Structural Foam for Aluminum Hulls

[jprev]

I was surfing the other night when I came across a nice site that built aluminum powerboats. This company used polystyrene in the hull instead of pour-able or sprayed two part polyurethane. The reasoning behind this was the poly, when wet, caused a reaction with the aluminum which caused corrosion of the hull.

I understand that most foam will not be subjected to a lot of water over its lifetime. But condensation and small leaks will allow some water in the hull. If this is accurate there are tons of aluminum boats on the market that use this methodology and would be subject to this.

Anyone heard of this before?

[Dutch Peter]

Originally Posted by jprev

I was surfing the other night when I came across a nice site that built aluminum powerboats. This company used polystyrene in the hull instead of pour-able or sprayed two part polyurethane. The reasoning behind this was the poly, when wet, caused a reaction with the aluminum which caused corrosion of the hull.

I understand that most foam will not be subjected to a lot of water over its lifetime. But condensation and small leaks will allow some water in the hull. If this is accurate there are tons of aluminum boats on the market that use this methodology and would be subject to this.

Anyone heard of this before?

You have to remember that for condensation you need the surface to be in contact with air! Sprayed polyurethane on aluminium will not allow condensation to for between the poly and aluminium.
Besides that, the polystryrene has to be cut into shape (atleast I think) and make a boat a lot more expensive!

A lot of threads are about aluminium and corrosion. If the aluminium is coated/painted, you don't use to many different materials (further apart on the electrolytic scale) and dry and cean the boat once in a while you'll be ok!

[Arrowmarine]

In my experience it doesnt matter what kind of foam you use, if water gets in between it and the hull, you will get corrosion. Unless your floatation foam is enclosed in a watertight chamber, It WILL get wet and eventually soak up a lot of water (which of course defeats it's purpose) This water comes from rain, waves, forgetting the drain plug:-)etc. I've been involved with scores of aluminum boat repairs
and seen this firsthand. Even on boats as little as 1 year old the corrosion can be pretty bad. The foam we use is a two part sprayable and it forms a skin when cured that is pretty waterproof. The problem lies in the fact that more often than not you have to trim this foam to allow for rigging concerns and as soon as you break that skin, it will soak up water like a sponge. The polystyrene (styro-foam) as Dutch said is very labor intensive. In fact Its a pain in the a$$!! But I'm undecided if it's a better or worse choice.
One thing I will add---- Most corrosion, even on 15 or 20 year old vessles, is never THAT bad. Usually just surface stuff. Ive never seen one eaten completely through .125 thick metal, although I'm sure it's possible.
Anyway, my two cents, Joey

[MikeJohns]

I always specify that the inside surface of a foam sprayed alloy hull should have a good and thorough paint preparation including a zinc rich primer and several coats of epoxy. No foam below the waterline. But still paint the bilge the same. Had one workboat that holed its unpainted Al. hull in a few weeks from an ignition key lying in a few cm of bilge water! Doesn't happen if properly coated.