Preventer line size/diameter - SailNet Community

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Old 07-17-2008
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Preventer line size/diameter

Dear listmates:
I'm rigging a traditional preventer on my Valiant 40, and am curious what size line you have used on your setups. I'm leaning toward double braid because it's the strongest across the same diameter line, but am not sure whether to use 1/2" or thicker.
Your thoughts are appreciated!

Richard
Grebe, V40
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Old 07-18-2008
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I use 1/2 inch on my main, which would roughly be the size of a Valient 40 main.

Ilenart
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Old 07-18-2008
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Richard-

I wouldn't use a double braid for this... if you can get some New England Ropes StaSetX, it would make far more sense. It is stronger than an equivalent diameter double braid. If you want a thinner preventer line, you could go with NE Ropes T-900, which is stronger and lighter than 1/2" polyester double braid. Use a section of nylon line to act as a shock absorber.
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Last edited by sailingdog; 07-18-2008 at 09:19 AM.
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Old 07-18-2008
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I would recommend a double braid for a little built-in snubber effect due to the stretch. The high tech lines don't have much stretch. That's why you wouldn't use them as dock lines. You don't want to transmit any more shock to your boom than you have to in the event of it trying to gybe on you.
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Old 07-18-2008
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I have 320 sq feet on my main and use 3/8 inch double braid, both shock value and because if I want to I can put it on my winch. My preventer is 75 ft of line with snap shackles that connect to the end of the boom - go all the way forward to my center / anchor / cleat with a block on it then back to the other side and around the winch; a separate line goes from the boom aft and is cleated off - this tensions my boom both fore and aft so it literally can not move in either direction.
To gybe I disconnect the aft line, release the winched line going forward to the cleat and back to the boom and center the boom, swap the ends (previously shackled to the boom end goes to the winch, winch end goes to the boom complete with a shackle) set the boom, swap the aft line to the other side and cleat.
That allows me to do a controlled gybe and not have to leave the cockpit at any time.
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Preventer details

Thanks for the helpful responses, folks. I am also curious as to whether you are also using a tackle with your preventers and, if so, whether 2:1, 3:1, etc. ratio. With a 450 square foot main (or so), I assume I would need some assistance in tightening up the line. Thanks again!

Richard
Grebe, Valiant 40
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Old 07-18-2008
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I actually use and recommend using a boombrake, rather than a static preventer line. IMHO, a static preventer line is more dangerous, since it can keep the boat pinned down and cause the cabin to downflood.
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traditional preventers vs boom brakes

I've seriously considered a boom brake like the Dutchman: a very well built piece of equipment and it does give you control over the gybe. But, it also creates a tripping hazard on both side decks and, worse, if you're going forward while clipped to a jack line, you've got to unclip for what could be a crucial moment. The tripping hazard isn't as serious with the traditional preventer and you don't have to unclip the tether while going forward. You're absolutely right, though, about the danger of flooding the cabin, etc. if you can't release the (now) leeward preventer line.
I'm also a bit concerned about breaking the boom if it swings around hard, though I don't know of any actual reports of this happening. BTW, the boom brake does need a tackle, at least for a larger mainsail, which makes me think that it would be a good idea with a traditional preventer as well.

Richard
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Old 08-16-2008
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Belt AND suspenders

I use a boom brake AND a tradional boom-end preventer.

A preventer is great, but you really don't want it to be the only thing restraining the boom when the poo hits the proverbial...

A preventer's principal role is to keep the boom (and mainsail) from flogging when running in moderate conditions, especially when the main is set by the lee. Under these conditions, the rolling of the boat can make the boom swing back far enough to backwind the main and start off an accidental gybe.

Regardless of how you set it up, during a planned gybe the preventer has to be let off and re-set on the lee side. SO, this is NOT a part of the running rigging that you can (or should) rely on when the going gets rough or you need to do things quickly. This was graphically illustrated to me recently when crewing on a fifty footer in the Sydney - Gold Coast race: with wind gusting to 45 knots we had to gybe, and opted for a "granny gybe" (tacking the bow through the wind) to reduce the strain on the rig. The crew responsible for the preventer was a bit slow throwing it off, resulting in a pad-eye ripped off the bow and a broken preventer line which proceeded to wrap itself around the prop... Thank gawd we didn't attempt a gybe!

Also, if you really get the boat out of shape and the main is backwinded, you need to release the preventer pretty smartly, and if you can't, it needs to be fail-safe - i.e the line should break at a lowish force rather than a high one (this is where the boom brake comes in). Imagine the 2 tonnes of force in a 1/2in preventer being released instantaneously when it breaks! I use 8mm polyester braid (stretchy) for my preventer and it runs all the way to the stem, through a snatchblock and back to a jammer on the cabin top where I can reach it from the cockpit and put it on a winch to set and (most importantly) ease off gradually when necessary.

A boom brake has a number advantages:
  • The forces on the boom are controlled by friction, so you can never over-stress the boom with a boom brake.
  • Depending on how you set it up, it can assist your boom vang by pulling down on the boom, or can even be an emergency main-sheet.

My boom brake is very simple - a figure 8 "rescue descender" used for rock climbing which i got off EBay for $45 and 20ft of 1/2in polyester braid. The line is shackled to the starboard toe-rail and runs through the figure 8 to the other toerail, through a block shackled to the toe rail and then back up to the cabin top jammers via a free sheave in the line organiser. I use a winch to grind it on. Because of the set-up, it effectively has a 2:1 purchase.

There's a photo of the boom brake on my blog at svsunnyspels.com.

The boom brake does create a bit of an obstacle on the side decks, but you get used to it pretty quickly. I take the point re. the jacklines. I run my jacklines OVER the boombrake lines, which also keeps the tether hooks off the deck, at least over that area. However, having an intermediate "catch point" in your jacklines is not such a bad thing - if you get washed along the deck you'll get stopped midway rather than dangling over the transom!
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Old 08-16-2008
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GMALAN, so you jacklines are not taut?

I agree about the boom brake, I have not used one, but SD has convinced me of the advantages.

In using a preventer, yes, it is possible to break the boom, it has happened, so care is need is calculating the loads and sizing the line.
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