Polar Diagrams – VMC

18 10 2009

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.

VMCeqVMG

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.

VMCexample1

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.

VMCexample1rotated

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.


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5 responses

18 10 2009
DeBilbaoPues

I am a SOLsailer with absolutely no experience in real sailing and I am trying to learn this art.
I wish to say to yoy a big THANK YOU. It’s a pleasure for me to read the (very clear) explanations you wrote. I am looking forward for more and more articles like these.

Forgive my terrible english. It’s not my mother language.

Best regards,
Iñaki (DeBilbaoPues)

19 10 2009
mk

Very good simple explaination!
Looking forward to new articles!
Thanks
/mk

20 10 2009
Arthur

Hi 76,
Great article!!! Clear explanation. Being an experienced sailor, although not off-shore, I noticed that I (sort of) already did the things you explain here. I still (virtually) sail on the feeling, or I use my own simple spreadsheet to calculate my VMC. I just need to enter CNM, HDG and BS and the output is given in numbers, in your example it’s made visible: A picture tells more than a 1000 words… (Or 1 number)

Anyway, I know lots of sailors will benefit from this!

Thnx,
Arthur

21 03 2010
Routing for non-routers « Vättern Shorthanded Sailing Club

[…] Publicerat av vatternsailing den mars 21, 2010 Här är en länk till en sida som beskriver en del teori som är användbart när man routar, SO76TP. […]

30 03 2011
Manually Compute the best VMC heading « Sailonline.org Tools

[…] might also want to read 76Trombones’ excellent post on VMC, which has much prettier pictures than […]

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