Thread: Metals/engineering question

I have a sailboat rudder shaft with some significant corrosion. I can't replace it right now, so I need to repair it well enough to make it through the season. The shaft is 2"OD aluminium tube, schedule 80. The corrosion is in a 1/2" zone and at its worst, the shaft is down to 1.82" in the corroded zone.

I'm thinking that the best solution for a temporary repair is to wirebrush the corrosion away and fill it in with JB weld, but more importantly, get a piece of aluminum rod the same as the ID of the shaft and bang it down in there to strengthen the whole unit and relieve the stress on the corroded area.

Questions: Is this an OK idea? Can I mix types of aluminum- I think the shaft is 6061, I was thinking of using 7075 for the solid rod. I wouldn't be welding, perhaps epoxying. Would contact between the dissimilar aluminum alloys cause corrosion? This is in a freshwater boat if it matters...

I think you're confusing pipes and tubes. Schedule 80 refers to pipe dimensions so a 2" pipe has an outside diameter of 2.38", inside 1.94" and a thickness of 0.22". Tubing is listed by outside diameter and wall thickness (or inside diameter).

I don't JB Weld will do anything for the strength. If possible I'd repair the corroded area by welding it.

Then ignore my mention of schedule 80. It is tube with an OD of 2", and a wall thickness of ~.385".

Can an aluminum welder add material to the corroded area the same way they could with steel? I know nothing about aluminum welding (and not much about welding in general).

post this on sailinganarchy.com

your idea sounds dumb to me.

JB weld won't do shit. Get it welded. Fuck, could you do with the ridder bring 1" shorter? Get the corroded section cut out, jam a piece of al rod in there, slide the other side over top (might have to file the seam off the inside if it has one) and weld the fucker together. same could be done, keeping the same original length, if you put a 1" piece of tube over the rod and welded it all together.

Not sure if that made as much sense in writing as it seems to in my head.

Not a boat guy so I don't have any concept of how much stress this particular piece of metal takes, but you're going to lose the heat treat on the aluminum if you weld it. It'll lose a lot of its strength.

I think jamming a solid rod down the inside of the tube will be extremely difficult. If the rod is close enough to the I.D. of your tube to make it structurally significant, you're not going to be able to get the rod very far in there without a press.

I would either 1) make a sleeve to wrap around the outside of the affected area, or 2) cut out the bad section of the tube and have someone with a lathe make you a plug to insert into that section. I would bolt it in so as to avoid welding / heat treatment loss.

Question is whether this corroded area needs to be flush. You didn't mention if it was a pintle-and-gudgeon rudder with a tiller but, assuming so, the only places on the shaft that need to be precise are the sections of the shaft that go through the holes in those parts that attach it to the stern and the fix would need to flush out. If it is a simple reinforcement you want on a section of the shaft at the waterline and not enclosed, cleaning and carbon fiber laid on it with resin would likely get you through a season in fresh water.

It's wearing away because the bearing rides there, right? Call Phil's Foils & discuss this with him. You may want to consider not sailing that boat in his condition. You may want to get fresh bearings as well. This has the potential for catastrophe & great harm to man, watercraft, shore & SAR Crews.

edits:
Here is the link to Phil's Foils, now Competition Composites.

http://www.fastcomposites.ca/site/marine/

The rudder is one of the most important parts of a sailboat and it needs to be addressed with particular attention to detail with a proper repair because, in skier speak, it is a high consequence failure when it goes (total loss of control, it goes only when it's windy or squally and it can drop the rudder to the seafloor and leave a 2" hole in the boat under the waterline which becomes a massive water ingress gusher or if it does not drop all the way out it will bang the hull and create an even larger hole). As you can see it's probably the highest consequence failure mode. So a JB Weld / sleeve / duct tape fix is not appropriate for the rudder post. The corrosion is telling a story that needs to be heard. Attention is needed in this area. The rudder post is very long and probably is inserted full length into the rudder. If the top is showing corrosion, the likelihood exists that hidden corrosion exists as well. Patching the top will weaken the post by point loading it where the sleeve ends. Now is a good time to make that thing ship shape. If you cant do it this year then not sailing could be the best move you make, after calling Phils. peace

How long is the tubing? I don't know a thing about boats (spent five years on wastewater treatment equipment though), but if only freshwater it seems like your idea to reinforce it with a solid shaft down the middle is pretty good (if the tube can't be replaced), as long as the gap between the shaft and the tubing is completely filled with epoxy resin making it essentially one piece. If it's only one season I doubt it would crap out.

