Monday, September 11, 2006

Sailor's Wind

The other day I jotted down all the ways I could think of (well, all I could think of in two or three minutes) to improve someone's sailboat racing performance. Several readers commented that I had made no mention of weather forecasting, understanding weather patterns, shifts, land effects on the wind, and so on. One kind soul even suggested reading Stuart Walker's book The Sailor's Wind and taking an online course about the topic.

I'm grateful for all of you for pointing out this blind spot in my thinking. It's not that I don't think about what the wind is doing, and what it's going to do, when I'm racing. But it is somewhat worrying that it didn't come to mind when I made that list. I'm going to try and explain this omission and it may come off as a bit defensive. But bear with me as I try to explain how I currently approach this whole issue and then, by all means, tell me if you think I need to make some improvements.

First of all, let me explain that most of the racing I do (in Lasers) takes place on relatively short courses in relatively short times. We are not talking Newport to Bermuda here. We're talking about windward legs of up to a mile or so (often less) and total race length of no more than an hour or so. So the wind effects that are important are on a relatively small scale both in distance and time. It's of little use to me to be told that the wind blowing on the coastline is probably going to shift twenty degrees to the right some time this afternoon. What I need to know is what the wind is going to do during the next ten minutes in the stretch of water half a mile upwind of me.

So although I do study weather forecasts available from various public sources, my gut feel is that they don't often help me very much. A dinghy sailor needs to understand micro-weather patterns and you're typically not going to find that from any regular weather forecast.

OK, you say, so you need to study Walker's book where he does explain the causes of various wind patterns and deal with small-scale wind effects too. Then you would be able to predict what's going to happen to the wind on your racecourse. Now Dr Walker is a superb sailor and is highly knowledgeable about many aspects of sailing, but in my opinion his prose style is often turgid and impenetrable. Take this for example...

It has recently been recognized that one of the most common form of large-scale, boundary-layer convection is the horizontal convective roll. HCRs develop when wind shear causes a rising column of heated air to twist into a horizontal helix. Continued rotation entrains additional air so that such helices, approximately aligned with the surface air flow, ultimately extend downwind for miles. Because adjacent rolls counterrotate, persistent bands of vertical flow (alternately upwards and downward) develop in the regions between them; bands of altered surface flow, backed or veered and strengthened or diminished by the direction of the adjacent vertical flow, develop beneath them.

Hmm. I think he's saying that some times the wind is "streaky". But I already knew that.

At the Laser Masters Worlds in Spain three years ago, the regatta organizers arranged for a professional meteorologist to brief the sailors before each day of racing. So a few dozen of us trooped into a meeting room each day and watched this guy's PC-based presentation of weather patterns and predictions. As I recall, half the time his predictions were wrong.

So let me summarize. Basically I'm saying that conventional weather forecasts, and even study of the theory behind weather, are of very little help for the kind of racing I do. However, practical observation on the race course, prior to and during the race, can provide a lot of valuable information about what the wind is doing and what it is likely to do.

But what do you think? Do you find you are able to gather useful information for your racing from a weather forecast? Does your theoretical understanding of weather make you a better racer?

Next post: ways of checking out the weather on the course...


EVK4 said...

I wasn't necessarily thinking of the Bermuda-Azores high's effect on weather on the Hudson but more the effects of land bending wind.

An example is SF Bay. In the summer, the wind is ALWAYS WSW. But the wind bends to almost S as it goes around Angel Island. In fact, a W wind might bend to almost S as well. I blame Global Warming, most people blame Point Blunt.

On a lake, there are probably similar wind characteristics if you have mountain gaps, large trees, hot sand beaches, etc. These would all create shifts that it's better to know about before you're on the water.

Or I might be all wet. And, on another topic, I don't think you could possibly change topics enough to make your writing uninteresting. I like your new direction even though I care almost nil about Laser Racing.

Carol Anne said...

