Michael,
Most strip canoes can be built with about 55 strips. A kayak with a 24" width might need 5-10 additional strips because of the deck. Your boat is so narrow you might get away with 55 strips. If you allow for wasteage from the saw cuts boards that have a combined width of about 21" should be enough to give you the 55 strips.
Charlie

: I am building a kayak of my own design and would like advice on how much wood
: is needed to build it. The length is 15' 6", the beam is 20",
: and the depth is about 12". The wetted surface area is about 20 sqare
: feet. Any advice that you could give would be very helpful.