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Old 10-10-2011, 06:49   #31
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Re: Preventers

Here one ARC broken boom:

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Mid-boom sheeting and no preventer used. From: Boom goes the boom | Elaine Bunting's Blog | Yachting World
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Old 10-10-2011, 07:09   #32
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Re: Preventers

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But this is not logical at all -- it's not the mainsheet which provides the opposing force to the preventer in a jibe. It's the sail. So the violent force from a gibe is transmitted to the boom at the clew and has nothing to do with where the sheet is belayed. Obviously the closer to the clew the point of attachment of the preventer is, the less bending moment is induced in the boom if you have a violent jibe while your preventer is rigged. Mid-boom preventers are a recipe for a broken boom for this exact reason.

If you saw broken booms on mid-boom sheeted boats, it was because of the mid-boom sheeting, NOT because of how the preventer was rigged. If you don't have your preventer rigged, or if your preventer slips or breaks, and the boom goes over, the boom WILL break if it is sheeted mid-boom. That's why I categorically don't like mid-boom sheeting, which IMHO is a lazy way to cut down the length of the main sheet, at the expense of safety. Where you sheet is not as important as where you rig your preventer, because a properly rigged and strong enough preventer -- rigged at the end of the boom -- will protect the boom from a violent incident with the mainsheet. Still, the mainsheet is the second line of defense, so I would never have a boat with mid-boom sheeting, personally.
Dockhead is spot on, with a very small proviso, that being the max bending moment would be induced with a loosefooted main. If the main is in a track, this will reduce the bending moment on the boom a little (uniform loading, as opposed to a point load)

Purely from an engineering perspective, the connection of the preventer must be at the end of the boom to eliminate excessive loading of the boom in an uncontrolled situation, especially with mid-mounted sheeting. I agree that mid-mounted main sheeting is a convenience thing, not due to some engineering advantage.

Just my 2c.

Bloke

PS: Just saw the picture from the ARC. Look at the section profile size of the boom, this yacht is spec'd for sailing around the cans. Check out any photos of Jessica Watsons 'Pink Lady' and see the spec size of the Mast & Boom...completely different purpose.
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Old 10-10-2011, 07:21   #33
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Re: Preventers

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Originally Posted by Dockhead View Post
Here one ARC broken boom: Mid-boom sheeting and no preventer used.
Two photos in that article:

The one you show is just simply a poorly designed and underspecified boom. It broken broke in a jybe (no preventer) at the mid-boom sheeting point. The boom section obviously needed a significant sleeve reinforcement for that sheeting attachment point. It might have stayed in one piece if they had had an end of boom preventer but then again it might not as it was obviously a weak boom section. Looks like a nice boat but someone needs to shoot the rig designer.

In the other photo (below), of a light weight multi-hull boom, we have operator error . . . they attached the preventer mid-boom when the boom was designed and reinforced for end-of boom sheeting. So, they are adding a load to a non-reinforced section of the boom with an extra long lever arm on a light weight boom design. It's not too surprising it would break. (It looks like a carbon boom and I am surprised it bent rather than shattering).

The moral is end of boom attachment is almost always better.
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Old 10-10-2011, 15:02   #34
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Re: Preventers

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Originally Posted by estarzinger View Post
We happen to use a spectra very low stretch preventer, which we find superior in boat situations.
Agreed. Generally I let the boom out somewhat beyond where I want it to be, with the main just touching the aft lowers. I take the preventer up (end-boom to bow and back to the cockpit) as best I can and then grind the main off the lowers with the main sheet (end-boom sheeting). The boom won't move even if the boom goes in the water, and certainly not just from back winding. The boom shouldn't move from where it is placed. Period. Dot.
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Old 10-10-2011, 15:27   #35
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Re: Preventers

my mizzen i prevent from mid boom. i dont use my main much, the sheeting on my main is mid boom--i would prolly use a line from boom end to hawse for that...i prevent my mizzen to hawse in cockpit. i learned the hard way not to prevent to taff in one of these boats , a staff is decorative only.......with preventer in cockpit i can appropriately allow boom to move sans difficulty.
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Old 10-10-2011, 17:42   #36
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Re: Preventers

I did what Auspicious did offshore. Cant say I got the snatch block all the way to the bow but it was well forward - ease the main more than I want then set the preventer then grind the main back to where it wants to be. Worked for me.
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Old 10-10-2011, 18:27   #37
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Re: Preventers

G'Day All,

Sounds like I am in the minority here, but this is what I have done on my last three boats, in all of which I have made long ocean passages as well as many many coastal miles:

I have rigged a three part tackle on each side, from a reinforced area about 1/3 boom length from the gooseneck to a heavy pad eye just aft of the shrouds. The fall of this tackle is lead aft to a secondary winch. I've used various lines on these tackles... tried 3 strand nylon at first, worrying about shock loads. This was FAR too stretchy, offering almost no control over boom position. Then tried doublebraid nylon which was much better, but still too stretchy. Finally ended up with regular doublebraid dacron. With the length involved there is adequate stretch to ease shock loads, and there's good enough control of boom position. Two of the boats had mid-boom sheeting, one had end-boom.

