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Ockam U Excerpt

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.


Full PolarPolars describe a boat’s performance under given true wind conditions.

In the figure, the curved lines represent how fast a boat can sail at a given true wind speed for any true wind angle. The squares mark the fastest point of sail upwind and downwind. These points are the so-called target speeds, the maximum Vmg (speed made good) points. An arrow has been drawn at the port tack upwind sailing point, showing that the boat can be expected to make 8.3 knots at 35° off the true wind. The boatspeed at that point is called the Upwind Target Speed.

Polars can be used to determine the best point of sail not only up and down wind, but in any direction. In addition, the shape of the polar provides further information. For example, the upwind sailing point shown above is the maximum Vmg point upwind. However, the curve is fairly flat there, indicating that one could sail 8.1 knots at 30° or 8.4 knots at 38° without sacrificing much Vmg.

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.

Shift gainIn the figure, A and B start off even because they are on the same ladder rung. Then the wind shifts 10° to the left, and A is suddenly 25% of the lateral separation ahead of B, and didn’t have to sand a single bottom to do it.

Taking advantage of these windfall gains requires knowledge of the Wind Direction. Those with local knowledge get their info by knowing what the wind does over time and interaction of the wind with the land. Observant types also take advantage of remote indicators of wind direction like wave patterns, smokestacks, flags and cows.

Remote indicators are helpful, but suffer from two problems; low resolution and wind indication at remote locations. True wind instruments report wind direction to 1° resolution and locally. They also do this offshore where there aren’t any cows.

So how do you know the wind is going to shift left instead of right? By playing the percentages. By observing the wind direction readout, you get a feeling that the average is 225° 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° 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.

Everybody knows about tacking on the header. However, there is a downside to tacking; your Distance Lost to Weather (DLw) in the tack. If the shift is too small or short lived, you will not make back your DLw before it peters out.  So tacking is a judgment call, based on how well you tack, and the wind shift statistics (see Break-even Tacking). When you can’t or shouldn’t tack, you can still gain from wind shifts by Wallying.

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 know exactly how much to pinch and foot

The meaning of Wally

SplainWallyWhen 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.

In the figure, the wind is shifted 10° right from average, moving the Vmg points. This means that the optimum Vmg point has a faster than standard Target on the lifted tack and lower than standard Target on the headed tack. This change actually makes your Vmg HIGHER than standard, because the curve rotates up relative to the Vmg point on the rotated polar. Therefore, if you Wally (sail the faster target speed when lifted and the slower target when headed), two things happen:

  • You go upwind faster than you would if you just sailed to your basic target speed.
  • Your separation automatically moves in the right direction to reduce damage from boats on the favored side, and increase gain on boats on the unfavored side (see Lateral Separation).

The Wally is rational “Footing to the headers”

How to Wally AND tack

Let’s say you’re sailing in an oscillating northwest wind with shifts lasting long enough to allow tacking.

  1. As the wind starts to lift you, you begin to foot. The greater the lift, the more the foot.
  2. When the lift hits maximum, you’re going your maximum foot.
  3. As the lift dies down, you begin to return to your target speed.
  4. When the wind hits the mean, you’re at target speed, and you tack onto the new favored board.
  5. Repeat.

There are two advantages to doing this:

  1. You’re always going faster upwind than you would if you just sailed your targets.
  2. Your lateral separation automatically changes so you gain more on those on the unfavored side and loose less to those on the favored side.


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 approximately 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, download our sample polar in WKS format.


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