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Old 24-05-2013, 02:31   #61
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Re: Boom Preventer

I have about 510 sq ft of main and just bought a Dutchman brake. I haven't rigged it yet, nor have I sailed with one. But I will test this when I go out all July and come back with view on how it fared.
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Old 24-05-2013, 19:31   #62
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Re: Boom Preventer

Quote:
Originally Posted by Andrew Troup View Post
The midlength of the reinforcement should be at the vang attachment point.

This is because the 'bending moment diagram', looking down from above, will be a triangle, whose base is the boom and whose apex is offset, perpendicularly from the vang attachment point, by a distance corresponding to the amount of side load *

The significance of this diagram is that it's a graph of how the loading affects the boom, in terms of making it bend. The peak of the graph coincides with the point of application of the force from the vang preventer.

So an ideal reinforcement would taper from a maximum at the vang tang, to nothing at the clew and gooseneck.
The old timers often attached mainsheets to wooden booms with spans that spread the load better and reduce the maximum bending loads on the boom. I have often thought of doing the same trick with mid boom preventers.

A quick example showed a reduction in the max bending moment from 4 kn to 3 kn, at the cost of some additional compression loads between the spans, which a normal boom should have no issues dealing with.

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Them old timers were pretty clever... For the engineers, is there any downsides to this, could the compression between the attachments cause any problems when you combine a strong BM with a high compression load as may occur in a boom in the water broach?

As for me I normally use a mid boom preventer run inside the lifelines. As long as the boom is solid enough. I don't like the idea of putting all that compression from the boom onto the mast that evans system could cause, but clearly it has worked fine for many years, and the stretch in the system might reduce any shock loads to advantage.

I did see a new Beneteau that lost it's mast when they tried to back the main with a preventer to get off a shoal. the mast collapsed at the gooseneck. as far as I know the boom was fine. This is why I like the hunter gooseneck struts, or some fractional raceboats gooseneck stays.

The biggest problem with twin mid boom preventers is that they obstruct the jacklines along the windward side (or the leeward side). One solution I have come up with is to have two sets of jackstays port and starboard, one inside the preventers and one outside them, so you can work both the windward and leeward decks without having to unclip to get past a preventer.
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Old 24-05-2013, 20:00   #63
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Re: Boom Preventer

Brilliant, Baldrick, brilliant. (My first thought on reading your typically thought-provoking post)

I can see the merits for bendy wooden spars, it's a winner.

However the tensions in the bridle, once you get up to the sorts of side loads a stiff, strong metal boom of large cross-section is expected to deal with, would be colossal, because the angle of spread would be getting up towards 180 deg

(a more acute angle might be OK for racing, but the lifelines and the proximity of boom to chainplates would put a low limit on this. A more acute bridle would also seem to me likely to be a pain for cruising, and fatigue might be an issue if the lazy preventer was not carefully managed so it never flopped about, and certainly never got caught up!)

The tangs would also have to be 'fl*pping strong', and would need something pretty impressive in the way of backing plates, both thickness and extent.

Furthermore, for a mid-boom vang-preventer, the tang nearest the mast is not really providing any resistance to slewing; it might as well be mounted ON the mast. So really it's almost a "Hawk" type, end-boom to chainplate preventer in drag, a kind of sneaky way of negotiating the transection problem with the lifelines, while continuing to apply the same thrust into the mast via the gooseneck.

That's my third thought. My second thought was that for a boom which is inherently stiff, like the typical alu cruising boom, the localised loadings at the two tangs might require more substantial reinforcement than the bending moment at the single tang.


..... but the twin carriageway, sorry jackstay down each sidedeck: It still seems as good an idea as when I first read it: you could make one green and one red on each side to show which should be used when on, say, port tack, given their propensity to intertwine like amorous serpents ....

