Last night the snowfall cancelled my plans (LMPA AGM), so I cozied up with some reading about fluid dynamics as applied to sailing. Below are a couple of pages from “The Art and Science of Sails”, by Whidden & Levitt. Years ago I would have been able to follow the math and do some of the computations, but those skills have atrophied like all muscles that aren’t exercised regularly. Thank goodness this book has lots of pictures!
Have a look at figures 4.9 and 4.10, the last paragraph on page 79, and the part in the middle of page 80 that describes two upwind race modes for modern sailing boats.
Here’s a simple summary:
- When the wind is light and the boat is not fully powered up, we should sail with both tell-tales flowing:
- The extra lift is needed, even at the expense of additional drag.
- We generate the extra lift with full deep sails, and power them up fully by steering a bit of a lower angle to get both tell tales streaming.
- When the wind is moderate to high and the boat is fully powered up, we should sail with the inner tell-tale lifted
- Now we have plenty of lift, but need to minimize drag.
- We minimize drag by flattening the sails and steering a slightly higher angle with the inner tell tale lifting