Combining Foam and Plywood construction in the Same Hull

That's a great site with some informative info and pictures! Thanks for posting.

 

Guogeon brothers

Guogeon brothers book on boat building is a must if you want to do this...

 

You shouldn't have a problem mixing materials, as long as the scantlings of each are appropriate to the stresses they'll see in their locations.

I'm curious what sort of curvature you're talking about in the flare of a 20 ft fishing boat. If you go with multiple thinner layers of plywood in the flare area you can pull off some pretty impressive compounding. You might have to build forms for at least that area to form up the plywood, but you would have to build forms for strip planking anyway.

 

I would be inclined to make the topsides out of plywood, too, using developable shapes and stich-and-glue. Then glue blocks of foam to the outside of the plywood, add a plywood deck to establish the outer contour of the flare, carve to shape, and glass everything in.

Basically, all the load-paths are taken by the plywood, including impact from docking, but you have the flexibility of the foam to achieve your desired shape. The foam in the flare becomes a huge fillet between the topsides and deck, reinforcing both. You lose a little volume on the inside because of the vertical topsides structure, but this isn't a very useable volume to begin with and the deck is probably handy to have.

This technique has been used with great success in amateur aircraft construction. Foam cheeks are added to a box fuselage to achieve a variety of aerodynamic shapes. Several sub-scale WWII fighter look-alikes have been built this way. They each had the same basic structure, but looked entirely different, due to the foam.

 

That is a cool idea - solves a couple challenges I've been wrestling with. Thanks for the tip!

 

That IS a cool idea! Thanks for posting it. The only question I have is how to ensure fairness and symetry of the foam cheeks as I shape them. Are there any tricks used by the aircraft builders that I could use?

Thanks again!

Charlie

 

I've seen foam blocks 'dished out' by running them at an oblique angle to a partially raised tablesaw blade (sideways to the kerf of the blade). By adjusting angle and depth of cut you can control curvature. Not sure if that would help in your case...