Floatation foam calculation

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jbo_c
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Floatation foam calculation

Post by jbo_c »

This sort of feels like a dumb question, but maybe not . . .

Since these are wood-cored boats(and, obviously wood floats), does the weight of ply factor into the total foam displacement needed?

Jbo

silentneko
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Re: Floatation foam calculation

Post by silentneko »

Not really. Remember the wood has weight too. Then you add resin and glass onto it. Fillets don't float either, nor do outboards, cleats, fishfinders.....

If plywood adds any buoyancy it is minimized by the other parts on a boat. Estimate your total weight, and fill your cavities. A half filled cavity can be a problem as the air pressure changes.
Built: 15ft Skiff, 16ft Skiff, Modified Cheap Canoe, and an FS17.

jbo_c
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Re: Floatation foam calculation

Post by jbo_c »

That sounds about like I was thinking. My plan was basically to treat the wood as neutral and offset all other weights with foam. We on the same page?

Jbo

silentneko
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Re: Floatation foam calculation

Post by silentneko »

That's OK, but extremely hard to figure out. You can't go by how many sheets of ply you use, because they are all cut down.
I would just weigh the boat whole and use that +10% or so as your number.
Built: 15ft Skiff, 16ft Skiff, Modified Cheap Canoe, and an FS17.

fishgitr
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Re: Floatation foam calculation

Post by fishgitr »

I recently got my coast guard manufacturer license. And from what I understand everything goes into factor. Engine weight, plywood, also the water that fills the boat etc. I believe 62 lbs a cubic foot or around that.

Another thing you can do that don't take up much weight or space is glass In your bulkhead that is required. Then glass another bulkhead about 4" inside of that one. Then fill that with foam. Also under your decks before gluing them in you can use 1/4" ply and build simple like 6"x6"x length of deck and fill those with foam.

On these production boats I will be producing soon, the spray rails will also be filled. This will help keep up right bouyancy.

TomW1
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Re: Floatation foam calculation

Post by TomW1 »

Jacques calculates what flotation foam is needed for each boat and shows where to place it. To keep a boat upright you place solid foam under the gunwales. Some guys fill all the compartments but there is no reason to do so other than waste money and add weight to the boat. But it is up to you as the builder to decide what you want to do. The one thing you do want to do is make sure the foam is to the very top of the compartment.

Tom
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OneWayTraffic
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Re: Floatation foam calculation

Post by OneWayTraffic »

I cut a hull plug from my boat near the transom. That was 6mm Meranti, 800g glass each side, and aluminium/silica filled epoxy on the outside. It floated, just. That's pretty much worse case, most of the rest of the boat will have less glass and more wood, proportionately.

Essentially the boat itself will float, the cooler and tote tanks will float, the crew should float, everything except motor batteries and fishing gear will float.

You also want to have some proportion of the boat above the waters surface, so you can get out of the water, worse comes to worse. Foam totalling the
same volume in litres as the weight of all your metal fittings in kg, plus 20% would be a minimum IMO. Or 1 cubic foot per 45 pounds of stuff very roughly. You want some above the waterline.

More is not going to hurt. I have twice that minimum with a fair bit above the waterline here and there.

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