How to calculate wind shadows

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

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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

We Made It!

After a grueling 10 day passage, we made it to the Caribbean (actually the top of the Caribbean)! We are safely anchored in Coral Harbor in St. John, USVI.

The plan was to sail out of Suriname, cross the Guinea Current (which would carry us West), then sail due North hundreds of miles to windward of all the islands, then turn and have a pleasant downwind sail into the USVI. This plan went to crap when I underestimated the power of the Equatorial Current!
The Guinea Current is similar in strength to the Gulf Stream, so we knew not to fight it, just sail North and get through it as fast as we could. The Equatorial Current is not as strong, but enough that we were being carried into the Windward Islands with most haste. To compensate for the current, I had to point out bow slightly East (about 0* to 30* on the compass). This meant that we were now beating into the trade winds. We spent 9 days battling the trades and the current as we sailed close enough to Dominica that they radioed us to tell us that we are not allowed to enter their country due to COVID-19 lockdown rules.
We finally had to cross over to being leeward at Guadalupe; so much for being hundreds of miles to windward! After Guadeloupe, we were able to keep to windward of the leeward islands and eventually, on our last day, make our way into the Virgin Islands on a beautiful downwind passage.
After all that time at sea, it feels so good to drop anchor back in US waters and just relax!

We made it! Our second transatlantic is complete and we are almost done with long passages!

In the Virgin Islands, everything is so close together that the longest sail you could make would take an afternoon. When we set sail to Puerto Rico, it will only be a few hours, as it is only 14 miles from St. Thomas to Culebra, and 20 miles from Culebra to Puerto Rico. Our next long sail will be from Puerto Rico to Cuba or Florida, then we will be back in the ICW where a long sail is a pipe dream.
Sleeping underway is not as restful as sleeping at anchor, and I’m ready for my nice long sleeps!