Deck framing with rigid foam

Go to 6:55 in this video
https://youtu.be/dXcqTxe4mzw?t=415

hmmmmmmm - - - as I say many times: his boat = his call.
would I do it ? no way. (in its present state).
BUT - that boat is only ten feet long - so I doubt very seriously he is going to have any issues.
(other than falling out of his boat).

in a larger boat, say 16ft, you would need to put a lot more time and thought into connecting
all the foam joints together with some expanding foam-in-a-can to lock it all together as one solid piece.
not to fill all the cavaties - just along the mating surfaces to lock it all together.
second factor would be the weight of the people walking and dancing on that part
of the deck - as well as the types of water the boat runs in.


I have seen waterbed frames made out of flimsy interlocking plywood panels
that when done correctly, will support thousands of pounds. It is all in the engineering.

I bet that'll work great - that lightweight criss-cross bracing concept is used in a lot of things. One big disadvantage to it in a boat though is having no under-deck storage.

The idea looks good - but what he's got there is only a demonstration of the concept - If I were making it I would cut notches halfway down each sheet and make them so they mechanically lock together then figure a way of adhering them to one another and put square rod sections of foam on the inside of each of the corners. The adhesive is the issue - I know you can use white glue on that type of foam without dissolving it, but that is water based and not suitable for a boat. There must be something out there that will do it I'm just not aware of what it might be off the top of my head.

You would want to know the compression strength of the foam and plan the square pattern according to the loads you would expect it to carry. Looks like he's got squares of 8x8 which probably work fine. Only issue is the type of foam he's using dissolves if accidentally exposed to gasoline - and mice love to chew through it. I don't think it's a permanent solution but it's certainly a very light weight one.

Paneling could be notched like this:



And then square rod sections of foam used to stiffen the joints like this (as if viewed from the topside):

I don't really like the sloppy vertical card house construction. I think you could do a lot more stable and durable framing by just using solid blocks instead of vertical walls. A nice big rectangular block down the center and then smaller blocks along the sides. It might use a little more foam, but I have serious doubts how long the foam edges will last before they started breaking down.

Pretty sure that the original Gorilla Glue will do the job. I'd call them first, though.

"The water activated polyurethane formula expands into materials to form an incredibly strong bond to virtually anything making Gorilla Glue your solution for almost any project or repair. Gorilla Glue is a 100% waterproof glue, safe for indoor and outdoor use and strong enough to stand up to the elements."

richg99

Its not a question of being water proof - its that polystyrene foam is very reactive and dissolves easily if exposed to most solvents other than water.

Here's something made specially for it but it's water based as well, not sure how well it would hold up to prolonged moisture exposure but it is listed as moisture resistant:

https://www.homedepot.com/p/Loctite-PL300-10-fl-oz-Foamboard-VOC-Adhesive-1421941/202020476

richg99 said:

Pretty sure that the original Gorilla Glue will do the job. I'd call them first, though.

Click to expand...

Gorilla Glue construction adhesive - comes in a caulk tube - works on foam boards. It also works on aluminum and almost anything else. This would be the correct adhesive for foam board. DAP also makes a foam board adhesive as well as Loctite PL.

no matter what kind of decking short of 3/4" you put on that edgewise foam you will be creating a lot of indentation wherever you're standing. That 200+ pounds will end up denting and degrading that thin foam edge very quickly. This type of egg carton frame works good on a waterbed for example where there is not a lot of direct pressure points but would not work well imo on a boat with thin decking above it.

Much better to spread that foot sized loading over a much larger surface like a block of foam ....

richg99 said:

Pretty sure that the original Gorilla Glue will do the job. I'd call them first, though.richg99

Click to expand...

It will work on polystyrene with no reaction and holds well; I did several test runs before using it to glue in some foam blocks along my transom. It's a water activated expanding polyurethane foam, it's completely waterproof when dry, and bonds exceptionally well.

Tubes of PL 300 adhesive is the most commonly used for foam insulation... quite a bit less costly than Gorilla glue...

https://www.google.com/shopping/product/7849952020071226409?lsf=seller:8740,store:4843279175670652763&prds=oid:15860415605801257195&q=pl300+glue&hl=en&ei=AiftWLneLob1mAHQy5iwCQ&lsft=cm_mmc%3DShopping-_-LIAs-_-D24-_-202020476&lsft=gclid:CNr-lceKndMCFYU9gQodxSsE2A#spf=1

Shaugh said:

no matter what kind of decking short of 3/4" you put on that edgewise foam you will be creating a lot of indentation wherever you're standing. That 200+ pounds will end up denting and degrading that thin foam edge very quickly. This type of egg carton frame works good on a waterbed for example where there is not a lot of direct pressure points but would not work well imo on a boat with thin decking above it.

Much better to spread that foot sized loading over a much larger surface like a block of foam ....

Click to expand...

You could set the honeycomb to end the thickness of a foam sheet lower than the plane the exterior surface (FRP, thin aluminum/plywood) would be set to and have a flat sheet of foam atop the honeycomb section beneath the exterior surface.

Playing devil's advocate here - If I were to add a casting deck to a small boat (10'ish) it wouldn't be elevated, I'd remove the center seat and put a sheet down on the ribs and add buttresses up to the sides for support - but the foam idea isn't a total fail.

Don't get me wrong, I agree it's a decent idea. That kind of foam is very easy to work with and predictable. I just think that a material that you can dent with fingernail is going to break down quicker than you'd like in that kind of minimalist construction. It's more than adequate if you can get enough of it under the decking.

He must have one hell of a set of sea legs on him if he stands on that deck in that narrow flat bottom boat.

You can use epoxy resin on the foam and cover with fiberglass, you will be surprised at how much weight it would handle and how strong it becomes. I built a buddy a foam boat that he put a 3hp Briggs on, thing was 1 2" sheet on bottom with 3 2" thick sheets 4" wide for bow, sides and transom. I did fiberglass a 3/4" thick piece of plywood to the transom and used 6" carriage bolts to bolt thru the fiberglass with a 3/4" piece of glassed plywood on the inside. He hunted out of the foamer for 4-5yrs and I fixed the bottom 3-4 times where he ran it into a stump or over a rock. That boat was still solid when we destroyed it after building him a wood boat.

We purposely tried to sink it and couldn't, we used it to haul 12-13 bags of Sakrete to fix a problem with with my elevated deer blind that was in a river bottom covered in 4-6' of water, carried that, me and him and his lab and still had freeboard.

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Ha ha ha, that was great, best jon boat set up ever! the battery was worth more than the boat. Ha Ha Ha!!

Seems this concept could be taken a step or two further and made into a kind of torsion box. But would require skins on top and bottom and parts to be glued into a single unit, so to speak.