Aluminum transom beef up

Block polystyrene if you can find a place to hide it, is probably better. I recall seeing an old alloy boat about that size that had foam poured down into the bowels, and it was so corroded as to be scrap metal value only. What had actually happened there I'm not sure, but the bottom became perforated.

 

You can frequently get by with very little welding, anything attached directly to the transom can be bolted.

There's no filling between the aluminum, square tubing is used with bolts going through it for mounting the motor.

Pour foam creates problems like already mentioned, plus can cause corrosion over time.

 

But no foam also makes this boat easy to sink.

Certain places in the world wouldn't even allow it.

So a foam plan or compartment plan is wise. How is the question unanswered.

 

My boat was done 20 years ago, here's a few things I would do differently.

It was an inboard jet when I purchased it in very poor condition.

I gutted everything just like you did.

I modified the transom to accept an outboard jet and kicker. I didn't do any welding, just bolted on aluminum supports.

I didn't plan on it lasting more than about 5-7 years after the rebuild, so that determined some of my methods. But here it is 20 years later and its still my favorite boat.

I used pour foam because of the idea that it would have a short term life span. I would have used sheets of foam cut to fit if I thought it would last 20 years. The pour foam did supply support and made the hull very quiet though. The boat is only used in fresh water, that makes the corrosion slightly less of a problem.

I also over built the deck and made it difficult to remove. I never expected to get in there again. It's glassed over honeycomb tied directly to the hull sides, this also increased the strength, but its heavier than it needs to be, and can't be removed without cutting.

While the pour foam and deck haven't caused any problems I know of yet, there's no way to actually check. Also, I can't do some of the modifications I'd like to do because it involves welding.

Other than that it's a great boat.

I'd do it again if this boat wasn't usable at some time in the future.

 

Flotation foam placed up under the gunnels might prove more valuable than foam under the deck in the event that the boat actually did fill up with water, and will not invite corrosion so readily, as gravity will help it shed water.

 

What causes the type of corrosion Mr Efficiency mentioned with a foam pour on aluminium? The foam detatches and traps a film or pockets of saltwater?

 

Yes, trapped moisture, over time it becomes very corrosive.

 

As ondarvr says, salt water is likely much worse, you might get away with it, safely, if the metal is properly prepared and painted, in fact that would be mandatory, I would think. But if repair is needed to a foamed area, welding is out, for obvious reasons, and about the best you could use would be pop rivets.

 

As far as the pour flotation foam....
We will use this boat 2-3 times a year and only in fresh water. Would painting the aluminum and installing drainage tracks covered with plastic still be a source for corrosion using the pour foam? Am I better to just use some kind of sheet foam or something and cut to fit inside my stringers? If so what's the best product to use? I want to build one time and enjoy for a lifetime

 

My concern is the stringers run front to back but there's also ribs that run side to side. Drainage seems difficult since the ribs that run side to side hold water. You might be able to see what I'm talking about in the pic

 

Corrosion of Aluminum and Its Alloys: Forms of Corrosion :: Total Materia Article https://www.totalmateria.com/page.aspx?ID=CheckArticle&site=ktn&NM=187
Crevice corrosion is the issue as per the article

Another quoted type of corrosion is called Poultice Corrosion but appears to be very similar to Crevice corrosion and results when aluminum is covered by another entity. Sand in bilge, teak on a deck.

BUT, many aluminum boat manufacturers use spray foam in their boats, certainly in the Pacific Northwest, EagleCraft, Coastal Craft and others use a spray foam. The advantage is noise reduction, insulation and a reduction in condensation on the inside of the hull.

There was even a jet boat manufacturer out of Canada that used spray foam but as Mr. E has pointed out, the issue of repairing such hulls with the foam installed requires removing much of the foam on the inside of the hull.

 

I think if properly painted inside, you are safe with pouring foam, corrosion wise. But the problem is access, if repairs are needed to the hull. That is a risk you might be prepared to take, though. It would quieten the boat, noticeably.

 

The paint tends to fail and then corrosion starts under the paint. Not many paints are water resistant enough to withstand 100% humidity for years on end.

 

Etch primer for adhesion, clear epoxy for waterproofness, you don't need UV resistance , I think he'd get away with it, but the hidden areas under the ribs etc are a concern. But still a job to start with a good clean surface to coat.

 

Bashing around in rough water causes foam breakdown, water ingress, if used on a river in freshwater, I think it would be OK.