VMG Sailing in shifts

19 10 2009

At one point in our sailing careers we were probably presented with a picture, or at least an explanation that looks a bit like the one below. A boat tacking upwind works it’s way back and forth on 45° angles, staying within the laylines shown by the solid blue lines. A common recommendation is often that you stay inside a pair of secondary boundaries (shown by the dashed blue lines) – the intention being that you won’t be likely to overstand the mark if or when the wind shifts. Being on the layline too early is rarely a good idea in RL racing. Not only will the shifts get you, but you are showing all of your cards early, and have limited tactical control over the fleet. In virtual sailing, we do tend to cut it pretty close on the laylines once we’re inside the weather update window because we have 100% foreknowledge of any relevant upcoming shifts.


Of course, we’ve moved on from 45°, and know how to adjust our TWA for maximum VMG! In a constant wind situation like the one shown above, any course you lay in on the VMG tacking angles will be equal distance, and equally fast. If we know that nothing will change, we can even head straight out to the layline so that we only have to tack once. I used to race (using word loosely) with a cruising boat that figured that any race with one tack was a good race…

We’ll need to be careful though… the most common misunderstandings of  upwind sailing are based in these “common knowledge” concepts.

Since we have the polar in the diagram, It’s a good opportunity to look at how it is showing us what we already know, so we’ve got some more arrows in the quiver later on. In a constant wind like this, our goal is to maximise speed directly into the wind and toward our next mark. So long as the waypoint is inside our laylines, we’ll tack on the VMG angles to get there – a situation that gets the nickname “pure upwind sailing” on SOL. Our polar diagram is lined up with the wind arrows, and our target is set to 0°. The horizontal red line is showing us the maximum points for VMG, and in this case you can see that both tacks are equal. You can choose either one and everything will work out the same. Armed with this knowledge, the next step for most sailors is to start looking around to see if there is more wind on one side of the course or the other.

What about those shifts?

I can honestly say that I’ve never sailed in wind like the sailing-school whiteboard example. There is always some variation, even if it’s a degree or two, or an extra tenth knot of wind. Around my home waters it’s more likely to be 40 degrees and 3 knots (in a 4 knot average wind mind you…). Oscillating winds present a huge opportunity to edge out other boats.

Most racing sailors know to “tack on the headers”. Figuring out how to do that round-the-buoys when you have only realtime data at your disposal is an art in itself. You need to keep track of what your averages and highest and lowest headings are, as well as trying to identify patterns in timing, while sailing as fast as possible. If you tack when you are first headed, you are usually tacking off of your lift, and if you wait too late you’ve been sailing the wrong tack for too long. You need to try to keep options open in case the wind changes more than expected, figure out if the shift is going to reverse of if it might be persistent and change tactics at the start and as you approach the mark! Most boats take years learning to get it right, and it takes hard work to play the shifts consistently.

Virtual sailors learning how some of this works have a big advantage in that we can see our weather at least a few hours out. This lets you concentrate on figuring out the best angles without the burden of trying to outguess the competition on when the next shift will occur. Learn to get it right here and you’ll have a big leg up when it comes to applying the same ideas on the water. If you are coming the other direction, one caution is that online sailors with RL race experience I think tend to underestimate the importance of playing the shifts perfectly – particularly if they race dinghies or sportboats, where boathandling is so important that crews don’t have time to spare keeping track of details.


Keeping things simple, the example above presents one whopper of a shift halfway up the leg. Although you could argue that it’s the an oversimplification, the same ideas apply to the tiniest of shifts – and the angles shown make it more obvious what is going on!

You don’t need the polar overlay to be able to figure out that one tack is heavily favoured, but we can use it to reinforce a few ideas. At the bottom polar, our two available tacks on the maximum VMG angles are shown by the black arrows again, with the red arrow indicating our upwind target, and the polar rotated to align with the wind arrows. In this case, even though the two tacks are “equal” when it comes to VMG, we can see that the Starboard tack option is the better choice for the first half of the leg as it points us in the right direction. The shift up course is conveniently placed to allow us to the lay the mark on on two fast tacks!

If the shift wasn’t there, this would dissolve into “pure upwind sailing”, and it wouldn’t matter which tack we took first as it would all come out in the wash by the time we lay the mark. The presence of a shift changes the game. Try to plot out what would happen if we sailed even just a little ways on port in this example… Everything is looking good at the beginning, and when you tack you are still “even” with the Stb-tack boats in terms of distance to windward. When they tack back you’ll cross, maybe even a little ahead if you had a little more wind on this side? But they don’t tack… they carry on until the shift kicks in, tack onto port and are laying the mark. Not only are you now behind them, but you’ll need a third tack to make the mark, and adding insult to injury the angle on it is going to be awful.

The “bump” on the polar that is closest to where you want to go is usually the tack you want to be on, barring compelling reasons for choosing the other one!



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