It means there is not a lot of curvature in the middle of the boat
compared to the ends.
Designers think of a boat in cross sections of (equal) length so the
mid-section is the one amidships. The ends of the sections are usually
called stations in case you ever come across that term.

What I imagine you are going to get is a shift aft in the centre of
balance of the boat due to the added weight of the extension. The boat
will float a little differently in the water which will unbalance the
sail and daggerboard(centreboard?). In a boat that small you can
counteract any directional instability with the rudder or by shifting
your weight to compensate. I doubt it would be a problem.


Ted W Lee wrote:
Thanks for your response Matt. Does "parallel mid-section" refer to the
flat mid-section of the bottom of a ship?
I have plans for a v-bottom boat (I like the lines of this particular
boat) which I would like to lengthen. It would be fibre glassed plywood.
Thanks, Ted.