Shalom (pic below), who has been making a quick job of learning to sail well had a couple of problems in the recent group sail around the Ta'al Lake Volcano island.
In the light winds it was about a six hour sail. Brian wrote on the Goose Facebook Group.
See the crease here Michael Storer?
It also creases the other way, diagonal down towards the downhaul. On the gybe from port to starboard the spar turned and folded making the sail triangular. I was able to correct this by reversing the gybe twice.
A couple of boats have done this - flipped the yard so that it ends up on the wrong side of the boat. See your sail is full of air in the photo, but you still have the crease in the direction showing inadequate downhaul.
From this angle is it also hard to tell where the corners of the sail are relative to the mast. If the sail has been set up too far aft so the sail corners are close to the mast, this might happen too. Let me check some of the other photos which might be at a better angle to see if the sail is a bit far back.
This next pic shows a lot of twist in a very light wind. The boom is lifted really high which shouldn't happen in such light winds.
That shows a too loose downhaul or one that has become loosened. The corner of the sail looks very close to the mast. Let's look at a photo of your setup on the beach.
On the beach your downhaul is already way too loose. Looking very floppy here. You can see the curve in the leach (back edge of the sail) - that is twist. Even though by the water you can see the wind is almost zero.
A quick check is to put your hand on the boom at the front end of the sail and push it down. If the crease is from a too loose downhaul it will disappear with a bit of downward hand pressure.
Compare with my boat (on the left) setup on shore on a moderate breeze day. The crease runs from near the downhaul to the top of the sail. I have probably overdone it in this situation, but if this crease disappears in the moderate wind of the day, then it is OK.
Enough downhaul so you don't get the bad type of crease in the sail. One in the other direction is OK if not too severe. More wind means more downhaul. Halyard loop on yard about halfway along the yard or very slightly forward. About 400mm of sail in front of the mast at boom level (for the 89sq ft sail - a bit more for the 105 sq ft one). Ease the boom out to 90 degrees before the gybe to reduce the twist as the boom crosses the boat and stops on the other side - a tighter mainsheet twists the sail more which may allow its fron. Certainly, the downhaul is the most critical line on the boat for both performance and to prevent the yard flipping to the wrong side of the mast from happening.
Here, Shalom is making good progress, but maybe he could ease the sail further out without it starting to luff.
A link to the original post on this topic.