Foam floor

[Mark20Twain]

I have a question about an option on replacing my boat floor. I am thinking a lot about wood rot after having to replace the transom and the floor stringers and want to do a foam floor. The boat is an old 1989 Mark Twain 20? really it is 21? but that is what it is titled 20?. I have replaced the transom and the 2 stringers it had 2x6x11 running the length of the floor. I replaced it with 2x8x11 knowing I wanted to notch the tops every 12?? to fit and run a 2x2 across the width of the boat to strengthen the floor. What I want to do with the foam is Home Depot and Lows sells 4x8 foam sheets 2?? thick and I want to cut them to fit the floor in the boat and then pull them out and lay them on the floor. When they are on the floor I was thinking of flipping them over and glassing them together on the bottom. After it is dry take the whole floor about six foot wide and twelve foot long now after it is all glassed together and put the glass part face down in the boat. Now I will have just foam facing up where I will glass tape the sides then multiple layers of glass for the floor. Well I made that sound easy hahaha lol? I know this can be done and all i'm just wondering if you think my big *** 240lbs stepping in and out of the boat and my weight on the floor would wear it faster than me just using ? or ? inch plywood 4x8 for the floor. The weight difference would be juristic verses using wood about 200 lbs. I would love to hear feedback

 

[tashasdaddy]

Re: Foam floor

it is a fantastic idea, and if you can perfect it, patent it. the problem lies, that the foam is soft, and will compress. so your top surface has to do the supporting. it would take a lot of matt and woven, and resin to get the strenght you need. they are using foam sheets, in architectural details on building, cheap to shape, and the put a resin based coating over it. when you go to the mall etc. look at the finishes. the problems is it is soft, and can be cut with a knife, etc. you would probably get by with a 3/8 sheet of plywood over the top of it. glue it to the foam also. i would try it but i really, really hate glass work.

 

[tmh]

Re: Foam floor

How would you attach seats to this foam floor?

btw, I have a Mark twain 1985 19' bowrider. Not many MT guys on this (or any) board. There is a MT discussion forum eb site somewhere out there, I haven't been on it in a while so i lost the link.

Good luck. MT made a solid boat so you should have a nice rig when done with the fix.

 

[fixb52s]

Re: Foam floor

That is a great idea. For the seat support, you could put a piece of something under this deck where the seats will attach.

Early boat deck cores were only balsa, so I can see that this would be a great new way to build a deck. Foam is a lot easier to cut than wood, and it would not rot. This, along with foam stringers could solve a lot of issues with older boats.​

 

[tmcalavy]

Re: Foam floor

If you really want to try a foam floor, why not put a thin skin on both sides of the foam for structural support, maybe vinyl sheet or as TD suggests thin ply? I'm also a BOB, tipping the scales at 265, and I used a standard half-inch floor in my aluminum runabout. It's well supported and anchored, but it is creaky. If I ever pull and redo it, I'll think about a 1-inch foam core with thin ply on either side reinforced underneath for seat anchors. Let us know how it goes, post pix along the way...never know until you try something.

 

[erikgreen]

Re: Foam floor

Foam floors are used in a lot of boats now. The big thing is that you really can't get by on the cheap and use the sheet foam from your local home improvement store.

In any composite construction with foam, you need it to do two things. First, be a base for the fiberglass to form around. Second, you need it to form the center part of the I-beam that keeps the two fiberglass skins separate. The strength of the whole deck depends on the foam holding the two skins rigidly apart a certain distance. For that to work, the glass must adhere to the foam tightly, with no slipping.

The problem with the low density foams like the pink stuff is that they don't have enough strength to resist coming apart. The epoxy or poly adheres to them well (when it doesn't melt them) but under stress the foam itself comes apart and they delaminate, with foam sticking to the glass skin and a new break somewhere in the foam sheet. Once that happens the strength of the deck is gone and it will fail.

There are a lot of good foam sheets in high densities that you can order that will work for this if you really want waterproof. They're way more expensive than plywood though. Check www.bateau.com for some price examples in their materials section - either foam or honeycomb board will work for you.

Remember, if building a waterproof deck was as easy as using sheet foam instead of plywood, we'd all be using that instead.

Erik

 

[oops!]

Re: Foam floor

i would be afaraid of it delaminating under the glass.....

 

[Mark20Twain]

Re: Foam floor

Wow!!! I got a lot more info then I thought it is much appreciated. That was another concern was how well the seats would mount to the floor and the delaminating. I got the boat really cheap and it is going on 19 yrs old so I don?t want to blow the bank on $200 4x8 composite honeycomb foam floors. Im 23 and I love the water like all the rest of us and don?t have the big cash to get a new boat so I went with the restoration way. Im guessing I will go with the 8 ply ? the have at my HD for about $100. I will try to get some pictures on here because even im impressed with how well the job is going. Im going on about a year now working on it here and there. I think I may buy a sheet of the foam and run a test before I make the final decision won?t hurt. Im thinking it might not do to bad because the largest space the foam will have to span is 12??x24?? the floor looks like a big grid.

