Flotation Foam calculations

[Sabin1269]

I’m to the point now of getting my deck installed on my ‘86 Starcraft Medalist 1901 and want to purchase the flotation foam for under the deck. Trying to figure out the formula for what I need. The deepest spot is 5 3/4” right next to the stringers and it will taper up to about 1/4”-1/2” gap between the bottom of the deck and where I will have it glued to the hull. Overall length is 13’.

 

[88 Capri (2022 SOTY)]

That will require geometry with which has been to long for me to remember but look for pythagarems formula for triangles and that should get you started before converting to cubic feet.
I am not sure if I spelled the formula name right.

 

[jbcurt00]

Measure 1 side as a rectangle:

X feet wide (from the centerline of the boat to 1 side) by 6inches deep by the length of the hull to be filled.

That will give you a good estimate of CuFt needed.

Basic boat, 16ft long (might only have 13ft foam filled) by 6in stringer down the centerline of the boat, by 5ft wide (full width beam).

13 X 2.5 X 0.5 = 16.25CuFt of foam

Ignores taper toward the bow, but you'd be hard pressed to get maximum expansion in the field that a pour in foam lists as its max filling volume.

 

[Sabin1269]

Well I feel like an idiot. Never thought of figuring it as a rectangle.... It works out perfect or at least close to perfect and puts me right around 9 cubic feet which looks and sounds about right. Now should I stick with the 2 lb foam or go with 3 or 4lb? Some of it will support the deck on the outside closest to the hull.

 

[88 Capri (2022 SOTY)]

Don't feel bad i didn't either lol

 

[jbcurt00]

2lb is what most manufacturers use for flotation foam.

It provides the most flotation per CuFt, because per CuFt it weighs the least, 2lb, which yields about 60lbs/CuFt of flotation value.

Order at least 10CuFt, more if funds allow. You dont want to run short and have to order it.....

Is 600lbs enough flotation value to keep your boat, motor and gear afloat?

A 1901 Medalist is fiberglass and 19ft long? Probably has a 120-150hp outboard?

600lbs seems a bit light and its 10% overage from your 9CuFt estimate...

EDIT: yeah, not enough by more the half
http://boatspecs.iboats.com/Starcraft_Marine__Medalist_1901__1986/bp/66b86886

At 1200+lbs for hull and I/O dry, you need 20+CuFt of foam

 

[chevymaher]

2 lb foam is plenty. You can walk on it and it does not give. No footprint nothing. It is pretty hard. If you had a rotten deck and floatation foam to filling it. You wouldnt ever know the wood was gone.

 

[Sabin1269]

Jbcurt00 I’m not sure where I could get another 10 CuFt anywhere. I’m just going off what was originally there when I tore it down. There is a bunch of spray in foam all around the hull up under the carpet and in the bow and I know I didn’t really remove much on the sides when I tore it apart.

 

[Ned L]

I think you have an error in your calculations. Yes, 20 cu/ft of foam will displace about 1200 pounds of water. I don't think the question ends there though.
it seems you need to displace 1200 pounds of water TOTAL. you are forgetting to include the amount of water displaced by the boat, gear, engine, etc. To be real conservative, if we say all that stuff displaces another 5 cubic feet, then you will be displacing a total of 25 cubic feet, or 1560 pounds worth of buoyancy.
Then there is the fact that you really only need to have enough buoyancy to compensate for or overcome the difference in specific gravity, or density. between the boat, motor, the 'stuff',........ and water ................ You said the the boat, motor and the 'stuff' weighs 1200 pounds, and we threw out that it takes up (conservatively) 5 cubic feet. Five cubic feet of water weighs 300 pounds. You need enough foam floatation to overcome the difference, which is 700 pounds.
There are other details involved, like how much of the boat is actually underwater versus above the water when it is swamped, and some other stuff too, but this real rough swag would seem to indicate that you could have as much as 1560 pounds of floatation, and as little as 700 pounds may be needed. If these numbers are anywhere close to correct then you would seem to have a reasonable margin of safety.
Or at least this is how I am seeing it.

 

[gm280]

If you really think about it, any time you have an angled opening to figure out the internal volume, yes, do it using a simple length x width x depth basic simple formula then you basically have the fill volume for both side of the haul from the center line. The area you are not filling in one side, would be pretty close to the area needed filled in the opposite side. Of course that all depends on the angle, but we are dealing with expanding foam and it will work out close enough.

If you really want to get absolutely perfectly accurate....well...you need a hobby! Try boating!