It is not a good idea to change a design that much.

Read these comments from someone who makes his living from designing and building boats.

http://www.woodworkforums.com/f33/scaling-down-128959/#post1253818

Unless you have done the changes using software, like the excellent free software available from this site http://www.blueheronkayaks.com/kayak/index.html , that calculates stability and drag, you will almost certainly get an unpleasant shock when you put the kayak on the water.

I realize that you could change only the length rather than all the dimensions, but the fact remains that the effect of altering a design as much as you suggest is not what you might think.