 

[Mark20Twain]

Re: Foam floor

Also Tmcalavy, the plan is to do both sides but I want to fit the foam in the boat perfect then take it out do the bottom outside the boat. I will then take the whole deck as one big floor piece and put it back in after running a bead of glue along the top of all the supports. Then the top will still be foam and I will glass tape the sides then the whole floor. Im just not sure on how much it will flex so I will have to try a section and see. Then again I can just use wood and it last another 10 years probably outlasting the shell of the boat.

 

[tmh]

Re: Foam floor

If you have time to dry it out, PT ply is the way to go, IMO. Many say no to PT because it doesn't cooperate with resin coating, but I used it on mine and it seemed to work fine. However, i dried the 1/2" 4x8 ply for maybe 5 mths over fall/winter and had it screwed to the wall of my garage to prevent warping while it dried.

Let us know what you wind up doing.

 

[ondarvr]

Re: Foam floor

Polyester resin will attack the foam and it will melt away, so you would need to use epoxy (more $$$). You would have no issues with flex, the panel would be very stiff, the problem is the foam isn't very durable, so there's a very good chance it won't hold up all that well.

You can buy the correct foam though.

 

[oops!]

Re: Foam floor

However, i dried the 1/2" 4x8 ply for maybe 5 mths over fall/winter and had it screwed to the wall of my garage to prevent warping while it dried.

Let us know what you wind up doing.

thats one hull if an idea for drying pt ply

 

[SFT2]

Re: Foam floor

Glass over foam is the stereotypical composite structure. There are a great many homebuilt airplanes out there built exactly the same way and they're a HELL of a lot stronger than a boat deck needs to be. For example, I designed a foam, glass, kevlar, and carbon spar, 22' long and roughly 2x6, that was stronger than a similarly sized solid wood part, but at 1/4 the weight. I don't remember the exact numbers, but it was designed for a 1000 pound load at 12 g's (12000 pounds) distributed over it. However, there is no foam at Home Depot or similar suitable for this type of construction. You need more density and consistency.

I was considering a composite deck in my Maxum that I wound up selling with the old deck in her, but a bit more complicated. 12" wide panels the width of the deck wrapped in glass and vacuum bagged, then bonded side by side and skinned top and bottom with more glass. It winds up with vertical glass webs every 12" and makes the pieces small enough to be managable when laying them up.

Mounting stuff is easy. For a 1/4" screw just drill a 1/2" hole through the deck, bond in a dowel, and glass over both sides. Naturally you need to know where everything will be going before you permenantly attach the deck to the stringers and hull, but that's not too tough.

It wouldn't hurt to put some kevlar in the upper skin, either. It's a nightmare to cut, but would help a ton with impacts. Without doing any math, I'm thinking a 1" foam core 12" wide with the corners rounded to about 1/4" radius (sharp corners and composites don't play well together) wrapped with 2 layers of 8 ounce twill weave glass at a 45 degree bias, then bonded side by side and skinned with a layer of 24 oz. biaxial on the bottom and 12 ounce kevlar under 8 ounce twill on top. The majority of the load is on one side, so putting more glass on the bottom, where it's in tension, makes more sense. If you know exactly where stuff will bolt in before starting any layups you could put a 6" square of kevlar on both sides of the foam in those spots, and bond a dowel plug in. Then put it all together, get it in the boat, and drill the plugs for brass inserts to hold machine screws.

Yes, it's complicated, but it can be fun, too I'll be doing exactly this when I replace the deck in my Baja in a few years. Not so much to get rid of wood as to reduce weight, though.

 

[dthrckt]

Re: Foam floor

really old thread...but i'm reading it because i'm planning to put a floor in my aluminum traveler.
i think the idea is sound. you could easily test the compression of the foam by making a small test piece and stepping on it. but, if the foam was not completely encased it would absorb water.
i think i'm going to use the sheet foam to make exact fitting templates and then cut the floor boards from the textured plastic 4x8 sheets i saw at home depot. finally I'll fill under the boards with marine expanding foam. probably the 4lb stuff http://www.shopmaninc.com/foam.html
if i fill my boat with water up to the line where the floor will be and then drain/measure i should be able to tell how much to use. now, how much to put in each section to get the proper fill is another matter...
guess i'd better think long and hard on this because one that foam expands under the floorboards, getting it out will be TOUGH...

 

[redfury]

Re: Foam floor

Truly is an old thread, but one that has merit as there never was any testing/reporting done.

I had had the thought of using a high density poured in foam, but my thoughts were along the lines of pouring the ENTIRE cavity of the boat up to the level of the floor, shaving it smooth, and then glassing over the top of that with blanks of wood/similar buried into the foam and under the glass for seating mounts. The entire under deck area being 100% foam ( short of a small bilge that would be completely sealed ) would theoretically be water tight...the foam would have no water to absorb.

