Functions 2

Functions
Page 2

Real world sailing

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Targets
Polar curves predict how fast the boat will go in a specified set of true wind conditions. Given a true wind angle and speed, they predict the boatspeed to be expected. A typical polar curve might look like the figure. (More on Polars)

Target boatspeed & angle

Targets are the second most important performance function after wind direction. They provide a consistent number that coordinates helmsman and trimmer to get to optimum performance quickly.

A target is the boatspeed and angle at the maximum (or minimum) Vmg. Since polar speed changes with true wind angle, it is not a stable function when beating or running. Targets on the other hand, only change with true wind speed.

In the figure, at 10 knots true, the fastest speed to weather is 6.25 knots at 44°.

Why not use Vmg to find the optimum upwind point?

The reason is that boatspeed response is slow while true wind angle is fast.

If you head up, Vmg increases immediately but the boat takes a while to slow down.
Result: Vmg goes up then gradually down.
If you head off, Vmg decreases immediately but the boat takes a while to speed up.
Result: Vmg goes down then gradually up.

To a helmsman, Vmg always appears to recommend heading up. Targets give a stable number that the crew adjusts boatspeed to, and there is no true wind angle involved.

Another problem with Vmg is wind shear. For every degree the wind twists, Vmg changes about 0.15 knots tack to tack, much more than boatspeed would. This often makes the previous tack's Vmg unusable on the new tack.

Targets require True Wind, and a polar file installed in the T1 processor (or in the 037 polar module attached to the 001 processor), or on an onboard computer outputting to the instrument system.

Wally

is a modulation of target speed that takes advantage of small or short wind shifts. A full description is included in Sailing in Shifty Wind.

Wally requires True Wind and a polar file. For the T1 processor revision 20.05 and later, Wally is built in. For older revisions and the 001 processor, Wally is output by BIF #4a of OckamSoft.