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What is Vmg

Maximizing Vmg helps you win

What is VmgVmg strands for Velocity Made Good. It is the component of your speed in the direction of the true wind. This is the number that you want to maximize to get to windward as fast as you can. There is also a downwind Vmg (best speed away from the true wind).

Obviously, heading straight into the wind means 0 boatspeed (so, Vmg=0). Heading on a beam reach also means Vmg=0, because, although you have plenty of boatspeed, you’re not pointing into the wind (because Cos(90)=0). The correct angle to sail into the wind DOES exist, but there are no signs of it; nothing feels different between pinching too much to footing too much.

This is where the polar comes into play. This is a graph of boatspeed (as a radius) versus the true wind angle at which that boatspeed happens.

It is called a Polar because this method of plotting the radius vs. angle is called a ‘polar plot’. Polar plotting automatically calculates the effect of the Cosine, so the must ‘up’ point of the curve is your best Vmg to weather (6.1 knots at an angle of 41º), while the most ‘down’ point is your best Vmg downwind (5.85 knots at an angle of 149º).

Best Vmg

This image is for a particular wind speed. A full polar includes curves curves for many wind speeds. Also notice the ‘cusp’ in the polar where the spinnaker and jib performance cross.

A simple tool: The target speed list

Worlds Most Useful InstrumentThose points of boatspeed at maximum up and downwind Vmg are called Target Speeds. These numbers can be sealed up into a convenient table and pasted on the bulkhead.

The Vmg dynamics problem

Weakness of VmgNowadays, instruments can display Vmg in real time. Why not steer the boat so as to maximize the readout? The problem lies in the dynamics. The boat turns fast (therefore the Cosine term changes fast), but the boatspeed changes slow. If you head up, Vmg goes up at first. But then the boat slows down, and Vmg goes down. So, you head down. But Vmg goes down before increasing.
The result is that the dynamics of steering by Vmg always tells you to head up, not down because of the different responses of angle vs. boatspeed.
In other words, use polar numbers (target speeds) instead of a Vmg display.

Vmg is the key metric for getting to windward or leeward the fastest. However, looking directly at a Vmg readout will cause you pinch. That is why everybody uses target boatspeed instead.

 

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