Tuesday, July 6, 2010

VMG Primer

A key to this year's Mac is to maximum VMG (Velocity Made Good). This means finding the fastest combination of boat speed and angle to the wind. We'll work on this during our next Beer Can race and during Saturday's practice sail.

This article, edited, from http://www.endeavourowners.com/dscsn/handling/perform.html, is a good description of VMG.

What is VMG?VMG is velocity-made-good, sometimes referred to as speed-made-good. Here is a simple example of VMG. Your boat is going at five knots, according to the knotmeter against a current of two knots. Your boat's VMG, or speed over the ground, is three knots. You turn the boat around and your knotmeter still reads five knots, but the current is now pushing you along at an extra two knots, so your VMG is seven knots.

Sail a mile or more to leeward. Set the GPS GOTO function to that mark. Scroll to the GPS screen that reads VMG. Now start beating toward the mark. Try various course angles, sheet tensions, and genoa lead and traveler settings, adjusting your sail trim to suit.

Dedicate a crewmember to monitoring the GPS, recording the course, sail trim, and wind speed, and the VMG resulting from each change. You will soon see which combination of course and sail trim results in your boat's best speed made good to weather in that particular wind/wave combination. Your speed potential will probably vary quite a bit with different wind/wave combinations and course angles. Does your boat do better while pinching or footing? Within a beat, pinching is sailing a little closer to the wind, while footing is sailing a little less close to the wind?

Do the same thing downwind, to find the best gybing angles for your boat and sail combination. Get a mile or two directly upwind of a GPS waypointed mark. Start sailing directly to that mark with your usual downwind sail combination. Note your speed, then come up 10 degrees and check the GPS for the change in VMG.

Even though you are sailing slightly away from the mark, the chances are your VMG has improved, meaning that you are moving more quickly toward your destination, even though you are traveling more distance and will have to gybe to make the mark. Come up some more and again note the change in VMG. Keep experimenting with VMG and make notes of what angles work best.

Reprinted from the America Online Sailing Forum Newsletter
GSTDPeterO is Peter O. Allen, Sr., a retired association executive from Rochester, New York. He has sailed and raced dinghies and various racer/cruisers for more than 30 years. He is a US Sailing Certified Club Race Officer. He and his wife currently sail Canto, a '68 Pearson Wanderer, as well as a Laser. He has been designated as the principal race officer for the Sunfish Pan Am Games Trials, to be held in Rochester July 9-11.

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