The only issues that came/come to mind are expansion rates of the foam vs. fiberglass and if weight would become an issue. Foam can be gotten in a high enough density that you would need a hammer to make a dent in it, but the overall weight would become an issue. A 2 pour process with half being lightweight foam and the second pour being the high density foam could alleviate some of the weight issues, but one has to wonder if the heavier foam on top would cause problems with the foam below. Also, foaming the entire hull bottom would cause the area below deck to become extremely rigid, so what stresses would be placed on the fiberglass above the deck level...I would almost suspect you could expect long term stress fractures to form in at least the gel coat.

 

[erikgreen]

Re: Foam floor

Worth mentioning here that a former (well, current, but he's mostly over at boatdesign.net and bateau's message board now) named KnottyBuoyz.. he did some testing with a vacuum infusion setup on laminating the pink foam and posted his results in a thread here.

Basically he verified what has been said, the pink foam doesn't have the strength to resist delamination... the resin and glass hold up, but the foam separates with a layer of foam ending up attached to the glass that's broken away from the core.

I actually thought about using the 2 lb pourable floation foam for a core... it's much stronger than the pink stuff even though it's the lightest polyurethane pourable you can get easily.

You could just make a thin, flat mold for a "sheet" of it and pour it out, spreading the liquid with a coarse brush ala roll and tip, and maybe using a plastic covered sheet of ply on top to ensure it spreads out and has a smooth, flat top surface.

Then you have to de-mold it and cut it to size... it's brittle stuff, but if you could do that then you would have a 4x4 sheet for maybe $25 that won't delaminate... but you do have to make special allowances if you want to mount anything on it.

If you're looking for an even stronger glass core than foam, use thin plywood.. it's much stronger than any but the highest density foam, it's much cheaper, much easier to find... of course it won't resist water as well unless you use PT, but you can make a very light plywood core deck that's extremely strong.

If all you want is a deck that is strong and waterproof, you can actually do this:

* Make a 4x8 or so rectangular mold from masonite or arbourite covering plywood. Make sure the edges are slightly raised and also covered.

* Wax this mold with partall paste wax all over, three coats with drying in between

* Using poly resin and alternating layers of cloth and mat (start with cloth) put about 50 oz. equivalent cloth weight fully wet out into the mold (3-ish layers of 1708 will work, or two of heavier cloth).

* Make sure you use a roller to get all the bubbles out and force the glass flat

* Using cardboard (either half tubes or angle bent pieces) make a series of ridges on the wet glass spaced about 9 inches apart (lay the half tubes flat/open side down on the glass side to side or end to end)

* Place one layer of 1708 or equivalent, wet out, over the cardboard ridges/tubes

* Let the whole thing cure

* Pop the panel out of the mold. You can then use it like any other flat deck piece, cut to shape and glass in place then fair and paint or cover with carpet

It'll be lighter and stiffer than a thicker plywood floor but just as strong, and totally waterproof.

If you get the hang of molding this way you can spray a layer of gelcoat first and your new panel will come out white and clean. If you shape the mold first with an anti-skid pattern you'll get that too.

If you're willing to try using "exotic" materials, give molding your own poly resin glass shapes a try... once you get proficient at it you can really do a lot of cool stuff, and you get a ton of confidence about repair and restoration.

Erik

 

[redfury]

Re: Foam floor

If I had the space to work something like that, I'd try it Erik . In the end, I think the argument breaks down to this- If you want a lightweight deck that is impervious to rot, you had better be prepared to either 1: pay for it, or 2: put a lot of labor into it, and pay for it. Otherwise, you are going to go with the cheaper solution and roll the dice on the length of time the deck will hold up over time.

Personally, I plan to wrap and seal my wood with CSM, bottom and top, with a layer of CSM over all the panels once they are put in the boat.

I've put a good deal of consideration into using the spray bedliner for my flooring for 2 reasons. 1: easy to clean. Spray it down into the bilge and out the back of the boat...just seems to make sense, especially comparing it directly to carpetting. 2: noise control. I fish, and the sound deadening factor is better than leaving a gel coated floor, and I don't have to worry about hooking the carpetting. As long as I get the final layer of glass smooth, the bedliner will look just fine.

 

[Mark42]

Re: Foam floor

The hard top project is pink foam with epoxy. I did some tests wrapping 1708 around foam and beating it, etc. I have to agree that it would not be a good choice for a deck where there are high pressure points that can compress the foam. For the hard top project, it is working well, and even after compressing the test pieces a little, they did not delaminate. They did delaminate when compressed more than about 1/4" because the foam is too rigid to rebound. When I say "delaminate" what actually happens is the foam tears apart leaving a thin layer of foam on the epoxy. To help with adhesion and torsional/twisting forces from breaking the bond, the foam was punctured with a pencil point on a 3x3 inch grid. The resulting holes filled with epoxy when doing the layup and gives more grip.

But the bottom line is I would not make a floor out of pink foam unless it was just used as a mold and had many layers of glass that supported the weight and the foam is sacraficial.

How about the PVC 4x8 sheets? That sounds like a great product for decking.