That's a nice set-up on the 17's mast step, a sort of hybrid bail and
pad-eye, making for the lowest possible attachment point and, so, the largest
possible boom-mast-vang triangle area.
With a sufficient height of boom above that attachment point, it'd be hard to
go wrong with a 45 degree angle between boom and vang -- though you can find
forums in which even those in agreement on the angle are at odds at how you
measure it (from the fore or aft side of the mast, the top or bottom of the
boom, and do you count the angels dancing on the head of a pin). This kind of
detail usually seems to involve a one-design or club rule.
For the sake of argument, the extremes would be uselessly weak and
unacceptable. You wouldn't attach the boom bail 6 inches from the mast, nor 6
inches
from the aft end of the boom. The question is, then, given the length of your
particular boom, where is the ideal location for the boom bail to maximize
the DOWNWARD purchase on the entire boom? I've never seen a recommendation to
allow the angle between boom and vang go under 30 degrees. After that,
you're not pulling DOWN on the boom so much as just cinching it up tight
against
the gooseneck.
I'm on a 15 and, so, can't provide you a template for a 17 [So why am I
posting this? Oh, just felt like talking about boats late in the evening],
but I
just measured the distances, and 24 years ago -- for reasons long forgotten by
me, but no doubt based on some good "authority" -- I installed the mast bail
bolt 4 inches up from the cabin roof (had to clear the sides of the mast step,
and the boom bail is through-bolted 19 inches from the forward end of the
boom. I believe the mid-point of the boom's vertical thickness rides about 22
inches above the base of the mast step. So the height of the mast leg of the
triangle nets out at 18 and -- adding gooseneck and half the mast thickness --
the boom leg is roughly 23 inches. So I inched out on the boom a bit
farther than a strict 45-degree equilateral triangle would have dictated.
Split
the difference between 45 and 30, it looks like. It's always worked out
super,
but then, there really isn't much relative force exerted on the boom and
mainsail of a 15. In fact, a boom vang is overkill on a 15, and I don't have
much occasion to use it. Kind of nice for wing-and-wing in a breeze.
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