by Roger » Thu Mar 22, 2007 6:24 pm
The purpose of a boom vang is to keep the boom from rising resulting in the mainsail spilling air. This can happen on any point of sail and in any wind conditions if there are gusts. It is more likely to happen in broad reach or running running downwind conditions and heavier winds with gusts.
On all points of sail however, it helps keep the power up, so I would therefore recommend it. You can give it some slack in close hauled and slack wind conditions however so that your sail can assume a more powerful shape to use what little wind there is more effectively.
The mainsheet itself in these conditions keeps the boom down, but as soon as it is way out to one side, there is little downward pull on the mainsheet so it is easy for the boom to kick up, when there is no boom vang.
It is a relatively cheap add on that increases sailing comfort in broad reach and downwind conditions. Its like having a pitching wedge in your golf club, a nine iron will do the job, but a pitching wedge does it better.