Hiding in the lee of an island is a great way to protect yourself from punishing winds. The land blocks the sea from building waves that will rock your yacht as well as blocks the wind m, allowing you to anchor rather peacefully in some horrible conditions!

When you are looking at a new place to go cruising and need to find happy hiding holes that you can tuck into if the wind builds, how can you figure out how far away from land you will still be protected?

Navionics will tell you the elevation of various points on a landmass. Some are in feet, some are in meters, but the math all works out the same.
The equation is simple:

Elevation in Nautical Miles x 20 = Distance of wind shadow in nautical miles

For example: 564 feet (elevation) / 6076 feet (number of feet in 1 nautical mile) = 0.09nm tall

0.09nm x 20 = 1.85nm wind shadow

In other words, if you are within 1.8nm of that point of land, you will experience much less wind as you will be shielded from it.
Another example would be 168m right next to it:

168m x 39 (number of inches in a meter) = 6552 inches / 12 (number of inches in a foot) = 546 feet / 6076 (number of feet in a nautical mile) = 0.089 x 20 = 1.79nm wind shadow

To make it easy, just do these calculations:

If the elevation is in feet:

Elevation / 6076 x 20 = Windshadow

If the elevation is in meters:

Elevation x 39 / 12 / 6076 x 20 = Windshadow