Replacing foam... How much do I actually need?

[redarndt]

My boat according to NADA weighs 1075 lbs, the 100hp motor weighs 340 lbs, and the kicker motor weighs 90 lbs.

My total is 1505 lbs for the boat and motors.

The plate says I can add 825 lbs and still be under weight. Now I'm at 2230 lbs. If a 2"x4'x8' sheet of polystyrene will displace 340 lbs of water, that means I'd need just under 7 sheets of foam to float the boat if it was completely swamped.

I don't think I can fit 7 sheets under the deck! Is it even worth replacing then?

 

[redarndt]

http://forums.iboats.com/forum/boat...und-pro-v-1800-dlx-rebuild-rewire-and-remodel


Above is the link to my restoration thread

 

[Woodonglass]

This is from one of the Flotation Foam Suppliers...

Get your calculator ready, you will need it. A cubic foot of polyurethane will float about 60 pounds of "dead weight". The wood parts of your boat will probably float, so you don't need flotation foam to offset that weight. The fiberglass parts of your boat will barely sink, so you really don't need much foam to offset the fiberglass- maybe one cubic foot of foam per two hundred pounds (or more) of fiberglass hull. The metal parts of your boat are what you really need to account for. A small (4-6hp) outboard may weigh 45-55 pounds. A 50hp outboard will weigh about 200 pounds.

So a 16 foot fiberglass skiff with a 50 horse outboard will need about six cubic feet of urethane foam to keep it afloat. A 12 foot plastic kayak will only need one cubic foot. A 30 foot fiberglass sailing sloop with a diesel engine and lead keel would need about 150 cubic feet of foam. Actually, very few 30 foot keelboats have positive foam flotation, but it's not out of the question- especially when you consider all of the air pockets that would exist, as well as all of the wood interior components that provide some positive flotation.

 

[Watermann]

Not everything in the boat that weighs something will sink, wood, seats, fuel, coolers and the persons weight. There's no way anyone would be able to stuff more than a few sheets of 2" foam in an 18' boat.

It's probably best to keep everything in your rebuild thread so guys don't get confused with multiple threads for the same boat. People who come later on will be able to use your rebuild info to help them with their boat that is if they can find it.

 

[jbcurt00]

825lb weight capacity includes fuel, fuel is lighter then water, and people are included in that 825lbs at 150lbs/person. Its a max weight and max # of people which ever comes 1st. So if you only have 3 300lb guys aboard your boat, even thkugb its rated for 5 people, you're overloaded.on weight at 900lbs.

Coolers are likely to float too as are some other items of 'gear'.

Are there any other areas on the boat you can add foam? Inside hatches, benches, consoles or gunwales?

Is 5.5-6 sheets more manageable?

 

[redarndt]

Good points. I was going to try to jam 4 sheets in there from the start and see how it goes.

I'll keep the questions in my restoration thread from now on and also link the side topics I've created in that thread.

Thanks for the input guys! Put my mind more at ease.

 

[tpenfield]

When I do these sorts of calculations (which is not too often), I include the displacement of the boat itself, which will be about 1/2 or 1/4 the weight of the boat/components. Because, even though the boat/motor alone will not float, they do contribute to the overall buoyancy.

So, if you have 1,000 lbs of boat & motor, you can figure that they will displace about 300 lbs of water. So, that is worth about 1 of your foam sheets. That will get you down a bit in the number of sheets. Also, you can determine if you want the boat itself to stay afloat, or also figure in that it will keep people and gear afloat.

You may be able to get creative and find other places to put the foam rather just below deck.