How close must a boat be to a set of measurements on a blueprint to be considered a copyright infringement or to be considered building a second boat without paying the necessary fee?

I'm considering building a second stitch and glue kayak, but want to alter the plans somewhat. I plan to add 12" to the length, reduce the beam 3", increase the depth 1", move the cockpit back 1", change the cockpit coaming shape, and add a little more fullness in the bow. Is this far enough removed from the original plans to be considered my own?

John