20' SeaCraft heavy motor

[Bushwacker]

Originally Posted by Offshore Asset

I agree there. The hull is very thin. Could I get away without a coring material and doing a layer or two of 1708? I am thinking that with the bulkheads I put in under the deck, and a big front bulkhead for the anchor locker, it should be pretty stiff. The floor will be going directly to the hull sides as well as maybe 2 vertical supports on each side. I've seen balsa used as coring. Just uneasy about putting wood back into the boat after I took out so much of it that rotted. Any other suggestions on foam coring materials for the hull sides? Is divinycell the standard nowadays?

Carl Moesly built his boats very much like the airplanes he flew for about 25 years . . . light, but very strong! The problem with trying to increase the stiffness of a panel with only a layer or two of cloth is that stiffness of a panel is proportional to the thickness of the panel CUBED! A layer of 1708 is about 1/16" thick. So if you compare 2 layers of 1708 to
one layer of 1/4" balsa core with a layer of 1708 over it, the balsa core will be 27 times stiffer than than the 2 layers of glass, and probably not much heavier! A little bit goes a LONG way when you start increasing core thickness!

The bulkhead in the anchor locker will only stiffen the front 18" or so of the boat, and it's already fairly stiff up there due to the flare in the bow and wide deck, so that won't do much. As far as core materials, end grain balsa core actually has 10X more shear strength and better compression strength than most all of the foam materials out there, plus it absorbs resin very well so it bonds better than most foams. It's good stuff as long as you don't drill holes in it. I've read a number of Dave Pascoe's articles who is a very knowledgable surveyor who has done a lot of research on core materials. Although he's a big fan of balsa, he also likes core-cell, which is one of the newer materials.

One thing about adding bulkheads is that you have to be careful not to create hard spots in the hull. My neighbor is a professional yacht builder and composites expert, and he says the best way is to use a foam fillet shaped like a trapazoid between the bulkhead and the hull. Then when you glass over it, it will create a gradual transition between the relatively flexible hull and the much stiffer bulkhead.