Gain from wind shifts without tacking
The Wally is a technique for gaining advantage of wind shifts when you can’t or shouldn’t tack. When should you not tack? See Break-even Tacking.
The Wally was invented in Fremantle where the wind shifts were regular, but too short to exploit (there were also significant tactical reasons not to tack).
The basic idea is that when the wind shifts back and forth at least once during a leg, you should change your objective to maximizing Vmg up or down the average wind instead of the current wind. This change of perspective always increases your speed to weather or downwind relative to maximizing Vmg on the current wind. And it also pumps your lateral separation in such a way that wind shifts always help you and hurt them. See Lateral Separation.
Since there is no cost involved, you should always Wally when you are not tacking
Here you see a polar rotated to align with the wind shift. The standard target points also rotate but there are new target points maximizing Vmg up the average wind. On the lifted tack wally targets are faster than target speed, and on the headed tack they are slower. In both cases you will notice that your Vmg (up the average wind) is better than standard targets. Thus, no matter whether you are in a lift or header, Wallying increases your Vmg relative to standard target speed.
The performance component of wallying is typically worth about 5 seconds per mile.
If you are so inclined, the method for calculating Wallys is discussed here.
However, there is also another reward to wallying; pumping your lateral separation. The gain by manipulating lateral separation can be as much as 10% of lateral separation.
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