Your point about the gooseneck struts throws a more coherent light on the 'gooseneck loads' which I raised above, evidently rather confusingly.
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Old 24-05-2013, 20:21   #64
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^^ Andrew. The span i was thinking of was only about a meter. Not over the whole boom. This reduces the load on the fixings and the bridle. With a 60 degree angle the loads on the parts of the bridle and the attachment points will still be less than the loads on a single point fixing. Your point about additional tangling is good. I guess it depends on what you have around the boat. On snowpetrel I had the diesel heater flue right in the way. So I couldnt have permenantly set up preventers.
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Old 24-05-2013, 20:29   #65
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Re: Boom Preventer

^^ just looking at the diagram I posted above I got the angle wrong for the loads... Should have been 45 degrees not 60. Duh! This would mean the bridle would drop about half a meter beneath the boom, for the one meter span, and the loads on the fixing points should not be more than about 3 KN compared to 4 KN in the no span setup. But it's probably worth making each leg capable of taking 4 KN to account for weird loading scenarios with the windward preventer. The angle could be reduced to probably 30 degrees without to much harm if needed.
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Old 25-05-2013, 02:54   #66
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Re: Boom Preventer

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Originally Posted by carstenb View Post
I have about 510 sq ft of main and just bought a Dutchman brake. I haven't rigged it yet, nor have I sailed with one. But I will test this when I go out all July and come back with view on how it fared.
I had one on my much smaller main. It worked very well, I could gybe it under control by carefully slacking it, or lock it and hold the main exactly where I wanted it by keeping it tight.

The only tip I have is to have the tails led back to winches on the port and starboard side so you can always pull the lee side down and forward. I had my control line set only off the port side, and it worked fine on starboard tack. On port tack I used to find when I tightened the control line to lock the boom in place the whole boom would move aft slightly, no major harm but it was annoying from a sail trim point of view.

On smaller boats (under 35 foot or so) a climbers figure 8 works quite well as a boombrake to slow a gybe.
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Old 26-05-2013, 09:25   #67
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Re: Boom Preventer

Thanks for the tip.

I looked at a climbers figure 8 (much cheaper!), but I couldn't find one big enough for my boat.







Quote:
Originally Posted by Snowpetrel View Post
I had one on my much smaller main. It worked very well, I could gybe it under control by carefully slacking it, or lock it and hold the main exactly where I wanted it by keeping it tight.

The only tip I have is to have the tails led back to winches on the port and starboard side so you can always pull the lee side down and forward. I had my control line set only off the port side, and it worked fine on starboard tack. On port tack I used to find when I tightened the control line to lock the boom in place the whole boom would move aft slightly, no major harm but it was annoying from a sail trim point of view.

On smaller boats (under 35 foot or so) a climbers figure 8 works quite well as a boombrake to slow a gybe.
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Old 26-05-2013, 10:05   #68
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Re: Boom Preventer

And why not utilize SnowPetrel's bridle connected to two lashings around the boom each encasing a soft shackle? This would seem to avoid the need for a massive tang and spread the load around the boom a bit. And when you remove it, there's nothing hard to bang about.
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Old 26-05-2013, 10:20   #69
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Re: Boom Preventer

I'm on a different page than Andrew and Evans--a preventer should break before something more important breaks. In my opinion if you gybe during strong winds, the main should come across--spend your money beefing up the gooseneck. The main should always be reefed down so that a crash gybe will not likely break anything--if it will, you should have it down completely.

My preventer for a 650 sq ft main was a four part 3/8 inch tackle with cam cleats, and the weak link was the cam cleat. One day I hit an uncharted reef on the North Queensland coast and had to crash gybe to get off--the preventer broke just when I needed it to, and I was able bounce my way back to weather without losing the boat.

Strengthening parts will have unintended consequences--on one boat we were breaking spinnaker poles, so we went to a heavier pole section--and lost the mast on a roundown 100 miles offshore.
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Old 27-05-2013, 08:08   #70
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Re: Boom Preventer

I guess i am on an even different page..

I have a simple view of the preventer.. to prevent the boom from moving when gravity overcomes the wind pressure... this most typically happens offshore in swells when on a broad reach in light to moderate winds. This is not a high load situation...

My mainsheet is attached to the boom at three places, and a 25ft dockline with a carabiner is attached to the central hoop, looped through the midships cleat and cleated off in the cockpit...this provides forward (albeit at a non-ideal angle) and downward (prevent the vang creaking incessantly) restraint.

In the event i must gybe, it is a managed gybe.. not a crash swing and shout duck gybe..lol
i remove the preventer, centre the traveller, bring in the main, gybe the boat & genoa, ease the traveller, ease the main, replace the preventer...
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Old 13-11-2013, 10:34   #71
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Re: Boom Preventer

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Originally Posted by carstenb View Post
Thanks for the tip.

