Why foam?

I have seen its a popular approach to pour expanding foam in the cavity under the sole of a s&g boat. Why foam? Does it float better then an empty cavity?

I can understand that it helps support the sole evenly but apart from that why is that better then just air filled undersole cavity?

 

Put in as a safety measure, not to hold the sole up. It's for people who like their boat to stay afloat when the unexpected happens.

 

I, myself, am not a proponent of pour in place foams. Most pour in place foams are of the open cell type and apt to become waterlogged. While they may reduce the free surface effect, their weight absorption makes them too heavy to consider. Closed cell or macrosphere foams work much better for this...at a much higher cost.

 

So the main purpose is keeping things afloat if the boat breaks apart and the undersole cavity is compromised? Will the two part resin foam that is widely used in the builds on most forum keep things afloat then or soak water like sponge?

Is one part spray on polyurethane expanding foam better option and what about polystyrene shaped to fit?

Mr Efficiency, which one do you use under your soles?

 

I have poured PU foam under the sole in a boat I have. Been there a long time, don't think it has absorbed any water, but I cant access all areas, just a couple of inspection ports. I've had boats with block polystyrene slabs in it, seemed OK, not water soaked. Poured PU is a closed cell foam, if it wasn't it would be like mattress foam, and just fill with water. Whether the cells are 100% all closed is another matter, and excessive vibration and shocks may well cause some breakdown. However, the "skin" that poured foam has seems to hold up well over time. If you slice into it to trim it down, may not be so good.

 

Ok, will have to cut down the poured stuff to make for an even floor. Polystyrene is looking best atm. I have used it previously as boat bull core and its stood up very well

 

FWIW it will keep it afloat long enough in an emergency. The cheaper boat flotation gets waterlogged after years.

 

4lb/cu ft PU pouring foam allegedly has much better water penetration resistance, but the penalty is the extra $ and weight.

 

You can always use polyethylene foam, which is soft, but still closed cell. It is very durable and can be deformed to squeeze into awkward places. Expensive, and doesn't come in thicknesses more than about 4 inches from what I have learnt, but can be glued together successfully.

 

I have some PE foam in 2" back in Oz, love the stuff. Makes for the best sleeping surface! Its the blue and white swirl from Clark Rubber. $200 a sheet would be way too exy. My sole is cut and dry fitted, I want to move on with the build

Thinking just epoxying in whatever polystyrene I can scrounge up

 

Just talking yesterday to a guy who was lifting the spongy ply floor out of an alloy boat, underneath were slabs of PU foam, no sign of being wet.

 

I am also incorporating an insulated fish storage bin under the front deck. I assume polystyrene sheets would be best for this. I can get a foam only box here, comes with a lid and no sheathing. Thinking of making a ply bin that will take this box and epoxying it in. And sheathing it with glass on the inside. Same with the lid. Epoxied to the hatch and laminated with glass. Might be a pia to get it all fitting after laminating

 

It is worthwhile to note that packing foam under the sole requires it to be well secured to the underfloor structure of the boat, or it could just pop off, if the foam's buoyant properties come into play, such as if, or when, the hull takes on water. It is no use for saving the boat then, though you personally may be able to float off on a piece of foam !

 

Saqa, for the ice box, you can't just laminate over the foam and expect it to be tough enough unless you build up a glass thickness of at least 3mm. This uses up heaps of glass and resin etc... A thin layer over PS will puncture easily. A better option is to build a ply box inside and outside of the PS and epoxy coat it for protection against moisture.

Floatation foam is old school IMHO... The most likely worst case scenario is that you'll capsize or swamp a boat like this. A sealed compartment will work just a good as foam in this regard. You don't often see it in production boats because the place they put the foam is never completely sealed, so water can always enter. It's easier and cheaper for them to use foam rather than the extra work and build sequence/method of water tight compartments.

 

The foam also adds substantial ridgity to the bottom and it makes the boat quite...sound deadening. Good for fishermen.

The object of foam is floatation. Get some foam up high, under the decks..and get some foam aft to keep the weight of the engine from sinking the boat.

Many folks prefer to keep foam off the ceterline..the area between your two longitudinals ....and use limber holes in frames and a drain plug aft in case of water ingress.