potential of a grounding issue, even if it is just for peace of mind.
On next week I'll be in Vigo too! I'm visiting a boat but unluckily I think 'll be in too much hurry otherwise I would look foe Athena :-)
The rudder shoe is connected to the rudder post, which is connected to the bearings, and connected to the helm drive and your autopilot actuator. (reading that is more fun to the tune of the "leg bone" song)
Any dissimilar metals or inadvertent current flow in any of that could set things up for that a reaction at the point.
Agree - Regards
There is a bearing or bushing between the rudder post and the shoe.
@@mckenziekeith7434 That is immersed in saltwater...
I agree with Richard, mixing different SS types, will cause Willy nilly corrosion, it happened to me with my bobstay!
I had corrocion on an outboard motor, at the bottom of the leg.
It turned out to be caused buy the aluminium reacting with the copper in the antifouling. Where there are two differnet base metales,
You will get electrollisis devleping. You can try applying ad barriacoat on the stainless steel may help prevent the prblem.
Gerard.
Parts do not need to be connected to each other by line to be exposed to the processes of corrosion. In sea waters they are connected automatically because sea water is electrolit. It exactly what happens in a battery.
Hope various orka's have no problem with the white sicaflex, they might object to the black-white scheme... đ
I would place that anchor drain cover at a different angle. Facing back not down at an angle.
You have to love Mads explanations of everything. The doohickey, doodads just to name a few. You go Mads. And Ava we havent had a history lesson yet???
Maybe the stainless bolts/washers are of a different alloy as the schoe...... ( Within the stainless spectrum)
Next time you pull out check to see if stray voltage is going down your rudder pedistol. Ground the meter at the battery and check the rudder shaft. Then ground it at your ac ground and check it.
Its either that or you had hardware that wasn't quality stainless.
I call BS of the annode causing it.
im afraid i cannot offer any advice on the rudder shoe. i avoid such issues by storing the boat on a trailer in my front yard. yay! but in other news... MY MERGULL ARRIVED!!! its going to be the tippy top piece of my christmas tree this year! probably every year! thank you thank you!
Love your channel!
quick in and out, great update 2xđ
I would guess the washers were not the material they were advertised to be and this setup a galvanic concentration cell.
Somethings happenning to seawater when it gets hotter. More agressive. Stainless is not a good material. If its passivated it's around copper in the galvanic voltage sequence. If the warm seawater get it depassified it convert into an anode. My own experience with stainless steel parts as nippels and valves is they last about 3 months in the return plastic pipe from seawater coling to buildings. This is hotter than the intake water.
Regarding the shoe, it does seem like an electrical corrosion. Suggested (reversed) path: shoe
Stainless shoe problem guess: pre installation improper prepping. Not polished and or cleaned with acids, not passivated welding spots.
It's just possible that the shoe being so close to the stainless rudder post in a conductive medium of salt water, and the post connected to cables and auto steer components, there was enough transient voltage leaking into the shoe to eat it. If you have any prop coating left over it might pay to coat the stainless with it to help isolate it...
When reversing a long keeled yacht I have found it helpful in the past to gun it briefly in reverse then shift to neutral and steer while coasting. It worked well for my traditional William Garden 33 timber yacht.
Iâm pretty sure that any below-the-waterline use of stainless steel is always a bad idea. As others have noted, stainless must be exposed to air in order to remain stainless. If itâs immersed all the time, it will without fail corrode badly. It has nothing to do with stray current or use of an anode.
Go with a brass ring when you can the zinc will absorb the current more then the stainless, also note stainless steel need air the alloyed mix with all components maybe incorrect.
(U may want to info manufacturer as this can be a danger fully documenting and sending it back for testing and refund)
Great video
Athena is looking really good for two years in the water, a tribute to you and copper coat. Prop speed works really well on our part of the east coast of Australia so I am looking forward to seeing how the alternative coating endures, it looks pretty much identical. Looks like a classic case of crevice corrosion on the stainless rudder shoe , I would be inclined to change that for bronze when you get to the US.