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Old 22-05-2013, 18:15   #31
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Re: Boom Preventer

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Originally Posted by Capt.Don View Post
I read somewhere that you should add a light breaking link to the preventer to that would fail before sufficient loads are placed on the boom, gooseneck.... As I recall, this was 4 or 5 lashings of light 1/8" line formed into a loop, connected between the deak/cleat and boom.
A better approach (with toerail vang-preventers) is a system of staggered lashings from the attachment point on the sidedeck.

The first might be a couple of turns, the second longer loop of four turns, and a third, longer still, of six turns (say). By the time the third lashing comes under load, the boom is just short of the cockpit, and the load from wind or sea no longer has sufficient leverage to break anything.
The progressive breaking of the first two lashings absorbs enough energy for the third lashing to hold.

There are tidier ways of arranging this (eg a stack-pack of zigzagged webbing, sewn cleverly) but the lashing one is easy to arrange, test and modify.
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Old 22-05-2013, 18:20   #32
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Re: Boom Preventer

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Originally Posted by Paul Elliott View Post
I think he means that the "short arc" preventer won't let the boom rise if you start dragging it through the water.

That's true, but (As you say) the vang will not let it rise either, so it will probably break in any case if it sticks hard in the water.

However, because it has to clear my hard dodger, my boom is way high and generally does not get anywhere near the water.
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Evans

Unless your boom is shorter in relation to your beam than I recollect, it seems to me the preventer alignment you describe (from near the end of the boom to chainplates) would create a fair bit of thrust into the gooseneck and mast if the main were caught aback?

Perhaps, probably more a 'peeling load' on the gooseneck than a compression load. All I can say is that things have been engineered so that it works, and has not broken (yet). I can see how moving the block to the bow would reduce that a bit, but (a) it would eliminate the downward 'vang' pull of the preventers (Which I find useful), and (b) does not allow an angle for 'inside the lifeline preventing' (useful in light air/big sell), and (c) could wrap/load the preventer line sideways on the stays in an accidental gybe (probably not a real problem, but would be an unexpected loading angle on the stays).

So, I could move the block up to the bow in a minute and have plenty of line to do so, and would love to hear some good argument it would be better, but so far I had thought the chainplate location worked better.
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Originally Posted by Capt.Don View Post
I read somewhere that you should add a light breaking link to the preventer to that would fail before sufficient loads are placed on the boom, gooseneck.... As I recall, this was 4 or 5 lashings of light 1/8" line formed into a loop, connected between the deak/cleat and boom.
usually called a fuse. I have mixed feelings about them in preventers, and don't currently have one. Fuses add some uncertainty into a dynamic situation - is the preventer going to hold or is the fuse going to break and let it gybe? I have decided that I like to know for sure the preventer is going to hold, and the risk (on my particular boat) of a broken boom (which the fuse addresses) is pretty small.
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Old 22-05-2013, 18:25   #33
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Re: Boom Preventer

I've dipped the boom during aggressive spinnaker runs, especially where a quartering sea is making things interesting. If not the full "death roll", we've experienced the high-speed round-up, dragging the boom in the water. Too often this is immediately followed by a round-down (due to over-correction my the helmsman), a broken spinnaker pole, and the boom in the air, held up by the preventer with the wind-filled main pinning us down.

We need to be able to unload the spinnaker (by this point often underwater), and gently release the preventer, which drops the boom and gets up back on our feet.

It's loads of fun! And definitely loads of strain on the boom, gooseneck, mast, etc.
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Old 22-05-2013, 18:35   #34
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Re: Boom Preventer

I've used (and do use) lashings at the preventer deck-attachment point, acting as fuses. My difficulty with these is the shock-loads from light-air slatting (one of the times I really like to use the preventers). By the time I make the lashings strong enough to withstand 24 hours of heavy slatting, I'm guessing that they are too strong to function as desired. I do tighten everything to reduce the motion under slatting conditions, but we're dealing with irresistible forces and immovable objects here...
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Old 22-05-2013, 18:53   #35
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Re: Boom Preventer

For discussion purposes . . . .this shows a chainplate preventer and a bow preventer with the boom at its 'running' angle.

The chainplate preventer is 12 degrees to the boom angle and the bow preventer is 19 degrees to the boom angle.

Obviously this depends on what angle you run the boom at, because we have swept back spreaders and full battens we tend to keep it in a bit (the battens are rubbing on the stays at this angle). On Silk we might have squared it a bit more than this. But the below shows our actual usual operating angles.


