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I want to install a boom vang and I need to figure out where to attach it. Right now the only place on the boom is a bail somewhere in the middle, looks too far aft (and flimsy too). On the mast there are two holes with bad threads. I would have to re-thread them but I would rather not make new holes, if I don't have to. What kind of loads are we talking about?
Are bails good enough or I need to use different hardware?
At what angle should the vang be set?
Does the image from Harken look like a good solution?
It would help if you said what kind of boat you have. The two holes on the mast are probably not sufficient to support the base plate of a boom vang on anything but a very small boat. The loads are proportional to the size of the boat, and without knowing what boat you have, not much can be said.
As for how far along the boom the vang should be attached, that really depends on how heavy a spar you have. The heavier the spar, the further forward a boom vang can attach. The vang should be 45-60˚ from vertical or so. Any more than 60˚ and it won't be very effective, anything less than 45˚ requires more leverage than you'll likely want to use.
It would help if you said what kind of boat you have. The two holes on the mast are probably not sufficient to support the base plate of a boom vang on anything but a very small boat. The loads are proportional to the size of the boat, and without knowing what boat you have, not much can be said.
As for how far along the boom the vang should be attached, that really depends on how heavy a spar you have. The heavier the spar, the further forward a boom vang can attach. The vang should be 45-60˚ from vertical or so. Any more than 60˚ and it won't be very effective, anything less than 45˚ requires more leverage than you'll likely want to use.
Two holes is two holes. If the bail is used (and I believe most folks use them) the thinnest, or better yet weakest, component rules. Not knowing his actual setup, I'd say the holes at the base are sufficient if they are drilled out and re-tapped to facilitate the next larger screw, provided that this screw is larger than the bail. The other question would be whether or not the bail fasteners are thru-bolted. This changes things some. My suggestion, which may not be feasible, would be to step the mast and thru-bolt the bail or attachment plate at the base of the mast. I currently own a 25 foot trailerable and a 29 foot not-so-trailerable. On my 25 I needed a way to attach a pad eye to the top of the mast to attach a line specifically for stepping. I thru-bolted it, lock nuts 30 feet up, inside the mast. Ask me how I did it.
You'll probably want to have the mast attachment point of the boom vang through-bolted through the mast with a compression sleeve to prevent you from damaging the spar by over-tightening it.
A bail would work nicely for the boom attachment point, but again, you'll probably want it through-bolted with a compression sleeve.
A vang undergoes tremendous forces, so whatever you do, you want it well secured. Most of the ones I have seen use bails on the mast and boom with SS rivets holding them in place, but I only have experience with smaller boats than yours. A through bolt with a compression sleeve is a good idea but would likely require taking the mast down. A through bolt without the compression sleeve would be fine for now, just don't over tighten the nut and definitely use a nylock nut. That existing bail on your boom does not look like it will do the job.
I can install the bail with a compression tube just above the mast step (similar to the first image). I can also drill holes through the mast and the mast step, then thread the mast step holes so I can use bolts to attach the bail. In the second case the position of the bail would be lower.
Which one would be a better approach?
As long as the mast plate/base has sufficient thickness and height for your intended threaded holes, that would work. However, it will complicate dropping the rig as the vang bail would need to be removed first. A sleeved throughbolt just above the base plate would avoid that issue.
I drilled the holes in the mast but also drilled oversize holes in a small aluminum channel that the proper sized nuts just fit into. I epoxied the nuts in place using the pad eye and some screws to make sure the alignment was perfect. Then I laid the mast "face down", taped the channel to 3 lengths of PVC (with a rope thru all of them in case they separated inside the mast) and slid the pipe in. A mark on the pipe told me when I was there. I had to juggle the mast a little and use a thin screw driver as a drift pin. Once the first screw is started, I got the second on in and sent them both home. The nuts are nyloks so they won't back out. Even if they weren't, since they are in the channel, they can't spin off. A tapped plate would do the same thing.
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