Is the head
of the sail all the way to the top of the mast
? You have to loosen the downhaul and really crank on the halyard
. There should be nothing in the connection between the halyard
and the sail head
except a knot
. Has someone added a shackle there? There should be a line or a mark on the mast
about the level of the boom gooseneck. You should be able to raise the boom above that line then pull the gooseneck down towards the line with the downhaul. If the head of the sail is too low both the leach tension and the luff tension will be incorrect. Being able to control the luff tension is as important as being able to control the leach tension.
If you loosen the downhaul can you slide the boom up and down? I've seen more that one boat where some heathen has screwed the gooseneck to the mast.