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Performance Functions

The Ockam system performance functions provide the information you need to get the most out of your boat.


is the most fundamental input to the Ockam system, and is the first thing the crew looks at when trying to make the boat go faster. It is also used by many many other functions.

Why you should use boatspeed sensors instead of GPS for boatspeed.

10KnotPolarTarget Speed

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.

The 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.

Current Set & Drift

The Ockam system computes current by comparing COG and SOG with Boatspeed, Heading, and Leeway. Calculated current may be overriden by the C= command. Current is used in laylines and opposite tack.

Current requires boatspeed, heading and COG/SOG.


is the component of boatspeed in the direction of the true wind. When beating (or running) this is the number to maximize (or minimize) for best performance.

Because of the dynamics of ship’s handling, Vmg is not a usable helming function (see Targets for an explanation).

A more practical way to sail optimum Vmg is to use target speed.

Vmg requires boatspeed and true wind angle.


is like Vmg except in the direction of the current rhumbline. Sailing maximum Vmc gets you toward the mark faster than sailing directly there, but at the cost of going off rhumbline. On a long course, the wind will shift and allow you to tack and consolidate back to the rhumbline.

On short courses, this may not be possible. However, you can still sail Vmc on an upwind leg by doing the wally.

Vmc requires boatspeed, wind direction and waypoint range & bearing.

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