I'm not all that familiar with the ball setup on the P18 - but if it's threaded like the Nacra mast base system, chances are the ball is turning on the threads of the mast post. Be sure that your ball is very tight on the post. Secondly, you can seat the ball in the base of your mast. With the mast down, apply mold release to a mast ball (hair spray might work but you need to test it first), mix up some Marine Tex, shape a thick round waffer, put it in the mast base, put in the mast ball firmly, and tape in place. After an hour, remove the ball and you should have a perfectly fit socket in the base of the mast. You might also want to invest in a new mast ball before you do this so you get a good shape.