The easiest way to explain the concept of velocity shifts is to start with an example. Imagine you are sailing upwind, close-hauled on starboard tack. It’s a very puffy day, with lots of increases and decreases in wind velocity. However, the wind direction remains exactly the same during all the puffs and lulls.
As you sail along, the wind you feel on your face is called your ap- parent wind. It’s a vector sum of the ‘sailing wind’ (the wind you would feel if you were sitting on a floating log) plus the ‘boat wind’ (caused by the movement of your boat through the water). As long as the sailing wind and your speed remain constant, the velocity and direction of your apparent wind will also remain exactly the same.
Now, all of a sudden, the front of your jib luffs, and you wonder how it’s possible to get a header when the wind direction is perfectly steady. But this wasn’t really a change in wind direction – what happened was that you sailed into a lull and got a “velocity shift” that looked a lot like a header.
When you first hit the lull, the true wind drops but your speed is still relatively high, and this makes your apparent wind move forward. This ‘velocity header’ will last only until your speed drops to match the new wind strength.
A similar effect happens when you sail into a puff. Your boatspeed doesn’t increase right away, so initially the puff moves your apparent wind aft. This is called a ‘velocity lift.’ As your boat- speed increases to match the new wind strength, however, your apparent wind will return roughly to its original direction.
As veteran Finn racer Gus Miller illustrates, the wind you feel while sailing (your apparent wind) is a vector combination of two things: 1) the sailing wind (true wind including effects of current); and 2) the wind created by the movement of your boat through the water. © Robert Deaves
Since velocity shifts are caused by the time lag between changes in wind velocity and corresponding changes in boatspeed, they will be more significant on boats that take longer to accelerate or decelerate. In other words, the heavier your boat, the more pronounced your velocity shifts will be. A light boat like a Laser adjusts quickly to new wind speeds, so velocity shifts are small.
Recognizing velocity shifts
It is sometimes difficult to recognize whether a change in your apparent wind is due to a velocity shift or to a real change in the direction of your sailing wind. It’s important to know this, because you must often respond differently in each situation. Here are several ways to figure out which kind of shift you have:
1. Look ahead. You get a velocity shift only when the wind speed changes, and this is something you can often see on the water to wind- ward. This is probably the most valuable method of gauging velocity shifts because it allows you to anticipate what’s coming and to be proactive about changing gears.
2. Feel the pressure. If the shift you get is due to a change in wind velocity, you should be able to feel this. For example, a velocity lift should come with more wind on your face and more heeling.
3. See how long it lasts. One big difference between a velocity shift and a ‘real’ shift is that the former is temporary and lasts only until your boatspeed adjusts to the new wind speed. So if you can’t see or feel different wind pressure, wait a few seconds to see if the shift disappears. It it does, it was probably due to a change in velocity. ■
This article originally appeared in David Dellenbaugh’s Speed & Smarts, The newsletter of how-to tips for racing sailors. If you want to sail faster and smarter, log onto SpeedandSmarts.com.
A resident of Easton, CT, Dellenbaugh was tactician and starting helmsman for America3’s successful defense of the America’s Cup in 1992. He’s a Lightning World Champion, two-time Congressional Cup winner, seven-time Thistle National Champion, two-time winner of the Canada’s Cup, three-time Prince of Wales U.S. Match Racing Champion, and a winner of the U.S. Team Racing Championships for the Hinman Trophy.
