FREE UPWIND BOAT SPEED

2011 (c) Doug Peckover

How would you like to add 5% to your sail area? Instantly. For free. There’s a go-fast trick in Butterflys that few people know about. It’s something that’s normally only found in the most advanced sailboats. Let’s start with a quick review of what makes a sail work.

Looking down at the sail shape with the wind coming from the top (in green), a jib is really efficient because there is no mast at the front, so there is little turbulence or drag as shown in red (while this turbulence is on both sides of the sail, we’ll focus on the more important leeward side). Jibs cut through the wind really well.

Lasers have a problem because the round mast is at the front end of the boom which creates lots of drag, again shown in red. All Lasers have this problem so there’s not much we can do about it.

 

A tight vang on a Butterfly has the mast turning with the boom. The mast is an extension of the boom and not the sail, so it creates about the same amount of drag as a Laser. Not very good. We can do a lot better.

The vang lever at the bottom of the mast can be pulled or pushed if the vang is loose. If the lever is pulled to windward (pointing towards the skipper) it creates a huge amount of drag. Pam and I joke about this being an air brake at the end of a tack. This is really, really slow because the most important part of the sail is now stalling.

 

But if you push the vang lever away to the leeward side, the mast lines up with sail and creates much less drag, as shown on the right. This is called “over-rotating” the mast and it’s fast for three reasons:

• The drag is greatly reduced.
• The mast is now an extension of the sail, adding almost 4 square feet, or 5% more sail area!
• The tension of the sail leach pulls the mast back. But the oval shape of the mast means that it wants to bend sideways and not backwards. With an over-rotated mast, it actually bends the mast to windward (see the left picture below). This creates a three-dimensional shape which catches more wind, especially in puffs when the top of the mast would otherwise fall off and spill the puff! Great for lake sailing, which is what a Butterfly is designed for.

 

Over-rotated masts were pioneered in the 70’s by Frank Bethwaite from Australia. I had the pleasure of learning this from Frank when I lived in Sydney.

So, next time you’re sailing upwind, have your vang loose enough so the lever can be pushed away 20% to leeward. Your upwind boatspeed will definitely improve. Free boatspeed!