These tackles are permanently rigged, and our practice is to secure them any time the wind is aft of the beam. Besides acting as a preventer, they act as a powerful vang which pulls nearly straight down and thus doesn't load up the gooseneck like a normal vang does. When one does want to gybe, a couple of wraps on the winch allow you to ease the boom across gently, even in very strong winds. When running very deep under windvane or autopilot one suffers the occasional accidental gybe,often from rolling... and nothing happens! The sail fills on the "other" side, but the boom stays put and soon the vane or pilot corrects the course, the sail pops back and all is well.

The worries expressed about dipping the boom don't seem to have much basis in most current designs, nor in the ones from the 60's and 70's like my previous two boats. And I believe that in conditions where one might conceivably dip the boom, one would be deeply reefed. This means that in most boats the boom would be riding much higher than with full sail, and that even if the end dipped, there would be no sail out there to add drag.

I wouldn't presume to advise anyone else to use this system, but can attest that in our ~150,000 sailing miles in these three boats we have had no boom breakage, have had the security of ALWAYS having a preventer rigged when we were sailing off the wind, and enjoyed easy controlled gybes when required.

Cheers,

Jim

PS The boats involved were a Yankee 30, a Palmer-Johnson Standfast 36 and our current Jon Sayer one-off.
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Old 10-10-2011, 18:36   #38
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Re: Preventers

We rig it only on passages. From the outer half of the boom fore (our boom is only 11'. Tying the preventer at the wrong part of the boom is a first class ticket to break the boom.

But I rigged them in many different ways on other boats - depending on deck layout. Mostly target at being able to control them from the cockpit and not having them catch deck items when gybing.

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Old 10-10-2011, 18:40   #39
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Re: Preventers

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Originally Posted by olegunny View Post
With todays high aspect ratio mains I do not feel it is necessary to attach the preventer to the end of the boom. Valiant yachts (I think a fine off shore boat) attach the preventer at mid-boom along with the main sheet. I have never heard of a Valiant boom bending from dragging the seaway. The preventer lines are much less obstructive than at the extreme end of the boom.
Agreed, I am having a hard time getting my mind around the idea that if you dip a boom a preventer attached to the end of the boom is some how more forgiving than one attached to the middle of the boom.

Currently on our boat the main sheet attaches to bails at the end of the mast. I have read somewhere that attaching the mainsheet tackle to the middle of the boom results in less chance of bending the spar.

On Westsails preventers are often attached via substantial shackle at the aft chain plate, approximately midships. Having experienced sailing short handed at night in a rough, confused sea state with a single line preventer set up to the fore deck I would not use this arrangement again. When I needed to gybe the boat I was "Prevented" from doing this because I couldn't leave the tiller or risk broaching.

My refelctions on this led me to the following conclusions: Preventers are primarily used when the weather is up and the risk of injury to crew and rig increases. Attachment points need to be extremely robust, through bolted and hardware rated for the anticipated loads. Tackle that is led to the fore deck is too far to travel when adjustments or changeovers are required, puting crew at greater risk to handle. Preventers need to be able to absorb shock, incorporating rubber snubbers or some other method of being able to take up some of the initial shock load. Finally I will ultimately set up a preventer "System" that has lines running both to starboard and port that will allow operation from the cockpit. The design for the last setup I read about like this was attached to the mid boom with load rated nylon web straps.
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Old 11-10-2011, 05:30   #40
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Re: Preventers

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Originally Posted by olegunny View Post
With todays high aspect ratio mains I do not feel it is necessary to attach the preventer to the end of the boom. Valiant yachts (...)
Valiant has the boom way off the deck and relatively tall topsides too. And, being cutter, the boom is short. This all helps.

I am not sure how much Valiant (40) compares to Passport. But I know a Passport that broke her boom mid-Indian due to the preventer tied mid-boom.

Cheers,
b.
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Old 11-10-2011, 06:31   #41
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Re: Preventers

Let's suppose you wanted to break a boom. What would you do?

To break a beam or a boom with maximum efficiency and using the least amount of effort, you would do the following:

1. Fix one end of the beam or boom, say, to a wall, or perhaps, to a mast, using a gooseneck.

2. Create a fulcrum in exactly the middle of the boom -- a sawhorse, say -- or, tie a strong line in the middle of the boom, belay the other end of the line, and tighten it.

