You sail a keelboat and can’t quite put a finger on how much heel you should allow. Too much, and the boat starts going sideways. Too little, and it just lacks drive. You got advice from other sailors that goes something like this: “It’s easy — once you feel good, just look at the inclinometer and this is the optimal heel for your boat.”
We’re sorry to burst your bubble, but this advice is not that great.
The art of sailing fast comes down to solving an optimisation problem: how to both increase drive and reduce drag*. The problem is that heel is not an indicator of achieving either of those.
Aerodynamically, heel is correlated to the heeling moment generated by the sails. Nonetheless a high heeling moment does not equate to a great driving force: if you sheet all in on a beam reach, the boat heels tremendously, but has near zero drive. So heel is not an indication of the driving force.
Hydrodynamically, heel is correlated to drag of the hull. All keelboat hulls are optimised to have minimum drag at some degree of heel — they are not meant so be sailed flat. Yet again the situation is not clear-cut, because the foils (and also the sails) lose efficiency as the heel increases.
Heel is a result of other variables that go into sailing: wind, waves, angle to both of those, your boat, crew placement and sail trim. Various settings yield the same heel, but the relation is one way only. The settings that give us the same heel most likely don’t produce the same performance.
Our end goal is to increase speed, so if there is no direct linkage between heel and speed, why even bother focusing on heel? Absence of direct linkage also means that there isn’t a single optimal angle of heel for all weather conditions.
Therefore the angle of heel should only be seen as a part of feedback on the boat's operation, not as a direct measure of its performance.
As in life, it makes way more sense to focus on the things that you can change: the inputs. Since we can’t control the weather and swap our boat at every instance, it leaves us with crew placement, steering angle and sail trim.
*The physics of this statement is a bit weird, because at a constant velocity the driving force exactly equals the drag force. But, if drive happens to be larger than drag, the boat accelerates. Yet, to sail as fast as possible, we need to accelerate as much as possible, so the statement still makes sense.
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