Nick,

Sounds like diamond tension is way too loose and the mast is just flopping around. <img src="http://www.catsailor.com/forums/images/graemlins/shocked.gif" alt="" /> This would probably affect the jib luff tension and therefore the whole jib effectiveness particularly if there was any waves or slop. Try 30 kg tension load on the diamond wires as a starting point and probably work up from there. Someone must have tensiometer at the Yacht Club.

Once you have set your diamond wire tension then probably look at increasing your stay tension too.

Regards,

Peter