So what is this VMC business? We’ll stick to a simple upwind case for now, but remember that VMC is just a way to measure your speed toward a certain goal. A relatively common question on SOL is to ask why some boats are sailing “upwind” a little bit below the proper tacking angles as indicated by VMG. The explanation usually offered is that the boats that are footing aren’t trying to get to windward, but are sailing as fast as possible toward their goal, which is usually the next shift. VMC might enter the discussion. Occasionally, someone with years of race experience chips in to explain that there’s no such thing as VMC on “pure upwind” legs, or that you never see the pro boats footing. It is easy to understand the following:
Black is sailing VMG and has no trouble pulling away from Red, who is footing, or Green pinching. Upwind on most boats, you steer to your telltales and tack on headers, and at first glance this is what is guaranteed fastest. You can see the loss in distance that either boat not on the proper tacking angle is incurring just by looking at the polar. Max VMG is the furthest point forward… anything else is slow to windward. Though the footing boat might feel fast, it’s common knowledge that he’ll lose out in the end.
The problem is that “pure upwind” sailing doesn’t exist in the real world and rarely online. Once you introduce any variation in the wind the decision is no longer as simple as VMG. Why don’t we see the big boats footing? Well… you do if you know what to look for. Often the difference between VMG and VMC angles is a degree or so. That’s sheets cracked a quarter inch, or letting the tails of the leeward ticklers lift just a bit on a puff… Where it really comes into play is offshore, over long time periods. Boats with wide polars like those of a typical multihull benefit more.
Here is the same diagram we used for VMG, but with VMC relative to a 0° target indicated. The target is not usually going to be the mark. This scenario is set up to show a really clear-cut case.
The red track is our VMG boat, tacking on the same TWA 37° as in the no-shift example. Black is sailing angles of 50° TWA, a pretty healthy ways below Red’s track. Black is clearly sailing faster, but he’s losing ground to windward, just like Red did in the first example. How can he get away with it?
Fast forward to the all-important shift. Not only is black going to get to it first, but look at what happens to the “distance to windward” measurement once both boats are in the new wind. Black is instantly both further upwind (dramatically so, in this example) but also further up-course. He happily tacks over onto port and can lay the mark on the same fast 50° angle while Red can just barely lay it at all. Red is going to get rolled to windward – Black seemingly came out of nowhere.
The red target lines and “ladder rung” maximum VMC lines allow you to visualise what is happening. In these 30 degree shifts, maximum VMC is around a knot faster toward the average wind than VMG. The boat more than makes up for the extra distance sailed by sailing with almost two knots more boatspeed. Sailing VMC upwind usually, but not always takes the form of footing on the favoured tack, but if you were stuck for some reason, you can see from the polars that best VMC for the bad tack is a (some would say crazy) 27° or so – sailing slowly but not getting swept further off track…