Help me understand the Physics of foam support.

Hi all, I am about halfway through the build of a Bateau C17 and have the inside and outside glassed, stringers and frames glassed in. The basic scantlings are 400g biax both sides of 6mm ply with extra on the stringers, chines and frames.

I have some rigid closed cell polyurethane block foam that I wish to install under the sole. 100kg/m3 density at 25mm thick and I can get some 67kg block foam at 40mm thick.

I am looking at different ways to put this under the sole, mainly flotation but also as noise dampening and increased stiffness/strength though the boat should already be very strong.

The block foam is very cheap (offcuts off an industrial block) and comes in large sheets I can cut to size.

I've made a diagram of the four different ways I could see to glue this in the boat. I don't want to fully foam fill, as I've put limbers in the frames for ventilation. The diagram should be self explaining.

I am concerned about whether the foam will break over time with slamming and pounding, or whether I make the hull too stiff if I tie the foam into the sole , or if tabbing in the foam with glass is a good or bad idea.
Or maybe it just doesn't matter much and I can do it anyway I please.

 

To satisfy this requirement:

Is straight forward, as you can merely place it in blocks between stiffeners, as shown here in red:



But...this requirement:

Is at variance with the first one.
Since to become structural, the foam needs to be encased, i.e fully surrounded by glass. Such as shown here, the glass being the blue:



Thus, I think you need to decide which has the higher priority for you.

 

Thanks for that. I had considered gluing to the underside of the sole. I take it then that even 100kg/m3 foam will have little structural effect either positive or negative if glued to the hull unless I glass over it?

 

Correct.
The easy way to think of it is this..take a shoe box without the lid. hold it and twist it...easy!
Now repeat, but with the lid on...not so easy.
Whilst this is a demonstration of torsional resistance, it illustrates how a closed cell works...and that is ostensibly the same. You gain your 'stiffness', from a closed cell that has a shear path from one surface to another.
The shear path - in this case - is the foam.

 

Off-topic of your question, but every time I see a mention of limber holes I think of this (scroll all the way down):

The White Boat https://hvartial.kapsi.fi/white/white.htm

 

Not a problem with mine. The sole will be structural, and glassed in all around as specified. The limbers are also much larger, and sealed with epoxy and there will be inspection ports. I don't expect water to get in, but I'll sleep better with the inspection ports in. The inspection ports will themselves be under cover or in compartments to prevent water leaking in easily. The boat will be stored on a trailer, under a tarp cover, ports open for ventilation.

I was more concerned about how the foam interacting with the hull will change things. It seems that the only major structural effect will be if I build it up to the sole and glue securely to it, glass it in or otherwise seal up the sandwich. In which case, if I read things correctly it will simply act like an extra stiffener running between the frames. I'm not sure if a hull can be too stiff.

 

I, too, have been wondering about this. In the last build of a boat similar to your under sole arrangement, I used blocks of polystyrene shaped to fit in between the stringers and frames. I coated the blocks with epoxy to cure to seal it. Then I used aerosol spray foam filler material to line the hollow and fit the block in. I levelled the tops with a sanding board, applied a thick coat of epoxy/wood fibre bog and fitted the sole. In my thinking this would approximate filling with pour foam, but I have never really known if it did add anything structurally

 

The primary goal of adding foam is to gain enough buoyancy to keep the boat afloat shtf. Ideally, floating upright.

The issue I have with the block foam is number one, will you get enough. And, number two, is the sole stiff enough on its own. Expanding foams that fill the cavity beneath the sole have the advantage of reducing the spans of soles. While it may be true that expanded foam is not intended to be structural, per se, it is true that expanded foam under a thinner sole will stop you from deflecting the sole greatly underfoot.

For these reasons, I'd be inclined to use the block foam either as a filler with expanded foam, or to use the block foam where you know foot traffic will not happen or spans are short and only expanded foam under sections Underfoot and wider spans.

 

I was once in the "seal it up tight then enclose it permanently" camp.

Now I'm more inclined to accept the fact that water will eventually get into every nook and cranny and make allowances for it.

Leave areas for water to flow, protect foam from the water as much as possible, leave the sole easily removable, etc.

This may require some additional support in a few places, but makes life in the future much easier.

 

Pretty much anytime I'm asked, plan for water with limbers and pathways and a dry sump bilge area or garboard plug. Build for none. Then when the leak happens, you can go, hmmm. I got a leak, but not get freaked out by it. My livewell hose came off under the sole and I can't reach it to fix, so for about 10 years or better been drainin it to the bilge. The sole is wetter more often and is getting soft now because it really isn't supposed to be wet as much in part.

 

I would rather not use great quantities of expanding foam under the waterline. I could use a bit to help lock the block foam in place. My diagram shows a couple of setups (1) and (4) where the block foam is glued to the hull and then the sole is stiffened with wooden beams either glued to the foam, or above it, just running between frames.

I have enough though: 10litres of 33kg/m3 two part foam, which I will mainly use to fill above the water line in side compartments and odd places here and there, 100+ litres of block foam (I can easily pick up more) and 100+ litres of EVA foam that I bought for a different purpose but now isn't required. That's a total of at least half a cubic metre of foam. Since the hull material floats, I know it will be enough to keep the boat at the surface. I'm after upright flotation no more than half submerged fully swamped, with all air compartments breached (which should never happen.)

 

Rather than go into a long explanation of how to install flotation I will just refer you to this page on Level Flotation. Boat Building Regulations | Level Flotation https://newboatbuilders.com/pages/flot2.html
I am surprised that Bateau did not include instructions on how to do this because this size and type of boat is supposed to have level flotation.

 

building your own boat is not subject to the same rules

 

True, it's not, but if he ever wants to get it insured, or surveyed, or sell it ......... And since he asked about putting flotation in it he might as well do it the right way.

 

Thanks for the link; I hoped you would pop in at some point. I've actually read a lot of your posts here and there on block foam and Coastguard testing. That's one of the reasons I'm less keen on two part than I once was: I still bought some, but plan to use it more carefully.

I am fine with the Physics of flotation itself, I teach High School Maths and Physics, so have already considered densities and weights, COG issues if swamped etc. I've also read a fair bit of literature on installing it.

What I was really curious about was the exact effect it has on the stiffness, strength. fatigue and impact resistance of the hull if I glue it to the bottom horizontally, or vertically, or build it up to and attach to the sole, or put some glass tabbing in to hold it in place, and so on. It seems that the effect on the above is minimal unless I put the other slice on the sandwich so to speak.

The Bateau instructions merely specify foam as a recommended option and give brief quantities and locations. Some builders follow these, others prefer limbers or compartments. Since the boats will have a dry bilge if built to plans and built well this should work; but sometimes it does not.

I'm trying to go with a hybrid of both. Enough foam to float and ways for water/condensation to escape. I wanted to know more about the other benefits of foam. If I can install it below the sole in a way that enhances strength or stiffness for little extra cost or weight I'm all in.

Thanks again for the link: I've read some similar before but I will look at this one closely as well. I live in NZ so the USCG rules don't concern me, but Physics doesn't seem to care where I live...