For the long-distance race I was in this past weekend, the weather forecasts were helpful in that we knew ahead of time that the wind was going to switch from being more-or-less northeast to being more-or-less southwest, although the forecasts weren't good at predicting exactly when the switch would occur. Thus, when the wind switched, we were reasonably confident it wasn't a temporary thing, so we got the spinnaker up and kept it up (even Zorro didn't do that).

However, the weather forecasts don't do a thing with the very small-scale weather fluctuations on the water. The weather station is rarely nearby -- it's often at the airport, or on the rooftop of the local cable-television company. Especially on lakes in desert or mountains, the only good way to learn about local conditions is to sail a lot and learn exactly what the wind is likely to do exactly where. Avoiding holes and anticipating flyswatters are skills that come from experience.

Litoralis said...

I do think that a practical knowledge of weather forecasting is useful. Having some background knowledge of how a weather forecast is created gives a sense of whether certain forecasts should be believed or not.
A general weather forecast can be used to determine whether the wind will go right or left during the day. Not that useful in a single short-course dinghy race, but when you start to see the racecourse being shifted between every race, then the previous knowledge of a forecast can help jog your memory and remind you to keep the potential persistent shift in the back of your mind.

Probably more important than a macroscopic sense of the weather, is a mental framework for visualizing the wind on the micro-scale of the racecourse:
Can you predict where there might be a hole in the windshadow of a row of boats?
Can you visualize the effect of breeze separating from the water surface over a group of boats in light air?
Can you see that leftie shift as it comes down the course?
Can you tell which gybe you should be before you get to the windward mark?
Can you predict the wind direction at the time of the start in an oscillating breeze?
Can you see the geographic shift created near a shoreline?

I think these skills are learned through a combination of observation, on-the-water experimentation, and visualization. I think some of the things I learned in my weather forecasting class and my fluid dynamics class help in terms of the visualization part of the equation.

Tim Coleman said...

Weather forcasts are always wrong to some extent but they nearly always get a good deal of it right.
It is worth keeping an eye on the trends of the weather as this often gives you a better feel for what is most likely to happen on the day of the race.

The next thing to do is correlate what you think is going to happen, based on the forecast, to what seems to be happening on the race area an hour or so before the race and again just before the race as changes in that short time help to confirm or modify what your forecast predicts.

For example if the forecast predicts a veer and increase in wind strength and when you get to the race area and the wind is in the predicted direction and starts to shift and increase then you can be reasonably sure that it will follow the forecast.
On the other hand if it already appears to have shifted from the predicted direction and is already stronger than expected you know that it is not likely to shift further.
Practical upshot in the first case is you keep to the side of the course where the expected new wind is to come from. In the second case you don't need to look for the shift but you need to keep a lookout in case it does something else.

On top of this are local geographic effects on the wind direction and flow patern over the race area and its variation in strength and also the local effects such as sea breezes. These local effects can often be the biggest factors.

Anonymous said...

Walker is probably not the best author on this subject - his prose and weather knowledge were a product of the book being written virtually half a century ago! By far the best text book I've found is Frank Bethwaite's High performance Sailing, masses of info and easy to understand.

Having some theory to back up the Mk1 Eyeball is certainly useful - one easy one is to stop the boat before the start and compare the movement of the lowest clouds to the direction of the burgee. There's often a significant skew and all other things being equal staying on the side of the fleet closest to this shift often pays off..

Great blog - really interesting stuff.

Anonymous said...

I tried to decifer "The Sailors Wind" - anyone who can come up with a translation I'm willing to pay! I race every weekend in and around the Boston area and use a website - - to find out what the wind velocity and direction will be for the day. It's pretty accurate and is helpful to know whether the wind is backing, velocity, etc, over the course of the day. With that said nothing beats local knowledge. Once you learn the weather patterns and how a particular pattern is effected by geography, current, et al you can use that knowledge to become a better racer. Of, course you should always be looking ahead for that next good shift...

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