I did see a skin on
: frame kayak framed with a large bamboo, split down and bent for
: ribs and the length.

If you use a bamboo frame covered with fabric you'll have something which will be more durable than a foam / cardboard boat. Think about it for the project after this.

: If I were to build the Viking longship, I would build the keel and
: ribs (which look kind of like a wide wishbone) out of the foam.

Wishbone shaped ribs were made from one solid piece of wood. They'd cut trees which naturally forked and hew them to the propersize and shape. Your foam is nowhere near strong enough to work in that shape. Best we can do is use solid 'D' or 'V' shaped blocks of foam.

: I would have to scarf the pieces for the keel. Laminating
: cardboard on each side of the foam would give it additional
: strength.

Forget scarf joints. We'll use another method. Let's do the math: on. 6:1 scarf joint in 2" thick material you cut your diagonal 12" back. That's tremendous waste when your base material is just 28" long. Scarf on a second, third, or more piece and you need to cut off two triangles of material. That takes away 24" from each 28" slab. A cardboard lamination would need to be absolutely dry to work.

: The ribs would knotch down over the keel.

That should work.

Before we look at the longboat let's look at some thing that will support you. I'm looking at a 17 foot boat based on the 'Walrus' design in George Putz's book on fabric-skinned boats. It uses 6 frames set 29" apart. I'd like to keep the cardboard for the deck where it will add stiffness but not be constantly submerged.

The easiest way to get a strong boat is to have a solid block of foam running the length of the boat. You can laminate a foam beam 6" inches thick from three layers of foam pieces laid like bricks. This would be about 12" high.

Think of the boat as 5 sections. The bow section would be carved to shape from solid foam. The next section would have the beam running down the center and have slabs of foam for the sides and bottom. The center section would be the cockpit. To get a low sitting position the center beam would be cut down. To maintain strength the sidewalls would be double thick. Section 4 is like section 2 and section 5, the stern, is solid like the bow.

: Would the bambo work for kayak strips?

I didn't realize we had bamboo to work with. It can be very useful.

I'll draw up some pictures of these ideas and post them this week

PGJ