About that foam?

I have some foam board that is 60 lbs/ft. As dense as I can get from my building supplier. Home Depot is around 25-45 I am told. What might the difference be with regards to some of the foam board regularly used for boat making?

 

60lbs /cu ft? That can't be what it weighs. Boats builders use 2 lb for flotation and sometimes 4 lb for foam or as structural foam. You rarely see anything denser than that. Are you sure he isn't giving you a buoyancy figure. 2 lb foam would suppor 60.4 lbs per cubuc foot of foam.

 

Or perhaps it's 60kg per cubic metre...? Which would be somewhat less dense than the 80 kg/m^3 stuff that is probably about the most common density for structural cores..

 

I was told it supports 60 lb/inch. Compared to HDepots foam of 25-45 lb/inch. I can pinch the foam from the Depot and dig my thumb into it. The other stuff I have will do little more than dent. I haven't seen the foam used for boat construction but I am guessing it is really close.

Another idea I have is to route groves in the foam to create inverse stringers. That should add quite a bit to the strength, wouldn't you think?

 

You'd be far better served to simply use thicker foam, given the work involved and the extra epoxy required to sheathe the grooves (if that's what you meant).

 

look here. about 2/3 way down the page they give specs on structural foam used in boat building. This is fairly typical. http://www.generalplastics.com/products/product_detail.php?pid=21&

 

http://building.dow.com/na/en/products/insulation/highload60.htm

This is the material I am using. It seems to be rated at 60 lbs per sq/inch (cubed?) I would like to know how it campares to the foam board mentioned in an earlier post which has something rated at 24 lbs/sq/foot. It cost me 30$ for a 2x8 sheet 2" thick, I ripped it down. They also have something that is 100 psi cubed which apparently is used to land planes on.........

Also the groves I routed are round 1/2" x 3/8" and run like stringers underneath the hull. I was going to take a spoon and mold the glass inside, not fill them up. Wouldn't you thing that would be way stronger than flat?

Thanks

 

I do not know how it compares to boat building foam. However the weight rating you list is for what the foam can hold up without compressing or failing. The lower 25-45lb rating is for snow loads on flat roofs and such.

Closed cell rigid insulation (extruded polystyrene) for use under concrete ground slabs is rated that way too. The thicker the slab and heavier use the concrete the stronger the foam board.

Let's say you are insulating the perimeter of a basement, the under slab insulation will be holding up the concrete plus whatever furniture.

Let's say you have a tunnel on an airport tarmac which is insulated and giant aircraft pass over it, then that will have to be very strong foam.

Ike, nice link.

http://www.hamiltonbuilders.com/building_insulation.php

 

I have tried using styrofoam as mold some years ago, it melts under epoxy. I even had duct tape to cover the foam, still the epoxy found its way through.

Lurvio

 

Styrofoam melts under polyester resin, not epoxy.

 

Lurvio,
I have also used Styrofoam and covered it with epoxy often. It did not attack the foam at all, with a quick cure. GPR resins will destroy Styrofoam unless it is protected by a barrier like PVA or vinyl paint which I have had good success with many times. The density of foams in the vinyl foam types are 15 lb density used for structural stringers on 60' sport fish boats. These are very structural, obviously.

 

Ill let you all know...
Thanks

 

Well thanks guys for the clarification, it said 'hartsi'(finnish) on the can, which seems to translate as 'resin', I've had thought it meant something closer to epoxy.