But of course replacement is always the best plan for critical components. Your solid rod will probably cost almost as much as a new section of tubing.

I know shit about boats and it might show but why not take it out of the water and replace and/or fix what you have. I'm sure you can somehow fix what you have. Just an assumption but just about anything can be fixed.

Originally Posted by splat

If it is a simple reinforcement you want on a section of the shaft at the waterline and not enclosed, cleaning and carbon fiber laid on it with resin would likely get you through a season in fresh water.

water (salt?)+Carbon+aluminum=galvanic corrosion. Lay down a layer of fiberglass between the CF and AL.

OK. I asked on a couple sailing forums as well and the answers are all over the board. First of all, this is not a stern hung rudder, it's on a shaft through the cockpit floor. No real bearings either, just delrin bushings in the fiberglass shaft tube. The corrosion is at the waterline, here's some pictures:
http://imgur.com/a/6X8X2

The general consensus is to stick a solid rod or a tube with an OD the same diameter as the ID of this one with some aluminum filled epoxy. That's what I'll be doing for now. This winter, I'll tear the whole rudder apart and replace the aluminum substructure from scratch.

I know F-all about boats, but those pictures don't look good.

The problems are both that the thinned tube has lower strength and resistance to bending, and that the pitting creates stress concentrations where fatigue cracks are likely to initiate.

The enginerd bending stiffness of a tube is proportional to (OD^4 - ID^4). That means stiffness increases a lot as the tube gets fatter, and the inner diameter is not that important. If you work your numbers out, I think filling the inner diameter of the tube will only increase stiffness by about 20-25%. So it will still bend and repeated cycles of bending can cause fatigue. If the outer tube starts to crack I doubt the center rod will hold.

Limping through a month with regular inspection maybe, but the entire season? But, I know f-all about boats.

Originally Posted by coldfeet

I know F-all about boats, but those pictures don't look good.

The problems are both that the thinned tube has lower strength and resistance to bending, and that the pitting creates stress concentrations where fatigue cracks are likely to initiate.

The enginerd bending stiffness of a tube is proportional to (OD^4 - ID^4). That means stiffness increases a lot as the tube gets fatter, and the inner diameter is not that important. If you work your numbers out, I think filling the inner diameter of the tube will only increase stiffness by about 20-25%. So it will still bend and repeated cycles of bending can cause fatigue. If the outer tube starts to crack I doubt the center rod will hold.

Limping through a month with regular inspection maybe, but the entire season? But, I know f-all about boats.

Yeah- I asked an engineer friend of mine about the loss of strength and he gave a similar answer. I'm probably going to do it anyways. The corrosion has presumably been that bad for several years- it used to be a saltwater boat in a marina with lots of stray current. For the past couple years it's been a freshwater boat on a remote mooring. It probably hasn't corroded much if at all since it was moved inland, so I figure that it's been sailed pretty hard in the condition it's in. The corrosion is certainly not good, but with a bit of extra support, I'm gonna try to get another season out of it.

Edit to add: based on your formula, the tube has 60% of its original stiffness right now. By filling the inside diameter with a solid rod, I could increase that to about 79% of original stiffness.

Have you cleaned it up enough to see that it isn't pitted beyond the obvious corrosion? Usually when a smooth surface oxidizes, the oxide at the surface slows/stops further corrosion, but a pitted surface allows for continued electron exchange from behind the outer oxide layer, leading to flaking, more corrosion, and the loss of mass, similar to your pictures. Another possible scenario is that the bushing (or water surface) is rubbing the oxide layer off, allowing for continued corrosion.