I looked at a climbers figure 8 (much cheaper!), but I couldn't find one big enough for my boat.
The issue of cost of boom brakes is a common thread on the forums. Since I am a machinist with a reasonably well equipped shop a few sailing buddies asked me to make them boom brakes. So I am hand making 1/2 inch 316L stainless steel modified climbers eights for $65. I designed them for 1/2" ropes but they can take bigger. I think they are probably strong enough to tow a ship with.
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Old 14-11-2013, 07:52   #72
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Re: Boom Preventer

The North sea is not exactly the flatest sea on earth and I am not sure if it helps as my boats a bit smaller but I rigged the simplest 'boom preventer' I could after experimenting with various other designs.

I used the bases of that I had to tack quick, and I had to do it single handed.

As you can see from the pictures below I control the boom via the mainsheet/Traveler ontop of the wheelhouse roof, the main control halyard is to the Starboard side but I also have a single halyard to the port side.

Yes I have to use them both when tacking BUT I do not have any boom crashing over anymore.
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Old 14-11-2013, 08:01   #73
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Re: Boom Preventer

Quote:
Originally Posted by Mystic38 View Post
I have a simple view of the preventer.. to prevent the boom from moving when gravity overcomes the wind pressure... this most typically happens offshore in swells when on a broad reach in light to moderate winds. This is not a high load situation...

My mainsheet is attached to the boom at three places, and a 25ft dockline with a carabiner is attached to the central hoop, looped through the midships cleat and cleated off in the cockpit...this provides forward (albeit at a non-ideal angle) and downward (prevent the vang creaking incessantly) restraint.

In the event i must gybe, it is a managed gybe.. not a crash swing and shout duck gybe..lol
i remove the preventer, centre the traveller, bring in the main, gybe the boat & genoa, ease the traveller, ease the main, replace the preventer...
This is similar to our experience and method. We typically use a boom preventer when sailing directly downwind, wing-and-wing. The preventer keeps the main steady and off to one side, even if the wind shifts a bit or swells would otherwise cause it to swing. We simply run a line from a boom bail to a snatch block on the toe rail, then back to a cleat/winch in the cockpit. You can see pics/details of our simple system in the blog post we wrote last summer.

I've also consided the Gybe Easy, but we aren't often heavy weather so I'm happy with are simple preventer.
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Old 14-11-2013, 20:04   #74
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Re: Boom Preventer

Tynesider, thanks for the post and images. Beautiful vessel. I worked on a 60 foot shrimp boat the winter of 1987 on the North Sea. It was an adventure to say the least with open decks and constant mechanical problems. We are certainly spoiled this side of the pond.
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Old 05-05-2014, 07:54   #75
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Re: Boom Preventer

Here is the best installation instructions for boom brakes and preventers I have seen.

"Preventers and brakes are notorious for breaking booms so should not be installed like most people do.
If its a preventer you use to prevent an accidental boom gybe on a monohull , you will want to fuse it with a small line that will pop if the boom was to drag in the water otherwise you will have catastrophic failure from the water load , on multis it's different, if you drag your boom, you are in bigger problems.
Either system you may use will need to attach to the boom somehow, and most attach it directly causing all the failures you hear about because of the point loading.
Here is what I have been doing for years and have never had any failures because of pure mechanics .
I use a very stout high tech line, either spectra/dyneema, vectran or one of the other flavors and run it along the underside of the boom from front to back. This line needs to be pre stretched to take out any sag and it is lashed tight one end to the other. Anything like a brake or preventer line is attached to this line and can be free running back and forth along the length as it will find its true center . The brake/preventer now puts the boom in pure compression load through the tensioned line, and unless the load generated is incredibly high the boom will never break.
I use a preventer line when sailing downwind (Leopard 47) to vang sheet the main to get proper leech twist. I also never sail any deeper than 150 degrees apparent as this is useless on any boat especially on a multi where you will be going slower than the wind. It is better to gybe downwind and keep the speeds up a little, it is also safer in the fact you won't round down causing an accidental gybe.
Hope this helps you understand the principle." - Troy Bethel

I have now started production (lost wax casting) of 1/2 thick very strong inexpensive 316L SS figure of 8 boom brakes and taking order reservations. Please see my site at: Boom Brakes
I am looking for help with sales so if you think you have the connections to help please contact me through the web site. Thanks
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