# Flotation foam or not?

Discussion in 'Boat Design' started by captainstick, Nov 12, 2021.

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### fallguySenior Member

It goes without saying the poly tubes had to be connected to the hull some way.

How do you intend to build the structural bits?

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### bajansailorMarine Surveyor

That hogging of the hulls is a good reason for changing them!

Re the top mould for the new hulls, will this be a simple channel section?

Re your maths, even if you assume that each hull is a uniform 30' long, 3' diameter half cylinder (but 10' of each hull is a pointed bow section), with a draft of 18", then the extra buoyancy obtained by a 1/4" thick layer of fibreglass will be :
0.25/12 x (3.142 x 3/2) x 30 x 62 = 183 lbs (approx).

So for two hulls, the total extra buoyancy is approx 366 lbs.

Where the circumference of a circle is Pi x D and hence half the circumference is Pi x D/2 (where Pi is the constant 3.142).

So the extra volume obtained = shell thickness x half hull circumference x hull length.

And in fresh water the density is approx 62 lbs / cubic foot, not 64, but this is a minor point.

Be aware though that a 1/4" layer of fibreglass on the two hulls will weigh a lot more than 366 lbs, and hence will result in a net loss of buoyancy.
(Fibreglass has a density of approx 2,000 kg / cubic metre, while the density of water is 'only' 1,000 kg / cubic metre - hence why a sheet of solid fibreglass will sink to the bottom. But if it has a foam core, then the buoyancy of the foam might keep it afloat, depending on the core thickness)

I think that the current boat effectively has a bow pontoon section at the stern as well as at the bow - are you going to keep it like this, or will you have the hull cross section a constant half round the whole way from the middle section to the stern?

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### fallguySenior Member

There are some real problems I am starting to see here.

Structurally, a foam core or solid frp must be designed for proper fitment.

Solid frp is heavy as hell. Slow to build, hard to mould for existing structural needs...long list really

Foam core is not going to be made into a tube shape.

Why don't you jist buy some ready made aluminum tubes and modify them to fit the framework?

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### captainstickGreg

The top section is just a simple 6” deep rectangle. I plan on creating a mold out of plywood for those and lining it with poly plastic before glassing.

Your math more or less jives with mine, but the hull cross section shape is somewhat closer to a rectangle than a circle. I just threw a tape measure around it to get 11ft. But yeah, using the entire amount as increased buoyancy is misleading. 1/2 is even overly optimistic.

As far as total weight, the fiberglass is going to be approximately the same weight as the plastic was. The plastic is almost 1/2” thick and it’s all getting recycled. I’m still creating lots of sample layups, so the final design/weight is undetermined. I plan on creating a full size sample next week and I’ll set the actual weight of the boat on it to see how it holds up.

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### captainstickGreg

I couldn’t find any aluminum tubes that would work. I could maybe get some custom ones, but I just like doing stuff myself. I altered my plan from a few months ago of using the foam as a core - after I saw how mangled the existing foam was. My latest plan is to use the plastic hulls as a mold for fiberglass and probably nix the foam to save weight. There will be 8 sealed compartments over the length of the hulls, partly to add a lot of strength with the extra bulkheads.

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### captainstickGreg

I am planning on making a plywood mold inside the 2 aluminum angle brackets on the boat that were holding the old hulls. So I will have all the bolt holes precisely mapped onto the new top sections. Then I will reuse the stainless threaded inserts in the old hulls and epoxy them into the top sections of the new hulls. So it all gets bolted the same way the old ones were. I plan on using some of the plastic from the old hulls as shims to get the new hulls nice and tight.

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### bajansailorMarine Surveyor

The fibreglass will be a lot stronger than the plastic, hence you should be able to reduce the thickness of the fibreglass laminate a fair bit from your proposed 1/2" thick.
Especially so as you will also be laminating in the 7 bulkheads to create the 8 watertight compartments.

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### captainstickGreg

yeah that is precisely what i am hoping. Even if the weight is the same and it’s overbuilt, I’m okay with that. I’ve been known to bump a rock or two.

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### captainstickGreg

I forgot to answer about this. Yes the stern of the old hulls is actually a reversed bow section. I did this for 2 reasons… 1st, when I reconfigured the boat a few years ago I had an extra bow section, so it saved me some \$.

2nd, I liked the idea of having the rocker at the stern so I could raise my props above the bottom of the hull in shallow areas. Probably saved me a lower unit or two at lake Powell.

For the new hulls, I was thinking I would use only the 1st half or so of the hull the way it was. So there will still be a little rocker, but a lot more support than I had.

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### fallguySenior Member

Can't you build it so you can drill them out to fit?

I have done a LOT of houseboating. I'd never want all glass toons.

I'd prefer a barge hull or steel toons.

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### fallguySenior Member

You could plan for some sacrificial wood runners. I have them on my boat.

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### captainstickGreg

I will drill them out in the mold when I dry fit each section. The stainless steel inserts will have to get epoxied in as I finish assembling. I still have to find something to build up material inside where they fit. Maybe I’ll use coosa or just a lot of thickened epoxy.

As for the material choice, I’ve only had polyethylene. I doubt you would want that either. Fiberglass seems like a big upgrade, but maybe 10 years from now I’ll be back asking about steel or aluminum. I realize i could damage the hulls beaching on rocks, but probably less likely than it was with the poly. And for what it’s worth none of my leaks came from beaching the boat. They were all from cracks bouncing and distorting on the trailer.

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### fallguySenior Member

A pontoon trailer is supposed to carry the decking, no?

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### captainstickGreg

That’s a good idea

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### BlueknarrSenior Member

If all of the cracks are from the trailer.

Then fix the trailer as well!!!!

Bad bunks or poor driving will damage any toon regardless of material.

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