How is the shaft attached to the rudder? Could you flip it? Or even cut that section off, and extend the tiller end? I think I'd rather have the problem end on the boat vs in the water.

Something else you should consider. Aluminum does not have an endurance limit: Which means that no matter how small the force you put on it, eventually, with enough repetition, it will fail. and with the corrosion, the problem is worse. The shaft looks really corroded, so I don't know how easy welding it will be, and welding does little to reinforce in the short term, because it is a T1 bond until alloying agents spread into the weld.

you should definitely be replacing this. A repair of any sort will do little to strengthen it, and will likely only delay further damage.

I would listen to Anemic on this one. The rudder is too important to have some JB welded repair holding it together. You might be fine, or maybe you will loose steerage and capsize or cause an accident. It looks like that rudder is bonded to the shaft. I do not see a reasonable option other than replacement of the whole item. If possible, get one built with a stainless shaft.

I may be way off base here, but if you do get a rod all the way down into the old tube, I think I'd consider drilling into the glass/old tube/new rod and inserting a solid Al (or SS?) dowel or 2 and sealing it up. Maybe have to do the same thing at the top end if possible. If the outer shaft fails at least you would still have your rudder.

All this seems pretty janky to me considering the consequences.

Originally Posted by Climber Joe

and welding does little to reinforce in the short term, because it is a T1 bond until alloying agents spread into the weld.

Yes, welding such a large area of aluminum will put enough heat into it that the immediate area will be completely annealed. If it was 6061-T6 to begin with, it will end up as 6061-T0 with less than half the strength (35ksi yield vs 8ksi), until the entire shaft is re-tempered.

Sounds like you're on the right track with the inner reinforcing rod. Splat has some good thoughts, too.

Good luck

Before welding you should remove the corrosion surface of the shaft. For this, you should use good mixture of acid and solvent. Don’t expose this mixture in the air. You should never add water with acid. You should use always a container for making a mixture because it is a heat generated process. You can use a good varnish at the upper surface.

so how did the sailing season go?

Ha ha....I just saw the date of the original post.

But why in hell did no one ask him the size of the sailboat involved? That is the single most important question, as the forces on the rudders are CONSIDERABLY larger on a larger boat than they would be on a 19 foot day sailer/racing dinghy.

+++ on NOT using JB Weld for repairing. If using ANYTHING like that, I prefer using Marine-Tex. In a marine underwater application where strength and bonding are important, it is FAR superior to JB Weld. Keep JB Weld for misc. repair. I only use JB Weld for repairing non-marine related things on the boat...stuff down below...never on deck!

So, I too wonder what repair process you ended up with, what type of boat is it, and how did the repair hold up if you WERE able to take her out.


EDIT:
If it were me, I would have replaced the whole rudder stock. Even building your own rudder is fairly easy..I've done it. Completely photograph your old rudder, take ALL dimensions, saw/grind away all fiberglass from the rudder stock and internal rudder framework (whether it be aluminum or bronze), take rudder stock and attached framework to a machine shop so they can build new. When new metal stock and rudder framework comes back, cut out one piece section of Klegecell closed-cell foam or the even better fiberglass-reinforced closed cell foam [the blue stuff], and shape it to resemble the original shape (this is SHAPING foam-core and is actually really fun!). Once properly shaped, start laminating on the the fiberglass and resin. Mat first, then roving, more mat, then roving, over and over in that series until strong enough. You'll want at least 6 layers total. West system Epoxy is good, but expensive. You can get by with using polyurerthane resin. Sand down smooth. Paint with a decent barrier coat...and then paint with your choice of ablative or hard bottom paint. Put on boat. Don't forget the zincs on the rudder stock...if you would have had zincs on there in the FIRST PLACE, you wouldn't be having that problem!!, put boat in water and then ENJOY!!!! I rebuilt my very large rudder this spring. It is more than 7 feet tall and 3 feet wide at widest point !! Took a week for the rebuild.

Lotta sailors here...this is why a sailing forum or perhaps a general boating thread would be cool here.

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