Speed & Smarts - The Newsletter of How-To Tips for Racing Sailors

ISSUE 108: How Current Affects your Sailing Wind

When you’re racing in water that’s moving, the current affects not only your course over the bottom, but the wind you sail in too. One easy way to imagine this is to think about a boat on a river (see below). Pretend the current is flowing southwest at three knots. The true wind velocity, which is the wind felt by a person standing on shore, is zero.

The boat that’s floating down the river has not yet hoisted her sails. Do the sailors on the boat feel any wind? Yes. They feel a breeze of three knots coming from the southwest. This is called their “sailing wind.” If the river current suddenly changes direction and starts flowing northeast at two knots, they will feel a sailing wind of two knots from the northeast.

In both situations above, the sailing wind was generated entirely by movement of the water. Since there was no true wind, the sailing wind was simply opposite in direction and equal in magnitude to the current.

But when you have both true wind and current, the result is a little different. Suppose the person standing on the shore of the river starts to feel a light breeze from the northwest. The sailors in the boat will also feel this, but it will be combined with the wind they felt while floating down the river.

The same effect happens every time you’re racing in current. The actual wind you sail in is a vector combination of the true wind (felt by an observer on land or on the anchored committee boat) and the wind caused by the current. This means your sailing wind will change (in strength and/or direction) if there is a change in any of the following four variables:

• Velocity of the true wind

• Direction of the true wind

• Velocity of the current

• Direction of the current

These four variables might be different in different parts of the course, or they could easily change over time. For example, in the situation shown at the top of the next page, the true wind velocity and direction are steady all over the course, but the current strength and direction vary greatly. As you can see, this has a large impact on the sailing wind for boats that are racing up the first beat.

As long as current strength and direction remain constant across the course, every boat will have the same sailing wind so the current won’t affect your strategy. However, if the strength or direction of the current varies in different areas of the course, or if these change over time, then you will experience changes in your sailing wind direction and velocity.

So before you start a race or a regatta, it’s a good idea to 1) check out a chart of the racing area to see how geography will influence water movement; and 2) sail around the course area and look at current on different sides of the first beat.

A lot of times you can actually see areas of stronger current by looking at the ripples on the water surface. You will see more ripples (or rougher water) in places where the current is running stronger against the wind (or weaker with the wind). For example, in the diagram shown here, you would expect to see more ripples on the left side of the course where the current is running more strongly in opposition to the wind.

Sometimes it’s hard to tell whether the ripples are caused by current or by a puff of wind. When you’re sailing upwind, this doesn’t matter so much because it is always good to sail into more wind or into current that’s flowing more upwind. On the run, however, it’s a little tricky. It’s always smart to sail for better pressure downwind, but if the ripples are due mainly to current that might not be a great place to go.

The bottom line is that changes in current can have a profound effect on your sailing wind. This is not a factor when the current direction and strength are steady, but if the current is variable or starts changing it will impact your strategic plan. Variable current is significant not only because it affects your course over the bottom, but because it may bring a windshift or better (or worse) wind pressure. •

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ISSUE 108: How Current Affects your Sailing Wind