So, what can go wrong with aluminum?

[Jdege]

• Wood hulls rot
• Steel hulls rust
• Fiberglass hulls delaminate
• Fero-cement hulls also rust

What can go wrong with an aluminum hull?

[Lepke]

Aluminum corrodes. Depending on the alloy. I've seen commercial fishing aluminum boats that have never been painted and only get a dull surface oxidation. I also have seen aluminum boats made with surplus aluminum that didn't last 5 years.

[boatman61]

Electrolysis plays havoc with it..

[Stumble]

Aluminium also suffers from cycle fatigue that can't be designed out.


Basically they are all tradeoffs. The ideal hull would be titanium (seriously) but it was also be phenominally expensive to build.

[a64pilot]

Soviet Alpha attack sub had a Ti hull I think, I can't conceive of the cost, I believe Russia is pretty much the only source of Ti?


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[Matt Johnson]

Copper Nickel would be awesome! Not having to ever do bottom paint.... what could be better!

[Matt Johnson]

Here's the japanese guy that built a titanium sailboat:

An All-Titanium Yacht - Professional BoatBuilder Magazine

Matt

[Jim Cate]

There's a chap from the Pittwater (Australia) area, ex owner of Pasha, a beautiful old style aluminium race boaat, w ho has started building a big (~70 ft ??) cruising yacht out of 2205 duplex stainless . Last I heard it was languishing, incomplete, possibly due to costs escalating beyond expectations... certainly a believable theory!

Would be a good hull material: very strong, good welding, excellent corrosion resistance... what's not to like?? I h ope that he is able to complete the construction, for it would be a trail blazer for others to follow... or possibly avoid!

Jim

[Paul L]

There's a stainless steal boat in the yard here Whangarei. Looks like they have a lot of issues keeping bottom paint sticking to the hull.

[Stumble]

Originally Posted by Jim Cate

There's a chap from the Pittwater (Australia) area, ex owner of Pasha, a beautiful old style aluminium race boaat, w ho has started building a big (~70 ft ??) cruising yacht out of 2205 duplex stainless . Last I heard it was languishing, incomplete, possibly due to costs escalating beyond expectations... certainly a believable theory!

Would be a good hull material: very strong, good welding, excellent corrosion resistance... what's not to like?? I h ope that he is able to complete the construction, for it would be a trail blazer for others to follow... or possibly avoid!

Jim

Stainless would actually be a terrible hull material. The problem is that stainless exists in two seperate states, passivated and non-passivated. The issue is that they are galvanically non-compatable but becuse passivated can switch states when it is submerged in salt water you wind up with both passivated and non-passivated sections of the hull. Basically the boat will try to eat itself at the waterline.

There have been a few stainless boats built from time to time, but they have serious problems and cost a fortune. You really don't buy much except more headaches.


As for titanium... There are major deposits everywhere. The US, Canada, China, all over Europe, South America, etc all have commercially viable titanium deposits. if you normalize the cost of titanium compared to 316 stainless the price of titanium is about 1.66 times more expensive for the materials at a little less than half the weight.

[Jim Cate]

Stainless would actually be a terrible hull material. The problem is that stainless exists in two seperate states, passivated and non-passivated. The issue is that they are galvanically non-compatable but becuse passivated can switch states when it is submerged in salt water you wind up with both passivated and non-passivated sections of the hull. Basically the boat will try to eat itself at the waterline.

There have been a few stainless boats built from time to time, but they have serious problems and cost a fortune. You really don't buy much except more headaches.

Interesting points, Greg. Is the passivation issue so serious with duplex stainlesses? My non-expert thought was that is is not... any data on that?

I've only had a good look at one s/s hull. It was a fishing boat in NEw Caledonia, and it had been thrown up on a reef in cyclone Erika a few years back. When I saw it, it was lying in a sort of marine junk yard of hulls beyond repair, and I had a good look at the remains. I was impressed by a couple of things: the bottom paint was nearly all flaked off, and that the hull had shown extraordinary ductility while being beaten to death on the reef. There were huge distortions in the plating, with a lot of pretty small radius bends, and the only places where it had failed were at welded seams. I don't know what alloy it was, though. I didn't notice any corrosion, either at the waterline as you suggest, or other places that I had access to (maybe 75% of the underwater area of the hull, but I'll admit that I wasn't particularly looking for it.

