Great discussion. I have a topping lift on my Bristol 35.5 and the sailmaker advises a boom vang to improve sail shape. One of the units he mentioned is Hall Spars Quick Vang which for my size boat come in 12:1 or 18:1 purchase. Anyone familiar with this product please chime in as well as which ratio to choose. Thanks
The 35.5 and 31.1 are very similar in the rig so these comments should apply to yours.
What I have found is that without the boomvang, the boom has a tendency to lift as the wind increases, and/or as you move off the wind, spilling some air especially at the top of the sail.
This is actually quite a desirable mechanism in SF Bay as it enables the boat to cope with gusts and strong winds nicely.
Applying some vang, especially from close reach to beam reach, is like standing on the gas pedal. The boat heals a lot more, the main develops more power, and the weather helm increases a lot, especially in gusts.
So the upshot is I can see why Bristol thought it OK to leave it off a cruising design. If I was singlehanding, and expecting stronger winds, I'd leave it loose. When I have crew we tend to play the vang sheet, trimming it when wind is <20 knots and easing it out when there's more. Generally it's the weather helm getting unreasonable that will have me telling the crew to ease it out.
Another board member advised having a cam cleat and enough line to reach the wheel, so you can adjust it quickly when you're on your own.
Finally, I think 12:1 is fine for a cruising boat. Not having too much advantage helps to not overstress the boom.
PS, I just brought the boomvang line back to the cockpit, next to the winch for the main halyard. Let me know if you want pics on how to do this. Basically though I used the same blocks as Bristol used for the main halyard, placing them side by side, so it looks like a factory job.