The ‘Doppler’ Windshift Effect – by David Dellenbaugh



On a beat, the speed at which you converge with shifts (and puffs) is roughly equal to the sum of the wind speed plus your VMG to windward. In this case, that’s 10 (windspeed) plus 5 (VMG), or 15 knots. But on a run, the convergence speed is the wind speed minus your VMG to leeward. Here it’s 4 knots (10 minus 6). So on a run the shifts are coming at you several times more slowly than on a beat, so you will experience that many fewer shifts.

We all know what happens when a train comes toward us at full speed with its horn blowing, at first the sound is very high pitched, but it drops quickly as the train passes by and becomes quite low-pitched while the train speeds away.

The reason for this is what’s called the Doppler effect. The sound waves in front of the train are compressed very close together, which results in a higher pitch. Behind the train, the sound waves are much farther apart, resulting in a dramatically lower pitch.

The Doppler effect is a useful analogy for what happens on a windward-leeward course. When you sail upwind, it’s like being on the front of the train, since you are sailing toward the wind, you will get the shifts and puffs at a faster rate than if you were sitting in an anchored boat.

Conversely, on a run you are sailing away from the wind, so you get the shifts and puffs at a slower rate, that’s like what happens after the train passes the point where you are standing.

Though you won’t hear any changes in pitch when you go from beating to running, you may notice some subtle changes in the wind. To illustrate this, let’s consider an example, suppose you are sailing around on the starting line and you find that the wind is oscillating every five minutes.

As you sail up the first beat, will the shifts come at you at the same rate?

The answer is no. Since you are sailing towards the shifts, you will get them faster, perhaps every three or four minutes.

How about when you round the windward mark and sail down the run?

Since you are sailing away from the shifts, you will get them less often, perhaps every 7 or 8 minutes!

The strategic implications of this phenomenon are significant. For example, if there are 8 minutes between shifts when you are sailing downwind, it is possible you will only see one shift on the run and if you get only one shift on the run, it means you should treat that as a persistent shift even though the overall wind pattern is oscillating.

The ‘Doppler’ wind shift effect also explains why better pressure is so critical downwind. Since you are sailing with the wind, you won’t see so many puffs, but you can stay in one much longer than on the beats, therefore, getting into the puff and using it fully is critical for optimum performance.

On a beat, the speed at which you converge with shifts (and puffs) is roughly equal to the sum of the wind speed plus your VMG to windward.

In this case, that’s 10 (windspeed) plus 5 (VMG), or 15 knots. But on a run, the convergence speed is the wind speed minus your VMG to leeward. Here it’s 4 knots (10 minus 6). So on a run, the shifts are coming at you several times more slowly than on a beat, so you will experience that many fewer shifts.

Dave publishes the newsletter Speed & Smarts. For a subscription go to www.speedandsmarts.com 

Prepare to Race

On the morning of the race, you will check the local forecast again to see how the predicted weather has changed or whether it is behaving as has been forecast.

Get out on the course at least 60 minutes before the posted start time and sail as much of the first beat as you can, making mental notes of the wind patterns to establish which side of the course appears favoured and whether the wind shifts are oscillating or persistent.

Compare what you are seeing with what has been predicted and start to make your plan for the first windward leg, the main advantage of doing this is that if immediately after the start something changes, you will have the information to make a snap decision about whether to continue standing on or whether to tack.

Whilst sailing the first leg prior to the start you can establish whether your setting for the rig, sails and sheeting positions are correct (these would have been set initially prior to leaving the beach based on information available at that time)

Check that the current at different points on the racecourse matches with what you know about this venue from previous regattas or local knowledge research.

Even the best sailors benefit from lining up against another competitor prior to the start and many of us have a tuning partner but if you are at a regatta and your regular mate is not there, work out who might be beneficial to work with and approach them about the possibility.

So many questions can be answered by positioning your boat two lengths from a competitor and speed testing. These tests can and should be lined up in advance with a reliable competitor whose speed and abilities are known and someone you know will show up on time at the designated spot.

Almost always prioritize tactical and boatspeed research over boat-handling practice, you are not likely to solve bigger boat-handling issues in the short period of time that is available to you.

Finally, allow an 8 to 10 minute chill period before the start, and during this time discuss the upcoming race in a low-stress manner giving the team an opportunity to re-evaluate sail selection, and then to fuel up and hydrate.

You are now ready to tackle any eventuality after the gun goes and to make a snap informed tactical decision when something that was not predicted occurs.

 

 

Left or Right?

This is probably the most asked question in yacht racing, going fast is super important but it is no good being the fastest boat out there if you are going fast the wrong way.

That’s why, no matter how fast you are, you must consider whether you want to go left or right.

When racing, we must employ strategy and strategy is the plan you employ for getting to the next mark as quickly as possible.

A strategy is a plan that takes account of things like wind direction and strength, current, waves and the position of the next mark.

These factors are different every time you go out on the water and they change constantly while you are racing and often vary across the course as a result, the difference between going left and going right can be huge.

On the first beat, you must work hard to take advantage of changes in both wind direction and wind velocity, current and the geometry of the course. The existence or absence of waves is another factor you should consider, but this does not often make a big difference.

Before you can actually plan a strategy, you must observe the racecourse and collect a bunch of helpful information including data about wind and current and do this before you leave the beach. Look up weather forecasts and current charts and don’t forget to tap into the local knowledge of other sailors.

Get out to the course area early and start looking around and make observations about what is actually happening on the course, it may not be exactly what was predicted. After the race starts, don’t stop thinking about strategy, the wind and current are always changing. An added dimension is that now you have many other boats to help you see which side of the course is really favoured.

The best way to plan your strategy would be to view the racecourse from overhead but since that is not possible while racing, you must keep your head “out of the boat” and focus on the big picture.

Your strategic plan could be as simple as, “Hit the left side hard”  or it might be more detailed, like “Start 1/3 of the way down from  the RC boat and play the oscillating shifts up the middle right  side.”

Don’t forget to keep re-thinking your strategy during the race as you will be constantly getting more information about the wind and other strategic factors. 

It is not always obvious which side of the course is favoured and there will be times when you’ll have no idea whether to go left or right and in fact, even the best sailors don’t have a strong feeling about which way to go on the first beat in as many as 50% of the races they sail.

When this happens, what should you do?

Unless you’re sure the right or left side is favoured,  stay near the middle of the fleet and keep your eyes open, the beginning of the first beat is a great time for seeing what the wind is doing and which side is paying off.

Once you get some clues about which boats are gaining, head that way quickly. Of course, you will probably come out behind the boats that sailed straight to the favoured side but you took much less of a  risk than they did, and hopefully, you will still be in the top group at the first mark.

If you can do this every race, you’ll be successful.