Foam core to stiffen floor of dinghy

[TheImpulse]

I'm wish to use faom core over 4mm ply the fibreglass to stiffen the floor of my sailing dinghy. But I'm not sure on how to go about bonding the two and prevent the core from absorbing to much resin which would increase weight.

[marshmat]

What type of dinghy is it?
Foam core over ply? That's not exactly a common one, to be sure; I can't testify to how well it would work. If you are intending to wet-layup fibreglass, you would need a closed-cell foam like Divinycell or Core-Cell. These do not soak up the resin, although they do bond very well to it. A good epoxy such as West System would be a must for this; polyester would probably not do the job.
4mm is a pretty thin ply. Is it glassed right now? If it's not, or only on one side, glass alone could stiffen it quite a bit.

[buckknekkid]

we are currently installing a complete cockpit sole in a fibreglass sandwich from SpaceAge Synthetics

[frosh]

Foam over ply is unusual. Also foam will dent under your feet so unless the ply is on top of the foam you need a skin on the foam of sufficient strength to prevent this.
Woven carbon fibre has a stiffness modulus several times greater than woven fibreglass. It does cost more but a layer of 200 gsm carbon followed up with a layer of satin finish glass of around 100 gsm to protect the carbon will make a huge difference. If you do this, you need to sand all the paint or varnish off first.

[Ike]

You would be better off going with balsa core rather than foam. This would significantly stiffen the floor and would not have the bonding problems of foam core. However, the big problem with balsa core is water intrusion. So you need to make sure you get a really good saturation of resin into the balsa and into the gaps between the balsa blocks. Also you need a good bond between the core and the laminate. Vacuum bagging should accomplish this. The resin will not appreciably increase the weight and the balsa will give you a nice stiff light structure.

Balsa core is commonly used for stiffening hull sides and decks, and even used to stiffen the hull bottom on wood drag boats. (Not really a good idea to put balsa in the bottom of a hull though. Punch a hole in it and water soaks the balsa.) But it works well in decks and hull sides.

[TheImpulse]

I foregot to mention that I would be fibreglassing over the top of the foam to give the sandwich effect. Thanks for your help.

http://www.ausimpulse.com.au/index.html

[John ilett]

This should work fine. Ply foam sandwich is common enough on boat interiors and very stiff.

Prepare the ply and bond PVC (divynicell,herex) or acrylic (corecell,rohacell) foam to the ply with epoxy glue or resin using either weights or vacuum bag.

Then finish a layup on top with maybe 2 layers of 160g-200g fglass cloth and epoxy resin. Use a peel ply and then non skid.

Remember that often foam verses balsa are unfairly compared as balsa's are typically about 160kg density whereas foams are very commonly just 80kg.

Balsa's are stronger (when used correctly) and cheaper but can also have many processing complications that will make you wish you had used foam.

[JEM]

"Denting" the foam will be an issue. You'd almost need another layer of 4mm on top of the foam so you can walk on it. That, or plenty of the glass.

You might consider just using additional layers of glass or exotic cloth for the stiffness you need.

Also, glueing on some flat-bar style ribs made of wood is simple solution. Sort of like cold molding but easier.

[adamfocht]

Space Age Synthetics has a great product for floors, bulkheads, stringers, its called Thermo-Lite and has some pretty impressive properties, weight and resistance to rot being the best.

Balsa, foam, honeycomb, would all be good choices for what you have in mind, provided you have the understanding to use them.

you've definately come to the right place to learn how

[Ike]

If you really want to do this on the cheap use doorskins and balsa core.

In 1980, just for fun I took a boat building class. We built a little 8 foot dinghy in cold molded construction. For the veneers we used door skins. The are thin veneers used to replace the skin on wood hollow doors. You can get them in various types of wood and furniture quality if you want. They are about 1/6 inch thick. You can cut them to size with a knife or box cutter.

Anyway, Balsa or foam sandwiched between two skins and vacuumed bagged would make a nice stiff floor. And if it gets messed up to the point it has to be replaced you just peel the door skin off and sand it down and put a new skin on.