Wood on Aluminum Frames

[aleks]

For a 44 ft sailing boat construction, I want to make use of wood planking on aluminum framing.

What aluminum alloy is best for the framing in view of strenth, weldability and corrosion resistance?

To avoid unpleasant corrosion problems, what is the best material for

1) the fasteners;

2) the ballast keel?

As to my understanding, aluminum - ss/bronze - lead might be a terrible galvanic combination.

Is there someone who has experience with such type of construction or knows where to read about it?

Every contribution is appreciated.


Aleks

[cyclops]

Talk about thermal expansion rates of the wood and Aluminum over 44' and ambient temperatures. Birds of the same feather (" expansion rate ") still flock together. Why mix them?

[Dutch Peter]

Aluminium Alloy 6082-T5 or T6 is generally used for profiles in alu-boatbuilding (in my region).

To avoid corrosion maybe it's best to use alu fasteners too: http://www.nylonalloys.co.uk/homepage.htm

As ballast I would look at lead or, if possible, concrete.
Hollow alu keel filled with lead is also a possiblilty.

[aleks]

Thank you, Dutch Peter, for your advice concerning the alloys. The differences in the thermal expansion coefficients, quoted by cyclops, thank you too, are they an essential problem for a 44 ft boat? The idea is hard chine, with frames, chines and stringers and deck beams etc. welded.

[MarkC]

Hallo aleks,

www.legendaryyachts.com

Legendary Yachts builds 40' 50' footers with a galvanised-steel frame and keel then wood-strip epoxy, then veneers, then I think some glass for bump resistance.

While they have taken their construction information off their web site - you can do a search in www.woodenboat.com (in designs or building?) where the owner discusses the production proceedures. (You may have to use GreatGoogly to do the search - read the top of the page in woodenboat.com.

The owner of Legendary Yachts has a wealth of info about composite construction - for example steel with wood - what fasteners, what wood, how to seal, what the weights will be like etc.

[aleks]

Hallo MarkC, thank you for your information about the Legendary yachts' composite construction technique. Some wood on steel composite constructions are known to me, but I think that wood on aluminum might be different. The question is whether they simply did not use to do, or whether there are principle difficulties. If a boat is properly designed and built, the thermal induced stresses might be without any bad effects. Does anyone know something about existing wood on alu composites?

[Baldur]

A while back I ran into a guy who had built an 18' skiff using 2" bent 6061-T6 aluminum tubing for the frames and then planked it with marine ply. The interesting thing was since the frames were round, he had no need to fair them with the planks.
I dont recall what type of fasteners he used to attach the planks to the frames, but I do remember not seeing any exposed bolts through the planking.

[cyclops]

If the ply skin is all that touches the frames he is going to have the ply flexing up and down on a very thin area of metal and wood. A couple of good waves should start cracks there.

[Baldur]

You are most likely correct about cracks forming.
Come to think of it, I have not seen that boat motoring around here in a long time..maybe he sank...LOL

[Thunderhead19]

Moisture between the aluminum frames and the wood will become very alkaline over a period of a couple years and turn the wood and the aluminum into the consistency of cheese. I don't know what to tell you on this. It's less profound with steel because steel doesn't passivate.

[aleks]

Wood and aluminum turned into the consistency of cheese is not what is intended. Meanwhile thanks to everyone for your contributions.

[Dutch Peter]

Originally Posted by cyclops

If the ply skin is all that touches the frames he is going to have the ply flexing up and down on a very thin area of metal and wood. A couple of good waves should start cracks there.

That depends on the frame spacing, and the thickness of the ply. Boats have been build like this before and still survive.

[aleks]

Those boats still surviving, do they show something of the alkaline attack problem? Before starting a bigger projekt, it is essential to know whether wood and alu is a combination that fits or not. The shape and spacing of the frames and the thickness of the ply, that might be the next step.

[Dutch Peter]

Originally Posted by aleks

Those boats still surviving, do they show something of the alkaline attack problem? Before starting a bigger projekt, it is essential to know whether wood and alu is a combination that fits or not. The shape and spacing of the frames and the thickness of the ply, that might be the next step.

Aleks,

I was talking in general, responding to the to remarks regarding cracks forming in the hull. But responding to the alkaline attack problem, I guess you have to paint the frames than before you put up the planking.