Hi Pol,
I am sure you will be able to lengthen the mast 40cm, as you describe. If your mast is off the boat now, and you are keen on doing some woodwork, then go ahead.
However, if the mast is still on the boat and you have no other reasons for unstepping it, there is another possibility - shortening the lowest panel by 30-40cm. This is not risky, as there is no scissors involved. You will only lose about two square-meters, and these can easily be brought back in use, later. All you need is a yardstick, a thin felt-tip pen plus needle and some whipping twine. If the sail is still rigged on the boat, the job can be done within two calm hours, without taking the bundle down:
· Hoist the sail to get access to the lower edge of the lowest panel.
· Open the lashings holding the foot to the boom.
· Measure and mark up a new foot, 40(?)cm above the original (keep that round).
· Stitch on a simple twine hoop at the new clew and tack position (as on photo below)
· Lower the sail a little to let you lash the new tack and clew to the boom.
· Roll up the excess sailcloth and secure it with twine stops at , say, every foot.
· Secure this rolled up bundle to the boom with similar stops.
Job done.
If you later for some reason are to take the mast down, and then want to lengthen it, the lowest panel can be brought back to full size in just two minutes.
Just a thought.
Arne