Wind shifts -- How to calculate gain and loss upwind

If there's one generalization we can make about the wind, it's that it is always shifting, even when it seems steady. And whenever the wind shifts, some boats gain and others lose.

For upwind legs, there are three basic rules of thumb:

• 1) The boat that's closer to the new wind direction will gain in a shift

• 2) The amount of gain is roughly proportional to the size of the shift

• 3) The amount of gain is proportional to the distance between boats

Once you understand these principles, we can move on to calculate more precise gains or losses for any windshift. This calculation depends on the magnitude of the windshift, the tacking angles of the boats, and the relative positions of the boats.

It is easiest to consider what happens when two boats are on the same ladder rung and the wind shifts (see Situation I below). However, two boats are seldom in this position. As you can see from the chart and Situations II, III and IV, the amount of gain and loss varies according to whether the boats are abeam, in front of or upwind of each other.

If the number of variables seems overwhelming, let me offer one simple guideline that works in most situations: For every 5 degrees of windshift, you will gain or lose approximately 10 percent of the distance between you and other boats.

In future issues we'll talk about the tactical implications of separation, or "leverage."

I. BOATS EVEN

In a 15-degree back, the boat on the left gains 37% of the lateral separation (x) between the 2 boats (see chart below). If the boats started out 6 boatlengths apart, then the left boat would gain just over 2 lengths in the shift.
GAIN AND LOSS CHART
The numbers below show the rough amount of sailing distance that is lost or gained in wind shifts, expressed as a percentage of the separation (x) between two boats. All tacking angles are assumed to be 90 degrees, except for the second example listed under I, which is 80 degrees. Situations II, III and IV are diagrammed below.

SHIFT	          5     10      15	 20	 25

I. BOATS EVEN 
Tack angle 90	 12%	25%	37%	49%	60%
Tack angle 80	 11%	23%	34%	45%	55%

II. ABEAM
Lift/A Gains  	  8%	16%	22%	28%	33%
Header/B Gains	  9%	19%	29%	40%	52%	

III. BOW TO STERN 
Lift/B Gains  	  9%	19%	29%	40%	52%
Header/A Gains	  8%	16%	22%	28%	33%	

IV. UPWIND
B Gains	          1%	 2%	 4%	 6%     10%	

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