Plywood alternative to building sailboat interior furnishings

I have gutted and am completely rebuilding a rebuilding an Alberg 30.

I am wondering and researching information about building nonstructural interior components. ie Settee's, cabinets, partitions, lockers, etc. out of fiberglass coated foam. I am not sure if a 4x8 sheet of foam sandwich would be heavier than a 4x8 sheet of 1/2 plywood. Somethings I read suggest it weighs less. I would guess that 1/2" laminated look to be from my test layups with cheap foam are 3/4" thick. I laminated 2 layers of 1708 on a pink sheet of 1/2" formular 150 sheet of foam and it seems amazingly strong. I put 4x4's under the sheet about 1' from each end and walked on it. I am 285lbs and it bowed almost to the ground. some light cracking going on but didn't break until I bounced my weight and then it bent in half. So makes me think that making interior non-structural would be possible. If this is indeed possible what foam options are available to use that don't cost arm and leg.

Would this be a way to go or is there other options lighter and cheaper and not wood. Although the one I made seems pretty tough, I dropped a 2 lb weight from about 3' and it just bounced. Also trying to separate the layup from the foam seemed fruitless. Being that the stuff I am wanting to make is not a full sheet size but 1/4 or less in size of a full sheet.

The reason I am looking into this possibility is that I would like to remove all square and right angles that came with the boat and includes curves and mold impressions into the design. I realize that somethings possibly need to be wood, possibly the settees due to weight and pressure and impacts from falling.

Could a 1/4" foam sheet be fiberglass laminated to 1/2" and be as strong enough as 1/2 marine ply and weigh less than a sheet of 1/2" Okoume. This process would allow me to make curves and impressions.

Cost and time are not the issues for me what is the issue is reducing building weight. however, I am not looking for 1%er solutions.

 

The idea of glassed foam is very definitely practical, and will be a LOT lighter than plywood.
I have only done a few small projects in foam, and the amount of strength to weight is incredible. Cost was the main concern for me.
1/2" fibreglass skins is way too heavy. You don't need anywhere as much as that.
Go for say half inch foam, and two layers of 10 oz cloth on both sides for "furniture". Just do a small test panel, and see how strong it is. You will be pleasantly surprised.

Things to remember.
The skins have to be strong enough to prevent flex, as the foam is only a compression component.
Use curves as often as practical, (compound or conical) to make the best use of the foams strength.

 

I don't have much exprience with materials outside of wood and concrete, but my sense is that you should be able to build what you want with sheet materials a lot less than 1/2 inch thick. With a little reinforced frame work, and the curves (arching) mentioned by @rwatson, 1/4" plywood would be plenty strong and light.

If some sort of sandwiched composite is just as strong or stronger, you should be able to manufacture a very light weight interior. Just keep it simple and keep it clean.

-Will (Dragonfly)

 

Don't bother with xps 150. It will sag and delam overtime.

something like 12mm corecell is pretty light unless you load 24 oz of glass each side; at which point is starts to get heavier. 96 oz in a yard is about 10 ounces a foot; say 320 ounces 4x8 or 20 pounds plus the 7 pound sheet....still much lighter....lighter still under vacuum..

costs are higher by a long way for the corecell panel and you can't hinge it...so you also need to build in high density edges...

I built my settee with 3/4" ply. I am trying to offset some aft weight...if I have to make new panels from corecell; it will be in a remodel

A way to build a light panel would be to sandwich xps with 1/4" or 4mm plywood..

Here is my sad little heavy settee! All 3/4" marine ply.

 

You could build with quarter inch plywood and only brace the backside with wood strips where needed to stiffen it.

 

You can find lighter or cheaper but not both together. Your boat is heavy so there won't be a major improvement in performance by using exotic materials.

