: Thanks Brian. Am I allowed a couple more questions?

Of course.

: What is the normal size? (If there is a normal size).

There really isn't a "normal" size, per se, but 4" or so seems to be pretty common.

: How is it constructed or is it simply a piece of tube epoxied in?

Some people use thin walled PVC pipe glued or glassed in place. Some are made from molded fiberglass. There's no reason one couldn't be constructed of wood strips and 'glass. The shape doesn't have to be tubular. Fiberglass knee tubes are typically a half tube or rounded V shape. When glued to the deck, they form a tube. If you shape your knee tube correctly, all you need to do is pad it on the sides and you have solid knee braces.

: Does it have a lid or access port?

It certainly can, if you desire dry storage in the cockpit. The 4" round Kajak Sport hatch seems to be popular for this application. It's also possible to have access from the outside, so you don't need to pop your spray skirt to access the contents of the tube. The problem is that this might interfere with deck bags, chart cases, etc. on the deck.

: This sounds like a
: great idea that I haven't seen on this site before and it comes just in
: time for me to incorporate it into the double I,m building right now. If
: that works out out I'll do it in the Cape Ann Expedition that will be my
: next project.

It's been discussed here before, but not for a while.

: (Whoever said you can't build just one was right). Thanks again.

You're welcome. I hope it works out for you.