Why can sailing by the lee sometimes be fast? It's counter-intuitive that pulling your boom in to catch less wind on a run can actually make you go faster. While this post is to help middle-of-the-fleet sailors, others might benefit from the following.
From a design perspective, the Laser's mast was revolutionary. But from an efficiency perspective, a Laser mast is located, like all other masts, exactly where you do not want it to be.
On the left we see a boat with a jib when going upwind. The drag is in red and is hardly visible because the forestay holding the jib creates hardly any turbulence. Jibs are efficient because they have very little drag and they have a whole lot of lift. On the right is a Laser and there is much more turbulence because of the mast. So much so that the front of a main does not work properly which is why telltales cannot be placed too close to the mast:
There's something else that is important - having the wind flow across the sail is fast. This is why sailing on a reach is faster than sailing on a run. And why asymmetrical spinnakers are faster than traditional spinnakers. And why a house roof that is low is more at risk in a hurricane than a higher roof with steeper angles. The reason is that the wind creates a lot more lift when it goes across an object than when it’s just trapped by an object.
In a perfect world sailing downwind, masts would not create turbulence and the wind would always flow across sails. But this is not how we sail. As shown below, the boat on the left has its sail all the way out and is trapping lots of wind, creating lots of turbulent drag, and generating no lift. The drag is in the correct direction so the boat is literally being dragged downwind. The boat on the right has the boom pulled in and the main looks a lot like a jib going upwind - but there is no mast at the leading edge to create turbulence and the wind is flowing across the sail:
So the sail on the left is catching more wind while the sail on the right is more efficient. Sometimes more is better and sometimes efficient is better. Sailing by the lee is knowing when to shift from one to the other. Here are some tips:
- I try to steer with my weight, so bearing off is leaning out to heel to windward. As a result, the end of my boom actually torques back and up at the same time. It feels like I’m “scooping” more pressure and, when timed properly, it feels very fast.
- With the boom in, the pressure on the main sheet goes from hardly any pressure and wanting to jibe to really powered up and driving forward. Timing this power properly can be a great way to catch even small waves. Again, this can be fast.
- I like to set the vang so that the leach stretches and “breathes” back and forth about 8 inches. This is a completely natural and legal pumping motion, and a gift from the laws of physics.
- In addition, you can bear off or use your main sheet to pump the main once per wave. This is significant because you’re pumping the leading edge of the sail which is super effective because that’s where all of the lift comes from. This is the only time in sailing when you can pump the part of a sail that has all of the lift.