Basic Downwind Performance, Part Two - SailNet Community

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Old 11-06-2003
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Zack Leonard is on a distinguished road
Basic Downwind Performance, Part Two

To review Part One of Zack Leonard's series achieving the best performance downwind click here.


The downwind legs of a racecourse are when many boats fade, so here's your chance to make gains.

Have you ever sailed down the run pointing at the leeward mark and watched as some of your competitors jibed back and forth for no apparent reason? You probably chuckled to yourself at how worked up those other folks were getting while you were just taking it smooth and easy. Well it's time to wake up, because there are definitely times when jibing frequently, even while steering a straight course, can pay huge dividends on the racecourse.

When I was a freshman in college I sailed quite a bit against Brad Read, who was honored as College Sailor of the Year that year. I remember asking Brad why he was always able to pass me downwind. He told me that he and some of his fellow racers at Boston University regularly used the upper floors of a tall building that overlooked the Charles River where his team practiced to study the patterns and movements of the shifts and puffs over the river. Because of this, and his vast experience sailing in other shifty venues, he developed an intuitive feeling for the wind shifts and a knack for getting on the right jibe in the most wind. This is one of the most important principles of downwind sailing.

In shifty, onshore winds, or even oscillating sea breezes, the wind shifts back and forth, sometimes accompanied by velocity increases. We know that playing these shifts and puffs correctly upwind—what we call staying in phase—can produce large gains, and the same applies downwind.


It generally pays to stay close to the rhumb line downwind, but jibing onto each new header will pay big dividends.
In an oscillating breeze with steady velocity, the decision-making is easy—it generally pays to sail close to rhumb line, jibing onto each new header as it comes. So, how do you know when your boat gets lifted and it's time to jibe while sailing downwind? It's a little trickier than upwind, but if you have a VMG chart (velocity made good) and you are sailing by your numbers, then the compass will tell you when this occurs (see sidebar). You'll have to head up to keep your speed the same, but just be sure not to confuse this with heading up because you're in a lull!

Say you're sailing in a small, non-spinnaker dinghy like a Snipe, then you only need to head up until the jib won't stay set. When you get to that point, note the compass angle and check that angle occasionally. If you have to head up much higher for the jib to break, then you have been lifted and should consider a jibe. As little as a 10-degree shift can make a huge difference. If your boat sails fastest when it's eight degrees above dead downwind, then you will only deviate from the course to the mark by three degrees by staying in phase on the headed jibe. If you get out of phase, you can deviate from the direct course to the mark by 13 degrees. I'm not much of a mathematician, but the geometry looks a whole lot better when you sail in phase, sailing the headers down the course the whole time.


Keeping a wary eye astern will help you monitor the puffs as well as the competition.

In puffy, onshore winds, the shifts will vary more in angle and velocity, and this is when wind velocity becomes the more important consideration. I like to keep an open eye behind me all the time on this sort of days to watch the wind, because some serious money can be made if you know what it's doing. Just look for the darker or more disturbed water—that's where the wind is. You can actually watch it move over the water, and with some practice, you can get good at anticipating when it will be where your boat is.

You should always be concerned with where the next puff is coming from—a factor that may determine which jibe you want to be on for the first puff. Before you round the upwind mark and begin your downwind leg, have a look at the wind on the water and try to determine where the first puff will come from and whether you'll want to jibe right away or just bear away after you round the mark.

The trick to using the puffs is is to sail your optimum VMG course toward the side that the next puff will be coming from, get into that puff, adjust to the new optimum angle for your boat, and then decide which jibe will bring you closer to the mark. Staying close to the rhumb line particularily in the leg, is not quite as important in puffy conditions because there's so much opportunity to gain if you stay in the puffs.


Sailing in phase more often than your opponents just might give you that edge you need to eclipse them at the end of the leg.

Think about the downwind leg like a giant game of connect the dots, only you are connecting the puffs. In general you want to dig back toward rhumb line in a puff, but sometimes the next puff is coming down the same side of the course. This often means that one side of the course simply has more wind for some geographic reason and you need to program that into your tactical plan. In that case, if you already are closer to the puff than anyone else, then you can take a short turn on each jibe, trying not to diverge from the rhumb line too much, but also trying to stay on the side of the run where the puff will fill in. The closer you get to the leeward mark the more you should try to get back to the rhumb line, and this will influence your decision on jibing or holding your course.

The next time you're out sailing on a puffy day, give this technique a try and you'll find that it can really improve your performance on the downwind legs.

Digesting VMG

Performance sailors use the acronym VMG (velocity made good) to express boat speed made along the direction of the wind. The concept holds that for a specific sailing condition, there is a single combination of boat speed and true wind angle that equates to the optimum VMG. On any point of sail—upwind or down—the longer we can maintain the best VMG, the better our cumulative performance.


Here's a polar diagram for the SailNet 30.

VMG for specific boats is best understood by way of a polar plot or polar diagram. The information that supports these diagrams is derived from velocity prediction programs (VPP), which take into account the boat's displacement, sail area, and righting moment. In the example above, a SailNet 30 will make six knots sailing at a true wind angle of 45 degrees. Downwind, it should move at 5.85 knots at an angle of 150 degrees, and so on. The actual VMG is listed at the left side of the plot. (VMG downwind is expressed as a negative quantity.) Using this polar diagram, the crew on board has a fixed idea of the speeds that they should attain for optimum performance on every point of sail.

If you'd like to get the polar diagram for your boat, you can contact US SAILING at (401) 683-0800; it's likely that the folks there will have polars for your boat or one very much like it.



 

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