To foam or not to foam?

[keith66]

Over the years i have worked on a lot of boats with foamed hull parts, i hate the stuff! often the foam is waterlogged where it will cause problems. Buoyancy bags are good in small boats, available in many different sizes they are made by Holt Allen & Crewsaver in the UK, the latter are particularly durable.

[Wynand N]

With new compulsory buoyancy/flotation laws in my country, this is the buzz word in boating now.
My company install buoyancy and issue certificates according to our governing body SAAMSA and the only buoyancy aid approved by them is close cell expanded polyurethane foam - we use Resicon 44V20 type. Spaghetti noodles, polystyrene, bottles and other strange things are a taboo.

The difference this foam make to a boat is astounding in overall stiffness. We build a bass boat of 4.5meters (15 feet) in length and whilst the hull is still in the mold, we pour/spray the foam into the hull up to the floor level which amounts for about 310 liters of foam. It is then smoothed level, a 4mm plywood fitted over that and glassed over. Take note that NO longitudinal stringers or transverse frames/floor supports are fitted to the bare hull molding.
You can jump on this floor without denting it even a mm and the hull sounds solid from the outside.
One has to experience the stiffness and silence of these hulls through a chop on water compared with hulls without with foam fitted with floors/framing etc and the security that nothing can sink you even if you tear the hull open from bow to stern...
This bass boat only weighs 170kgs and she carries a beam of 1.95meter.

ZED, PU expanded foam weigh about 38kg per cubic meter (1000 liters)

[Zed]

OK...thks what's the expansion ratio? i.e. What does a litre of mixed material expand to? I had a friend that pumped it into the rails of his Datsun 1600 rally car, said that it stiffened the car a lot !

[Zed]

I remember now... it was 25/6 times volume mixed, 1l = 25l blown... that right?

[Wynand N]

right

[Fanie]

Originally Posted by ancient kayaker

I will have lots of buoyancy, about 500 lb on a 10 ft boat.

500lb is going to be a bit heavy for a small boat, don't you think, and where will you put all that foam

[PAR]

Terry you only need about 4 cubic feet of foam. If it's a structural foam (lets say 8 pound density stuff) this will be about 215 pounds of buoyancy. If you use a light foam, like 2 pound density you'll get 240 pounds of lift. This is enough to permit your boat to swamp, completely filled with water, yet the rails will be above water, so you can pump or bail it out. There is such a thing as too much buoyancy, which shows up in a capsize. The boat floats on it side at about 95 degrees to upright, but causes it to float very high, making righting it more difficult. Ideally you'll want about 1/4 of the beam immersed in this situation, so you can grab the dagger board and leverage the boat back to her feet, maybe in concert with pressure on the halyard.

[Ike]

Terry

I have been involved with flotation in small boats for over 30 years, testing for flotation, repairing boats without enough or none, and all other aspects. There are many solutions, but foam is by far the simplest. But, it has it's down side.

As has been said here, pour foam, also called two part or spray foam, has it's problems. The major problem is water absorption. This is almost always due to improper installation, that is, not following the foam manufacturer's instructions to the letter, including such things a the right temperature and humidity. All of this can be eliminated simply by using pre-made foam blocks. They do not absorb water. We did a two year testing program on block foam and no water was absorbed in 2 years of immersion.

Polyurethane is the preffered type. It is solid, reliable, and is resistant to oils, gasoline and other solvents. PolyEthylene is also a good choice. That is what pool noodles are made of. It too is resistant to oils and gasoline.

As Par said, 4 cu feet of two pound density foam will give you about 240 lbs of buoyancy. This is far more than you need. I have about 2 cu feet of foam in my 12 foot rowboat, and it is rated for three people and a 2 hp outboard. Actually I ran all the calculations and I only need about 1 cubic foot but I added a little to make it float higher when swamped. As with you I want to be able to right the boat and get the water out. As for the weight of foam, 1 cubic foot of 2 lb density weighs 2 lbs. 4 cubic feet would weigh 8 lbs. This will hardly make a difference on the weight of the boat.

Some will say air chambers are fine. Well, they do work fine as long as they don't leak. Many builders use them on small dinghies and other small craft.

I think I was the one who suggested ping pong balls. Many years ago, back in the late 80's I actually tested a boat that had ping pong balls for flotation and they worked just fine. But as was said, where do you get them in large enough quantities? Actually most anything that traps air, and excludes water will work. I have seen air bags, plastic bottles, coke cans, and other stuff.

In my rowboat I used Polystryrene insulation foam. It is 2 pound density and comes in 2 by 8 foot sheets 2 inches thick at almost any harware or home improvement store, and it's cheap. I cut the sheet into convenient sizes and then encased the foam in fiberglass using epoxy resin. The glass was used just in case I do spill some gasoline from the O/B. If you don't have that problem then you don't need to protect the foam.

So my vote goes for foam, but not pour foam. Use pre-manufactured block foam.

[ancient kayaker]

As the design stand at present I have about 4 cu ft air per side tank, which will have smaller internal air bags at the stern, plus maybe 1 cu ft of foam under the foredeck.

I can get the air bags from a store that supplies small boat fittings fairly close to home. A friend of mine happens to have a couple of about the right size intended for a sailing canoe, which he has shelved for the time being; I'll make him an offer. They are very tough and hard to puncture, designed to be removed from the cavity and used for keeping camping supplies dry.

The polystyrene foam is available locally; I have used it before and it is very easy to work with, fairly cheap too. I will use it under the deck and either make it a loose fit in an unsealed cavity or make it removable.

If I have an outboard it will be an electric trolling motor as outboards are not permitted in some areas I visit, so fuel resistance is not an issue.

The tanks perform other functions as well as buoyancy, they serve as seats, side decks and structural stiffening. Capsizing will be prevented by the buoyant mast. I am hoping righting her can be done from inside the boat, but I can check all that in my pool before venturing into a lake, including righting using the daggerboard or other means.

I am surprised at how complicated this simple sounding question became; there were a lot of suggestions and information and I greatly appreciate you taking the time to answer in such depth. thank you.