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Old 22-05-2013, 18:57   #36
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We have a Walder boom brake and like it, but it doesn't lock down the boom like my old toe rail preventers did. I often have to go forward and manually pull the leeward side of the brake down before I tighten the Walder or it tries to bring the boom back to center. I may switch back to the mid boom preventer we had before (we are mid boom sheeting).

Question I did have- If I'm using the preventers, doesn't this also pull down the boom like a vang? What reason would there be to keeping the vang? We still have a topping lift so don't need the rigid vang for that and can't see any other reason for one.

Matt
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Old 22-05-2013, 18:58   #37
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Re: Boom Preventer

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Originally Posted by estarzinger View Post

Thanks. I just looked and their biggest model appears to be rated 'up to 750 sq ft', which I suspect means its too small for Hawk, since we have a particularly high righting moment (and thus higher than average sail loads), but I will ask them.

So, does anyone know of a brake rather higher than the biggest Dutchman one?
I don't know of one bigger. While I plead little experience I found that our Dutchman needed to be tensioned up pretty good to do the job. I have the line lead back to a #44 winch. I find small adjustments in tension have big effects.

I thought it worked well but took some getting used to. Im still not sure I have the tension knob set right.

We had the lines led out to the toe rails, but the Wife objected to the tripping hazard. Now I have welded some tangs to the edge of the coach roof, hopfully they will provide sufficient separation. We will see, but not until I get her back in the water since replacing the sprit. Probabably end of August as I am off soon to pick up our other boat in Lewisporte and bringing her down to Delaware City.
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Old 22-05-2013, 19:09   #38
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Re: Boom Preventer

Quote:
Originally Posted by estarzinger View Post
For discussion purposes . . . .this shows a chainplate preventer and a bow preventer with the boom at its 'running' angle.

The chainplate preventer is 12 degrees to the boom angle and the bow preventer is 19 degrees to the boom angle.

Obviously this depends on what angle you run the boom at, because we have swept back spreaders and full battens we tend to keep it in a bit (the battens are rubbing on the stays at this angle). On Silk we might have squared it a bit more than this. But the below shows our actual usual operating angles.


Attachment 61175

Ah, the VISUAL thinker!

Helpful.

So, if you were to show this with a mid boom connection the angles would be much greater, and I think the boom would load up faster. Not only a more abrupt angle, but also less stretch in the line.

Not sure of the agvantage or disadvantages.

But also you can see how a boom break needs to withstand some pretty high forces, because it is so close to the boom, there is more lever arm on it. I think.
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Old 22-05-2013, 19:13   #39
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Re: not for big boats

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Your point about only using the preventer in light airs is well taken, and I would venture further to say that skilled sailors would never need to use a preventer other than in light airs.
Say one has a preventer rigged when the breeze is up and then they accidentally gybe? What does the boat do?

I am unlikely to ever know first hand as I would only rig a preventer is light air and rolly seas. Usually I simple steer up a bit to stay far from the slow and dangerous gybe sector. I've pondered the consequences but don't know. Anyone here had this accident?

In what position does the boat end up if the rig remains intact? Beam-to and luffing?
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Old 22-05-2013, 19:20   #40
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Re: Boom Preventer

^^

Boom on the other side, with mid-boom preventers to chainplates and bow. One of you mechanical engineers will have to explain what all the implications are.

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Old 22-05-2013, 19:27   #41
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Re: Boom Preventer

Evans

In my post about angles, what I was intending to suggest (but re-reading, I forgot to actually say so - sorry about that!) was actually that you might consider moving the attachment point further forward on the boom.

(rather than move the other end of the preventer to the bow, as you seem to have assumed I meant...)

If (as you say) your boom is never likely to dip, and as I seem to recall you have a reasonably generous boom section

(I helped you lift it back into position when I happened to come on board a few years back, and you were doing some maintenance - to the gooseneck?):

The bending loads on the boom will be no more severe than if you chose to sheet from that location. I'm happy to "Show my working" on that if anyone is unconvinced.

(bearing in mind that in situation 2 referred to in my ealier post, the loads are in the x-x plane

There would be minor operational advantages, if you could get the attachment point far enough forward, like being able to leave it permanently rigged, but to me the overwhelming advantage is that it much reduces horizontal axis loads into the mast - I don't know about you, but I would prefer the boom to be the first point of failure in extremis, rather than the mast.