As to cost comparisons between s/s and Ti for hull structures... the difference in material costs may not be great but fabrication costs would surely be far higher for Ti, wouldn't they?

As for me, if I went for a metal boat, 5000 series Aluminium sure seems to work well in practice!

Jim

[Stumble]

Jim,

It's the inter granular structure of autensic stainless that actually causes this to happen. If you look at the picture below when the metal become depasivated, at the microscopic level the ferrite and austenite portions are not galvanically compatible and actually corrode each other. It's called intergranuler corrosion, and is a serious problem.

This is basically the same reason why stainless chain is never used for moorings. For anchor chain it obviously isn't prefered, but it's never used for permanent moorings for exactly this reason.


A titanium hull... The labor would be a,a zingy expensive. Frankly I think it would be cheaper to build a swimming pool, fill it with argon, and build the hull inside of it while the welders are wearing space suits.

I am not close to knowing enough to guess how complicated the scantlings would be. Titanium isn't particularly stiff so you may actually need thicker plate than aluminium. But it's so strong you may be able to go with a different framing structure to take advantage of that. On the flip side unlike aluminium you don't have a tempering issue with welding Ti so You may be able to use thinner plate. It's just beyond my knowledge to predict it.

I do know the Navy is spending millions to find a way to weld ti for ships hulls. The guy who is heading up the research is here in New Orleans, and last time I talked to him (about two years ago), he said they were about to start large plate tests (25' long but welds). Assuming this research comes to fruition it may substantially change the equasion.

The Navy did a prediction on a ti hull and predicted a lifespan of +2,000 years.

[Snowpetrel]

Originally Posted by Stumble

Aluminium also suffers from cycle fatigue that can't be designed out (snip)

This is technically true, but if it conservatively spec'd (as any cruising boat should be) and properly built any fatgue issues are not really a concerning factor, or rather less of a concern than fatigue and hydrolysis are in a GRP boat.

I have seen fatigue in chainplates that were underspeced, and a few cracked welds on lightly built high speed boats and racing boats, but its not hard to build a cruising boat that has a hull with a near practically indefinate fatigue life from aluminium dispite the fact that it doesnt have a defined fatigue limit like mild steel.

One advantage over say grp is that fatigue usually occurs near or in the welds as a crack, so you have a chance to reinforce the area. Ive seen this in the keel structure of old aluminum racing boat. I believe the area has been rebuilt much stronger now. I certainly kept a close eye on it while I was sailing the boat! Whereas on a GRP boat its hard to see the gradual internal failure inside the matrix that leads to Sudden failure, as in polinya star.

Electrolysis is preventable, but a permenant worry. Internal corrosion is also a problem inside tanks and in dirty wet bilge spaces.

Any fasteners can be a source of significant corrosion. It is hard to keep paint on the cabin structures and the reccomended antifouling paints are expensive and poor.

Insulation for noise and, hot/cold and condensation control is pretty important outside the tropics.

The plus sides are excellant strength to weight, stiffness and very good toughness, together with a fully welded structure with the possibility of eliminating most sources of leaks by welding most fittings directly on to the deck. And of course not actually needing to paint the boat above the waterline.

[Stumble]

Jim,

I may be wrong, but I am pretty sure that most aluminium boats use a mix of aluminium alloys. 5086 (corrosion resistance) for the underwater hull, 6061 (strength) for non-submerged structural elements, 5052 for the hull and deck. of course not all boats may go into this detail, but if you are trying to maximize the benefits it's not a bad idea.

[Stumble]

Snowpetrel,

The fatigue issue was meant young in cheek. As is this entire thread. Building a titanium boat is a silly idea.

For a world cruiser (with no budget) I would build in either carbon fiber (fast catamaran) or aluminium (big monohull).