Ockam UOckam U is a seminar that teaches you how to use any true-wind instruments to win races. Some of the principles can also be applied to any kind of sailing, including dinghys. The course will teach you how to:
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What follows is a condensed excerpt from the Ockam U manual covering true wind, polars and the Wally.
True Wind and Wind Direction
True wind is the wind that passes over a boat as if it were not moving. When the boat moves, the wind you feel is "Apparent", a combination of the true wind plus the speed of the boat. Sails are trimmed to the Apparent wind, but boat performance is best characterized with reference to the true wind.
Wind direction is the magnetic bearing of the true wind. This is the wind reference used on land (wind is NNW at 20). This number is the combination of True Wind Angle plus Heading plus Leeway, and is the most important output of true-wind instruments as we will see shortly.Polars
Taking advantage of Changes in Wind Direction
Winning a boat race doesn't depend only on speed. Cunning plays an important role too. When sailing upwind, if the wind doesn't shift and two boats have the same performance, they can be said to be even when they are on the same "ladder rung", a line perpendicular to the true wind.
So how do you know the wind is going to shift left instead of right? By playing the percentages. By observing the readout, you get a feeling that the average wind direction is, say 225 degrees and gain a sense of how shifty the breeze is. Then, because wind always oscillates (more or less depending on the weather pattern), you will know that when the gauge reads 230, the wind is 5 degrees right, and you can expect it to go left in a while.
The wind direction readout does NOT replace local knowledge, it enhances it. People who know that the sea breeze clocks right during the day will factor that in, and thereby know that that 230 reading maybe isn't that far right, and the wind might shift further right. However, quantifying wind direction sharpens their knowledge and makes them more competitive than they would be without it.
Lateral separation can be manipulated by pinching in the headers and footing in the lifts. If you don't overdo it, Vmg isn't affected much, but lateral separation can be changed a lot. If you're headed and pinch, your separation is reduced on the boats to windward and simultaneously increased on those to leeward. This reduces the damage you will suffer relative to the windward boats when (if) the wind then lifts you. At the same time, your increased separation from those to leeward increases your gain when the lift comes.
With polars, you can figure out exactly how much to pinch and foot
The meaning of Wally
When the wind will oscillate at least 1 cycle before you get to the windward (or leeward) mark, then your objective is to make the best speed up (down) the AVERAGE wind direction, not the present wind direction. This change in perspective moves the little squares on the polar depending on where the present wind is relative to the average wind.
Why not just tack? There may be tactical or strategic considerations, but more importantly, tacking costs distance to weather. If the shift is too quick or too small, you won't make up for the loss. Wallying gains distance when you can't or shouldn't tack.
If you don't have a polar, there is a handy rule of thumb you can use to Wally. Sail at a true wind angle modified by half the angle of the shift. In other words, if you get a 10 degree lift, crack off 5 degrees (ie only come up 5 degrees), and vice versa.How to Wally AND tack
Let's say you're sailing in an oscillating northwest wind with shifts lasting long enough to allow tacking.
How important is the wally? It depends on the shape of your polar. If your upwind polar is pointy, speed gain and lateral separation are hard to achieve. On the other hand, if your polar is broad, there is little penalty for being a few degrees off optimum target angle, and lateral separation can be adjusted with little sacrifice in Vmg.
Catamarans have very broad polars, and two upwind sailing regimes - flying a hull by sailing off, or plunking both in the water and sailing narrow. Presuming the two regimes have equal Vmg, knowing the state of the wind shift and the location of your competition will help decide which regime to pick. Even if regime change is not an option, adjusting lateral separation is, because of the small penalty for footing or pinching a bit.
If you're interested in playing with this idea on your computer, go to the OS4 page and download the package, which includes a polar in WKS format.