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The affects of current on apparent wind

There are many mis-perceptions about currents and how to work in them. Especially in distance races such as the overnight Governor's Cup and Solomon's Island races, skippers often work towards deep water what the current is in a strong ebb, and towards shallow water in a strong flood. This makes some sense because distance races on the bay tend to be southbound races, winds in the summer tend to be out of the south (the famous Bermuda High). And the bay being somewhat east-west narrow limits the distance races to either north or southbound directions. But what's really happening here?

The wind is almost never the same at a point in time across or up and down the bay. Taking a look at Sailflow for the Chesapeake area (http://www.sailflow.com/windandwhere.iws?regionID=108&regionProductID=30&timeoffset=0), we can see that in different parts of the bay the wind is different. And as the wind changes, it changes differently in different places too. Same with the tidal currents. So part of the navigational problem for a distance race is the balance between the varying wind speed and direction and the positive or adverse affect of current across the bay and the race course.

But, so make that more complicated, we should also consider the effect of the current on the wind. How does that work? For me, it is easiest to think of sailing in a current as like sailing on a magic carpet - the carpet is moving (in the direction and with the speed) with the current. All the boats around me are affected in pretty much the same way - sailing along, but also being carried along by the carpet. But the carpet is also creating wind. The easiest way to think of this is, if there was no wind at all and my boat is sitting still on the water, and if the magic current-carpet is moving at 1 knot, then I would feel a 1-knot wind coming from the direction in which the current is heading.  So, to complete the picture, the affect of current is somewhat similar to the way apparent wind comes about with the motion of the boat changing the direction and speed of the wind. This can be calculated using a good calculator and a lot of patience. And certainly the instrument packages available on some of the high-end yachts include these calculations.  But, for the average racing sailboat, those instruments are out of reach. And part of the suite of automation tools Chesbay.net will offer includes this kind of calculation over the race course.

It's important to understand the impact when making the navigation decisions. Currents on the bay are generally light - not much more than 2 knots during peak ebb or flood. But in increase or lessening of the apparent wind by 2 knots may be very significant. Especially in light winds. Let's take the following example problem.

We're sailing south in our J35 "Uncle Joe".  Suppose that the wind is from the southwest at 6 kts. Let's say 210 degrees. In the western channel, the current is ebbing at 1.75 knots - let's say at 180 degrees. The effect of this is that the wind we feel will shift forward (coming from about 204) and increase in speed to about 7.5 knots. On the eastern side, in the shallows, the current is running at about .75 knots.  The apparent wind speed would be less (about 6.6 knots) and the wind angle would be closer to 207. Now, which side to go to depends on tacking angles of the boat and the rumb line - having the come forward to 204 may well mean we have to tack more often to beat down the bay. This may well more than make up for the slight decrease in wind. On the other hand, our forward speed is going to be faster in the deep water, and faster also because of the increased wind speed. So the decision becomes greater speed with more tacks or less speed with less tacking.

Tough decisions, but now based on knowledge.

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