Re: Vang on a dinghy
Here's my take :
Without a vang at all, and with the traveller always centered, this means that as you move from close hauled to a run, the downwards effect of the main sheet gets less and less. The boom then has a tendency to rise, and the more wind, the more it rises. The effect of this rise is to allow air to spill air out of the leach of the sail, losing some power.
A cruiser might not mind this loss of power, in fact it makes the boat easier to handle, as you have a negative feedback effect, the stronger the wind the more air you spill, but if you are racing you definitely do mind.
A good starting point is to sheet in hard on a close reach, then snug up the vang so it's just tight. Then as you go through the points of sail to a run, the vang is keeping the boom level, fighting the rise.
If you feel overpowered from a beam reach to a run, let the vang out. The boom rises and some air spills out.