Friday, January 20, 2023

Sail toward the wind shift

The hallmark of all good navigation is looking ahead. Under sail, the main thing we want to anticipate is wind shifts—currents and traffic are other important factors to plan around, but for now we look at the wind. We look at two practical examples of the general guideline when sailing to weather to sail toward the direction the wind is expected to shift toward. If the wind is anticipated to shift left, then we want to be on the left side of the course when that happens. 

We can often anticipate wind shifts based on the terrain, with no weather pattern changes—such as channeled wind bending into an open bay on one side of the channel—but we also get insights from forecasts. An approaching Low generally brings winds backing to the south, which in a narrow channel running N-S will force a southerly wind to a south-southeasterly in the channel. When a front crosses such a narrow channel, the S-SE wind makes a 100% predictable sudden shift to the S-SW because wind always veers at a front.

In the first example we have a south wind across the area during the start and most of the way to the finish, but there is a strong adverse current in the channel, which means we must choose right or left side to stay out of the strong current in the middle—current is always weaker in the shallower water along the edges. But we have a forecast that the wind will be shifting to the SE or S-SE as a low approaches. We do not know when, but definitely before we get to the south end.

S wind changing to SE. Here we show it happening just at the south end, but this is meant to illustrate where the boats are when the wind all along the channel went from S to SE.

We can choose the east or west side and then tack down the beach. But since we know the wind is shifting to the east (S to SE) we want to be sure to take the east side. See what happens when the shift occurs. The boat on the east side then has a fast reach right to the mark, whereas the boat on the west side has to keep on tacking to get there. This is a shift to the left, so we want to be on the left side.

Below we have another realistic scenario. The wind is now from the SE along the water way and it does not take long as you tack down the middle of the Sound to notice with binoculars that boats along the shorelines are not tacking, they are reaching right down the beach. They are taking advantage of the fact that wind on land is notably backed (shifted to the left) relative to wind on the water. This is a friction effect. And the transition does not take place right at the shoreline, but it extends out into the water to some extent and these reaching boats are riding along in that wind.

Thus in this situation, the logical route is along the shore to take advantage of that, and we could go down either shoreline. But in this scenario, we know from the forecasts that a front is going to pass over us before we get all the way down, so we have a logical side to choose. When a front crosses the sound the wind veers (shifts to the right) in a matter of minutes, so this wind going from SE to SW is shifting to the west, so we want to be on the west side—or put another way, shifting right, we want to be on the right.

And you see again what happens when the two boats meet that shift, one on the east side has to start tacking to the mark, and the one on the west side has a fast reach right to it.

SE wind veering to SW. Here we show it happening just at the south end, but this is meant to illustrate where the boats are when the wind all along the channel makes this change from SE to SW.

The summary is: if the wind is shifting to the right, we want to be on the right side; shifting left, we want to be on the left side.  To think through the shift direction, draw the wind arrows point to point as shown below.

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