My Boat has a Martingale?

You may remember that our boom snapped in two during the Zongo Cup. The wind had shifted after rounding Point Buchon and as I eased out the main sail (let out more line to swing out the main sail) I failed to loosen the boom vang. The boom snapped right at the attachment of the boom vang. The boom had a lot of corrosion around this spot, but not easing the boom vang didn't help matters. So, I decided to do some research on how to properly use a boom vang.

To my surprise, there are various names used for a boom vang. Currently, boom vang is used in the US and kicking strap is used in the UK, but and older term is martingale.  Basically, it is a pulley and/or a piston/rod systems that is used to level the boom and controls twist in the main sail.

Great, but when do you use it?

After searching around a bunch of books and web sites, I found a decent working description on the Sailing USA site:

"If the boom is over the water, one may want to first trim the lower part of the mainsail by using the mainsail's sheet , then loosen the boom vang until the aft (top) portion of the mainsail or telltales start to luff then tighten the boom vang until the luffing stops and the telltales (along the leech or back edge of the sail) are streaming backward." This is for sailing on a reach or down wind. Why do you want the sail twisted? The wind blows at different angles near the water or the bottom of the sail than it does 40 feet off the the water. In order for the whole sail to provide the maximum lift the sail needs to twist to meet the wind at the proper angle. 

If sailing close hauled or into the wind, the main sail doesn't really have twist.  The boom vang is used in strong winds to flatten the sail and reduce the lift force of the sail.  A flat sail can help avoid having to reef the main and will reduce excessive heeling.  

p.s. If we had been smart like those folks on the Bella Star we might have found the corrosion under the boom vang plate and fixed it!