Polar Diagrams – VMC

If VMG is just the windward component of speed, what is VMC and why is it useful? While VMG measures speed into the wind, VMC measures it toward an arbitrary target. The easiest way to define the target angle when using a polar diagram is to translate it into a TWA.

If we assign a target TWA of 0° or 180°, we can find the maximum speeds up or down-wind. These angles are the same as maximum VMG. On the sample polar above, one windspeed has been isolated for clarity. The red arrows show the target directions, and a line at right angles to it helps to show the point on the curve furthest forward. For the upwind case, the tangent point is ~37°.

Things get more interesting when you pick any other target directions. We’ll get into how you pick your targets in a later entry.

In this example, we’ve picked a target angle of 30°, indicated by the solid red arrow. Remember that this is measured from the wind direction or TWD. Being able to switch from compass angles to polar angles is a good skill for both real-life and virtual sailing. This example could apply to a situation where the wind (TWD) was from 25° and the CC for our target was at 55°… the important thing is the angle of the target relative to the wind.

The “ladder rung” for the 30° target is tangent to the polar at the maximum VMC – in this case a TWA of 50° shown by the solid black arrow. Just like sailing upwind, choosing the maximum VMC angle will move you toward the target the fastest. You can see in this example, it will also move you off track, and you’ll have to figure out a way to get back to windward if you are trying to lay a waypoint or mark. Later, we’ll look at how to use an expected windshift to accomplish this.

Notice that there is a second maximum for the other (non-favoured) tack, shown by the dotted red line and black arrow. In real-life racing there are often tactical reasons to sail the “wrong” tack – controlling other boats, or maintaining position at a start are common ones. Sailing “high” on the non-favoured tack is also part of the technique of “Wallying” in puffs and micro-shifts, where you sail VMC on one tack because the timescale of the changes in wind is too short to justify tacking on the headed wind. Sailing the slower tack happens less often in virtual sailing, but you can sometime get caught the wrong side of an update or stuck up against a shoreline.

Rotating the polar is a good trick to help visualise the angles and maximum values a bit better. If you are using a paper chart, laying the polar on top gives you an instant picture of how the fast angles relate to your  course and any marks, shorelines etc that might be in play. We’ll show how to do the same thing in SOL as well, where you have the added benefit of being able to instantly align the polar to the correct TWD using the onscreen wind arrows!

For the last few races, I’ve been posting a polar suitable for on-screen use in the “SailTalk Only” chat area of sailonline.org. I’ve now moved the existing ones to this blog (under SO76TP Polars tab) and will continue to add new boats as they are needed. The design of the polar is still a bit of a work-in-progress – suggestions on how to make them more useful are very welcome.