This standpoint is a golden standard in some teams. If the leech falls a bit leeward, it might be accepted. But if it hooks a notch to windward, it is a certain no-go.
Why do they think that? At first glance it seems obvious: if the air exits the sail directly parallel to the centerline, it must give us the maximum amount of forward thrust. Sadly — or happily, depending on your preferences — the physics of sailing is not always straightforward.
What if, for example, by hooking the leech, we get the boat to heel a bit and it happens to decrease hydrodynamic resistance. The aerodynamic efficiency of the sail may be reduced, but the net effect can be positive.
As another example: opening the leech prevents stalling, thus allowing a greater flow through the sail. Not to mention using twist in heavy air to limit sail forces.
These kinds of rules of thumb should be taken with a grain of salt, because more often than not they tend to limit your creativity. Why not just try and see which trim works and which doesn’t?
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