The cheapest stable boat tender possible.

1) You need to sand the XPS to remove the slick surface in order to get good adhesion and avoid delamination

2) Motor mount and other stress areas are made of vinyl trim board stock

3) 1/8" glass typical . . . 1/4" in stress areas.

 

I was thinking the same thing. 4’ wide seems a bit narrow for stability for a working platform though. I’d probably opt for an 8x8 platform if there’s enough access around the boat. 4 sheets of 2” rigid foam insulation, ‘cross hatched’ sandwich. Glued with something like liquid nails or another construction adhesive that’s compatible with the foam. Less than $200 total probably. Total flotation of over 1,300 lbs. If that’s not enough, slap another layer of foam panel on it.

If 8’ is too wide, stack and glue 2 of the sheets together, then cut the other two lengthwise into strips (1x8) and stack and glue them to create ‘pontoons’ under the 4x8 sheets. That would maximize flotation along the working edge. Might need to use straps or rope to lash the whole thing together. Flop a piece of cheap plywood on top to walk on.

 

ps is still easy to shear

I like to make it super rough with 36 grit and then hotcoat it with thixo first

 

I guess with 3 to 6mm thick fiberglass the XPS really doesn't matter and serves more as a stay in place mold?

 

Delamination of 1/8" of glass is probably not a major problem in a dinghy.

Avoiding exposed glass edges is also important.

All said, I would personally prefer going with a 6mm marine foam core. It would be lighter and using less glass would still not have delam/crush concerns.

 

Wouldn’t the weight be about the same. Most of these foams are running 3 lbs/cubic feet. Also the Owens Corning foamular 1000, the pink board, shows compression strength of 100 psi. I’m not sure how this compares to pvc foam, divinycell, but the glass and epoxy will spread the load.

I’ve made a hatch and a companionway cover out of the standard extruded styrofoam from Lowe’s. I made nice filets and epoxy coated the inside. The outside got 1 layer of 17 oz biaxle cloth. I’m 230 lbs and I can jump up and down on them. They’re 7 years old now.

 

If you seach for XPS in the forum then you'll find some good answers by the experts about this. The TLDR is that while compressions strength of XPS is ok the shear and tensile strength is too low.

It can't take the loads, especially not the force of a small impact. So you need to make the fiberglass laminate thicker which increases cost, weight and work. From what I've read any 80kg/m³ structural foam has enough shear strength for most normal fiberglass loads. So it doesn't make sense to use XPS. It can work but it's not the optimum in terms of weight and cost. 3mm laminate is like 3000gsm compared to something like 1000gsm you'd need for structural foam or 200gsm you'd need for plywood.

Except if you put it between plywood (or maybe veneer?), then the plywood gives enough impact strength and the XPS gives stiffness and light weight.

Or maybe if you need it for additional insulation in your boat, then it should be able to provide additional stiffness like those SIP / osb xps panels do but you need to engineer that correctly.

Or I guess if you want to glue together a dingy like the above by using cheap and easy to shape XPS and laminate it.

 

Big box store polystyrene under same testing for compressive strength is 15 psi vs Gurit Corecell M at 148 psi.

https://www.owenscorning.com/NetworkShare/EIS/23513-H-FOAMULAR-150-XPS-Insulation-Product-Data-She

You cited foamular 1000 which reports better compressive strength, but that is all special order stuff usually.

But the datasheets don't show anything for sheer.

Sorry for the edits. Bad grammar and clericals.

Point is to keep things clear for other builders.

 

Borrow something