Sailing Faster Than The Wind

Last week I had the opportunity to cross something off my bucket list: sailing faster than the wind. Now at first this sounds like a preposterous idea – if the wind is pushing you, then how could you go faster than it? It seems the equivalent of the Baron Munchausen pulling himself from a swamp by his own hair. Or powering a sailboat by blowing into the sails (maybe with a huge fan?).

Before providing an explanation, here is a brief video I filmed on the Lending Club 2 in the San Francisco bay (this boat was previously the Banque Populaire VII). The wind was blowing at about 20 knots (that’s nautical miles per hour, with a nautical mile about 15% longer than a standard mile) and the boat speed gets up to about 36 knots here (we did just shy of 40 knots that day).

So what is going on? Well let me start with the explanation that a sailor would give. The wind you experience on a sail boat is called the apparent wind and it is the combination of the true wind (which you would experience with the boat not moving at all) and the headwind from your motion. 

Now imagine for a moment that you are sailing a course that is at a 90 degree angle to the true wind. As your boat starts to move, the wind you feel now also has a component from your headwind (which is coming straight at you). So the apparent wind is no longer coming at 90 degrees but further forward, towards the bow (front) of the boat. And it is also stronger than the true wind. Now just imagine this as your new frame of reference. You trim (adjust) your sails to this wind and your boat accelerates further with the apparent wind increasing and moving further forward. This process continues until you sail as close to the wind as your boat will let you – meaning the wind comes from as close to the bow as possible on your boat.

For some boats, such as the trimaran Lending Club 2, the top speed from this process will be significantly faster than the speed of the true wind. How much faster can be determined via a formula that you can find in this excellent Wikipedia article. You may or may not find this argument convincing but there is plenty of evidence from actual sail boats achieving speeds greater than the speed of wind. 

Now here is a question: is it possible to sail faster than the wind when heading dead downwind, meaning with the wind coming exactly from behind? At first that seems completely impossible. After all, if you followed my explanation or read the Wikipedia article you will conclude that the apparent wind on the boat will be decreasing as you go faster downwind – after all your headwind now comes from exactly the opposite direction. So that would suggest that once your boat moves at the same speed as the true wind the apparent wind on your boat will be zero and no further acceleration is possible.

That intuition turns out to be correct for sailboats, at least for ones with conventional sails. But, surprisingly, it is not a barrier for land yachts using propellers instead of sails. In 2010, the Blackbird, achieved 2x the speed of wind going dead downwind. There is a great Wired article about it. And if you are wondering about the physics behind it, I recommend this epic forum thread.

Posted: 3rd July 2015Comments
Tags:  sailing physics amazement

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