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
might look like the figure. (More on
Target boatspeed & angle
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.
are the second most important performance function after wind
direction. They provide a consistent number that coordinates helmsman
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
hand, only change with true wind speed.
In the figure, at 10 knots true, the fastest speed to weather is 6.25
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
the boat takes a while to
Result: Vmg goes up then gradually down.
- If you head off, Vmg decreases
the boat takes a while to speed up.
Result: Vmg goes down then gradually up.
Another problem with Vmg is wind
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.
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.