 

I never planned on using formular 150 I only used it to test out how fiberglass and a core are when laminated together. That being said, I have been making body surfing boards with my son using dow insulation boards. and some of them are quite old now and still going strong. Actually, my Grandchildren are now using the same body surfing boards my son and I made for him when he was young. However, they are an object completely surrounded in several layers of fiberglass a bit different from a sheet.

I also saw Fall Guy's post about his project where he separated the laminate by hand from the pink board he used. For the sheet I made we used two pairs of large channel locks to pull it apart, it did come apart but it tore non uniformly down various areas of the center of the foam. It had very few areas that it separated from the fiberglass only. The only thing I see different is my son and I painted Glidden Gripper paint on the foam before coating in epoxy then fiberglass. Have no idea what would happen to this down the road strength-wise.

Any Ideas on 1/4" Okoume and fiberglass skin on one side! or both sides.

 

Wood is stiffer by weight than fiberglass. You can't beat plain plywood, specially with some light stiffeners glued under.

 

I've got to go with Gonzo on this one. Ply, well finished and a few cleats underneath. Rock solid and simple. Westley, you could use .25" Okoume but if it's interior joinery I don't think you need go to the expense. I used Araucoply (Radiata pine) 1/2 and in a few spots 3/4 ply exclusively for all the interior joinery on a restoration I did back in the 2008/10 area. It still looks fine with no evidence of rot. I DID coat all six sides with three coats of epoxy. In any areas subject to wear and tear I added a layer of 4 oz. fiberglass cloth. 2 coats of primer and two coats of Interlux Perfection finished the panels so they're pretty well entombed with resin and paint. You can also use HPDL (high pressure decorative laminate) on good quality plywood. Interior bulkheads done in white makes a boat look bigger inside and is easy to clean. Wilsonart and Formica also make some very nice material for countertops that simulates stone.

Araucoply has gotten harder to find recently, I don't know if this is due to COVID or something else. Lowes used to have it as a stock item as did Home Depot but I don't see it lately.

 

Thank you, Gonzo. Thank you for your good suggestions. Most of my boat interior will be white. I am not into wood maintenance anymore at all. A touch of wood here and there is all there will be, rest will be white with gray sole and light gray cabin roof. I

 

If you are going white, laminate is a good option. It goes on with contact cement and lasts for many years.

 

Laminate is heavy, but pretty maintenance free. Paint on wood does a very good job, also.

-Will (Dragonfly)

 

My plan is to paint all the teak bulkheads white. I have great results in the past painting teak. I first wash the teak down and scrub lightly with a soft wire brush with (sugar soap) I use Tesco and then a sponge rinse and then let dry. I do whatever sanding I need to do with 80 grit vacuum dust away, and fill in holes dents, etc. I use epoxy and wood filler. After sanding everything the last time and vacuuming dust away, I wipe it down with a deglosser and let it dry. I then paint a coat of Glidden Grabber. I think it's now called PPG Gripper, it's an all-purpose bonding primer-sealer sort of thing. I love this stuff. I have found that you need to make a second coat due to teak sometimes discolors the first coat. Again I love this stuff.
I have quite a bit of teak furniture at home painted in this stuff and some if it was fresh brand new teak. years later all it ever needs is a slight touch-up here and there. I used it in my old boat H30 on the interior and 8 years later still good as the day I did it other than areas I damaged doing this or that. I need to do some of this soon.. Tired of the dark interior while working it. LOL

 

I am surprised that no one mentioned Coosa board. I have done a lot of rebuilding hatches, power boat transoms etc with Coosa. There are numerous densities/price points for the sheets. It is easy to work with, and no rot. For non structural boat furniture, it could be covered with laminate/or painted. There is not an imperative to coat everything with epoxy, so there could be a time/cost savings vs using plywood. As has been said, it is hard to beat good marine ply in dry locations.

 

Go with MDO if its available in 1/4" thickness I know 1/2" is. Great ply, ready to paint, reasonably priced, and water resistant, used for outdoor highway signs. If available in smaller than 1/2" I would like to know where.