adding floatation foam

[wagonmaker]

Hello all....... It's been awhile since I have posted. The skipjack is coming along slowly. I have put up a pic or two concerning adding floatation. I have added some details to this little boat not called for in the plans, but I suppose our own preferences is one of the many reasons we choose to build.The stock plans didn't call for a forward hatch or any deck beam supports. I have added both. Since this is my first build I wasn't sure about adding foam for floatation or exactly how to install it.As you can see I added 3/4" polystyrene extruded foam board to the underside of the forward deck area.It was glued between the one by deck beams I made of 3/4" stock with a 1/4 " length of plywood dadoed into the stock. The foam was cut to fit tightly ,then edge sealed & I started the priming & painting of it. My question is this............Should the exposed bottom side of the foam be covered with anything to prevent deterioration or crumbling in time. This foam board is very strong & dense unlike the cheaper styrofoam type stuff. I was thinking of using maybe some 1/8" door skin if needed.I am just wondering if it will start coming apart at some time if left exposed on the underside of the deck.Any comments or suggestions appreciated. I am just wondering how much flotation this will add & should I take the time to add to the underside of the side deck areas as well. This being a small daysailer I am obviously not planning a transatlantic crossing. I plan to stay close enough to shore to just swim out if the ship goes down  ;D. I am just curious what is actually needed here. Thanks ..

[JeffM]

Seawater weighs about 65lb per cubic foot, so anything that displaces 1 cubic foot of water will produce 65lb of buoyancy.  That comes to 26 cubic inches to produce 1lb of buoyancy.  Do the lengthXwidthXthickness thing to get a good idea how much you have.  However, your foam under the foredeck won't produce any buoyancy until it displaces water--I'm betting you don't want the deck awash like that, so your foam needs to go lower in the hull (but high enough to provide a little righting force) , balanced fore and aft, etc, so you could reasonably float high and level enough to bail yourself out of trouble.  Keep in mind that the foam isn't weightless, so it must float itself as well as boat and crew.  I suppose it wouldn't take much foam to simply keep the boat from sinking.  But if the water's not warm, you want the boat to have enough buoyancy to allow the crew to get out of the water.  I would think covering your foam would be a mistake for the very reason you give: if it deteriorates, you'll want access to it.  Anyway, sealing up any compartment permanently risks hidden rot.

[Hirilonde]

so anything that displaces 1 cubic foot of water will produce 65lb of buoyancy.

Don't mean to seem critical but...  Anything that replaces 1 cubic foot of water will produce 65 lbs.  minus its weight in bouyancy   The rest of the calculations are correct.

1 gal = 231 cu/in.

1gal water weighs 8.8 lbs (salt water is less, but negligible)

1 cu/ft. = 1728 cu/in.

1 cu/ft = 7.48 gal

Styrofoam brand foam is really only affected by ultraviolet or being physically damaged.  If it is kept in the dark it will last almost indefinitely.  You may want to do some research on the brand you used to see what protection it needs.  If you do choose to seal it up keep in mind that if you don't totally water-proof the foam compartment you may be creating a space that will grow mold.  I suggest either a completely water-proof compartment or an open one that can breath.

[Mark Gudschinsky]

Something I have recently wondered about in the floatation department is the material "pool noodels" are made of. Recently at work some items came packed in small 1" thick sheets. Anyone know the correct name or term for this material, and where it might be got?

I think The flexible nature of the stuff would be ideal in some of the smaller craft.

[lewisboats]

Closed cell polyethylene foam

Steve