By far, this is one of the MOST contentious topics in sail trim. Are you in the “Speed Wrinkle” camp or the “Smooth is fast” Camp. How much twist is too much twist? Should you travel the boom up above center-line?

So, let’s start with some “universally agreed upon” (haha) notions and work from there. And for simplicity’s sake, let’s say were going to talk about a sloop rig boat using a main and a jib (no more than 120% overlap).

1. Start with the wind abeam (straight from the side) and the sails flogging.
2. Slowly trim both the main and the jib in until they have stopped flogging and have just achieved a shape that is smooth and rounded.
3. You should see both telltales on the jib (assuming you have them) fluttering back together and, on the leach of the main, you should see at least the lower telltales streaming back.

This is “good trim” in the most basic of definitions.

Now this is great, but even if you are sailing the same direction for hours on end, the wind may shift. It is important to watch for the flow of the telltales to give you an indication of how the wind is flowing on the sail. 

In the illustration below, courtesy of Sea Scout Ship 502, you can see the 3 different scenarios for Jib Trim.

1. On the left, this is what we are looking for. Both telltales streaming back together.
2. In the middle, the sail is trimmed too loosely and the inside telltale is starved for air. Trim the sail in or point the boat more away from the wind.
3. On the right, the sail is trimmed to tight and the outside telltale is starved for air. Ease the sail out or point the boat higher into the wind.

Finally, remember, when in doubt, let it out. Most new sailors, and about half of experienced sailors sail over-trimmed, or with the sails pulled in too tight.

After you get these concepts, we can talk about sail SHAPE which is a lot more alchemy and opinion.