Quote:
Originally Posted by estarzinger View Post
usually called a fuse. I have mixed feelings about them in preventers, and don't currently have one. Fuses add some uncertainty into a dynamic situation - is the preventer going to hold or is the fuse going to break and let it gybe? I have decided that I like to know for sure the preventer is going to hold, and the risk (on my particular boat) of a broken boom (which the fuse addresses) is pretty small.
I would urge those who share your reservations (as I do) to consider a progressive fuse - on a boat your size and for the duty, I would favour something more engineered, like a linesman's harness fall arrestor...?

I don't see any point in them if the preventer is led to the bow, however. The load on the preventer would increase progressively as the fuses failed, because of the angles getting worse...
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Old 22-05-2013, 19:30   #42
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Re: Boom Preventer

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Originally Posted by estarzinger View Post
^^

Boom on the other side, with mid-boom preventers to chainplates and bow. One of you mechanical engineers will have to explain what all the implications are.

Attachment 61182
Evans

The diagrams you draw tell enough of the story to allow a useful analysis of a preventer led to the bow, but not to the chainplates

in the latter case there needs to be a side view, because the forces are predominantly in a vertical plane, not horizontal as for a traditional preventer
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Old 22-05-2013, 19:54   #43
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Re: not for big boats

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Say one has a preventer rigged when the breeze is up and then they accidentally gybe? What does the boat do?

I am unlikely to ever know first hand as I would only rig a preventer is light air and rolly seas. Usually I simple steer up a bit to stay far from the slow and dangerous gybe sector. I've pondered the consequences but don't know. Anyone here had this accident?

In what position does the boat end up if the rig remains intact? Beam-to and luffing?
Provided the gybe is not the result of a major course change or windshift (like 30 degrees) and someone can get to the helm in time, it's just a matter of turning the bow back downwind to recover the situation. This is why preventers are an awesome 'peace of mind' strategy in a big breeze when there's someone on the helm. It doesn't matter so much if they're not great at steering generally, as long as they are coached to develop this single imperative 'conditioned reflex'.

Contrary to what people imagine, the force trying to spin the boat does not build immediately, and because of the boat's speed, there's considerable steerage way.

On a 20 footer, you might only have a second or less, but on a big boat it might be three seconds, or five even.

If nobody is at the helm it get more difficult to generalise. The helm might be on autopilot, vane or brake (governing what rudder angle is adopted) also: Fin keel, or long?

There is no simple answer: I have seen everything from a reverse hove-to approximation, with the main to weather and the boat heeling the wrong way (not comfortable, but not the worst possible) to the boat continuing around to heave itself to on the original (other) tack - which is ideal - or sail off on a reach on that other tack.

If the helm is jammed hard-over, it generally results in either a conventional or a reverse hove-to, unless there's a big headsail complicating things.

Worst case (and I'm extrapolating here because I haven't ever taken that long to respond, or had the guts to leave things to their own devices for that long) would be to carry on round and do it all over again.

This could only happen, it seems to me, if

a) the helmsman or autopilot reversed the helm several times during the cycle, misguidedly

Unfortunately, an autopilot is likely to do exactly this, because it assumes the boat is going forwards, not making sternway

.... or possibly

b) if the seas were so big that they took charge, overwhelming the 'control signals' from the wind and rudder
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Old 22-05-2013, 20:36   #44
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Re: Boom Preventer

When talking about light airs, boom brakes are useless I think? I never understood why people want those.

For heavy weather, you can't discuss preventers without discussing other related features like how the reefs in the mainsail are handled. Each extra reef should lift the aft end of the boom further up so as to create more distance from the water on a run with preventer set. Last time this was discussed most had never even heard of this.

We have a ketch so everything doubles. The preventers for the main are one continuous line that is tied around the end of the boom but through a small paddeye so it doesn't shift position along the boom. From there it goes to a block near the bow and the all the way back to a rope clutch on deck where it can be handled from the cockpit. We selected Lewmar rope clutches with a diameter line so that the weak point is the clutch which will slip. This prevents serious damage elsewhere.

The preventers for the mizzen are the same setup except it's more efficient as the blocks are at the main chainplates so a wide section of the hull for better angles.

Last but not least: since we switched our runners to Amsteel Blue line, we jibe into the set runner which catches the sail and dampens everything a lot.

cheers,
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Old 22-05-2013, 21:19   #45
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Re: Boom Preventer

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My personal preference tends towards reinforcing the boom midsection and fitting beefy tangs each side for a multipart purchase to the toerail immediately aft of the chainplates, permanently rigged (doesn't have to be swapped around the lifelines when bearing away from a close to a broad reach, for example)
Great idea! I'm wondering how much the midsection reinforcement would cost. Would you put it right where the vang attaches, or further aft?
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