There are two ways you can lengthen a kayak. You could increase the distance between the forms by a fixed amount to get the length you want.

There is a better way which gives you more volume for the same increase in length. If you want more load carrying capacity rather than just more length, then this method is the one you want.

Leave the end stems and the position of the first and second forms as they are. Increase the gap to the third form by a small amount and the gap to the next form a bit more.

The biggest increase in the gap between forms is in the middle where an increase in length gives you more volume than you would get with the same increase in length near the ends.

In "The Strip Built Kayak", Nick describes a similar method for scaling down a design.

I used that method to build myself a larger version of a kayak I had designed for my wife. It worked perfectly.