Boom Preventer

[Cruisin Cat]

Originally Posted by Capt. Don

A boom with a line stretched between ends is no different than a mast with a sail stretched between both ends.

....except that the mast is designed to have a line stretched between its ends because that's the only may to raise the main, whereas the boom is not designed to have this high tech, expensive and totally unnecessary line stretched between its ends because fortunately manufacturers (or most of them at least) know enough about vectors mechanics to recognize that the same result can be achieved by simply attaching the preventer to the clew end of the boom.

If you attach the preventer part way along the boom, then you do introduce significant bending moment to the boom at the point where the preventer is attached. This underboom line is probably a better arrangement than that, but not as good, simple or cheap as simply attaching the preventer to the clew end of the boom where you would otherwise attach the underboom line

[jr_spyder]

Interesting thread. A couple points come up for me:

I have an in-mast furling main, so the only attachment of the main to the boom is at the end (and the boom is end sheeted). This puts a very obvious compression load on the boom and it's quite clear to me that my outhaul line caries more stress than any other line on the boat, and that's pure compression on the boom. No problem.

Last year I installed a Dutchman boom brake and love it. It attaches to the boom just aft of the vang, about 1/3 down the boom's length. A thought mostly missing in this thread about a brake is that it's load carrying ability is limited - on purpose - so it allows slippage of the line through the brake when necessary. I can hardly winch mine down tight enough to get to zero slippage - and you shouldn't anyway. It's a brake, not a preventer. I like how it gently manages my boom in a lumpy sea and the fact that I don't have to worry about any points of over stress in my boom, even in the unlikely event of dipping it into the sea. Also, unlike a preventer, the brake is always rigged and at the ready. Just firm up the line when you head downwind, or in light airs and a lumpy sea, and the boom is under instant control. I'm fortunate to have auxiliary winches just aft of my cockpit that I can put this brake control line on, so I have very easy access. It's a great addition to our comfort and lower stress level about accidental gybes.

John

[Andrew Troup]

Originally Posted by Cruisin Cat

....except that the mast is designed to have a line stretched between its ends because that's the only may to raise the main, whereas the boom is not designed to have this high tech, expensive and totally unnecessary line stretched between its ends


because fortunately manufacturers (or most of them at least) know enough about vectors mechanics to recognize that the same result can be achieved by simply attaching the preventer to the clew end of the boom.

I agree with your conclusion but I cannot agree with your explanation - in many cases the extrusion used for a boom is barely distinguishable from that for a mast.

It seems to me that what Capt. Don's reasoning fails to take into account is that, having set up a tight mainsail luff from masthead to gooseneck, we do NOT attach a horizontal line to that luff, part way up, hook it up to a distant towboat and tell it to go away.