3. Apply a shock load to the other end of the boom -- say, jump on it, or fix the clew of a sail to it and let that sail get suddenly backwinded in a strong wind.

Voila -- maximum bending moment is applied to the beam or boom, and with the minimum amount of shock loading, it will snap like a dry twig.

You could get the same effect exactly by putting either end of the boom on two sawhorses, and jumping up and down in the middle of the boom (or tying a line to the middle of the boom and applying a strong pull on it). The point is that the distance between to the two opposing forces is what creates the bending moment, which is what breaks the boom (or beam).


If, on the other hand, the boom or beam is tied off at the same or nearly the same place where you are jumping or the clew of your sail is attached, approaching zero bending moment will be applied to the beam or boom (becoming exactly zero bending moment when the forces are exactly aligned), and it rapidly becomes impossible to break it, even with many tons of force -- the clew will rip out sooner than you will damage the boom.

That would be like jumping up and down on the boom (or beam) at the spot where it is supported by one sawhorse. In other words, the worst way to break it, if that is what you were trying to do.

What that means, is that it is really important structurally to have the point of attachment of the clew as close as possible to the point of attachment of the preventer. Any distance between these points creates more and more bending moment in the boom, and thus more and more probability of breaking it.


Selden, who manufacture most of our rigs, says this about it:

"

Preventer guy:
Used to prevent accidental gybes. Attached

to the outer end of the boom, and led forward to a cleat or led via a snatch block on the foredeck, then aft to the cockpit. The preventer guy must not be fitted to the centre of the boom since that could cause damage, especially if the end of the boom goes into the water as a result of rolling."



I agree with them. Attaching a preventer to the middle of a boom is structurally wrong -- and a recipe for disaster. In my opinion, one should never do it.
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Old 11-10-2011, 07:36   #42
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Re: Preventers

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Originally Posted by Jim Cate View Post
I have rigged a three part tackle on each side, from a reinforced area about 1/3 boom length from the gooseneck to a heavy pad eye just aft of the shrouds. The fall of this tackle is lead aft to a secondary winch. I've used various lines on these tackles...
Jim, any chance you could post pictures of this?

Thanks,
-SEC
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Old 11-10-2011, 08:13   #43
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Re: Preventers

For situations where the boom extends way off the deck and snapping the preventer at the end in may prove acrobatic:

- tie a line from the boom's end to a point that you can reach along the boom, then tie the preventer onto a block, or a spectra loop, freeriding on the a/m line,

- tie to a point as far out as you can BUT use an attachment that is BELOW the boom (or else on the opposite side of the boom - the preventer running UNDER the boom) - at least the pull will be more in line with the boom's stronger section.

I think it is good to remember the basic difference between a solid wood boom and an alloy extrusion boom - at all times. The extrusion boom can accept only marginal forces when they are applied on the side of the boom. Ask any rigger, or get an old piece of mast / boom and do the real life test - you WILL be surprised how little it takes.

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Old 11-10-2011, 12:29   #44
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Re: Preventers

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Originally Posted by sec906 View Post
Jim, any chance you could post pictures of this?

Thanks,
-SEC
G'Day Sec,

Sorry, but I don't seem to have any such pix in the computer, and we are about 7000 miles from the boat at present. Will be returning to Oz on 9 Nov, and if I can remember anything that long I could do so then. If you are still interested, send me a PM a few days after our return and I'll have a go at it!

And for all those theorists who believe that your boom will surely break if you follow this plan... well, none of mine have yet, and we have given them ample opportunities.

Incidentally, we do have a friend with a Selden boom which CAME with a mid-boom vang attachment, and to which he attached a preventer (with the local riggers approval). And sure enough, it immediately broke the boom. One should note that these booms are a rather thin walled deep box section, and in this case there was no reinforcement in the vang attachment area. I agree that this construction is ill suited for mid boom preventing. Many booms are much stronger in the horizontal plane than these, and many have reinforcement in the vang and sheet attachment areas.

Again, I'm not advising anyone to follow the plan, but it has worked well for us.

Cheers,

Jim
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Old 11-10-2011, 12:34   #45
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Re: Preventers

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G'Day Sec,

Sorry, but I don't seem to have any such pix in the computer, and we are about 7000 miles from the boat at present. Will be returning to Oz on 9 Nov, and if I can remember anything that long I could do so then. If you are still interested, send me a PM a few days after our return and I'll have a go at it!
Will do; thanks. Of course, it might be better if you just take me along and show me in person...
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