Preventer rigging


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Bill Balme
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My take is that you want a little give in the preventer to avoid any impact loads, so we use some very stout 5/8" double braid penants on the boom and then the main preventer lines are attached by bowline to the penant and are made of 1/2" sta-set double braid. Ours is a 44ft boat with 650 sq ft main.

Bill Balme
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Dick
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Bill Balme - 9/5/2019
My take is that you want a little give in the preventer to avoid any impact loads, so we use some very stout 5/8" double braid penants on the boom and then the main preventer lines are attached by bowline to the penant and are made of 1/2" sta-set double braid. Ours is a 44ft boat with 650 sq ft main.

Hi Neil,
I agree w/ Bill. Nylon, in reasonable sizes, might allow the boom to go full swing if the tip is dipped in a wave as it stretches a great deal. HM lines are like wire and would make transmit shock loads directly into the preventer system un-modified by any cushioning. Double braid Dacron/polyester in ½ inch is my choice (significantly smaller main), but size should be chosen commensurate with load estimates. Mine is regular untreated Dacron by a good manufacturer. Sta-Set is likely also fine: it has been worked to decrease stretch, but does not approach HM line, I believe. My experience w/ Sta-Set X is that it is stiff and hard to handle, less so w/ Sta-Set: fine for halyards, but less user friendly for lines needing handling like sheets etc.
It would be interesting to have a rigger weigh-in. If anyone is near one (I am not), one might ask and give a report. Make sure they are experienced with offshore boats and not just coastal cruisers.
My best, Dick Stevenson, s/v Alchemy

John Franklin
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When I recently overcame barriers to access the Forum, with Bill's assistance, he invited me to start making "controversial posts". So, I'll start here!
I am in fundamental disagreement that the best preventer arrangement is a line from boom end to the bow. I know it is a very popular system but I suggest below what I believe is a far superior system.

Looking at fundamentals, the primary purpose of a preventer is, when running downwind, to prevent an uncontrolled gybe. I would suggest that an equally important use is to restrain a heavy boom from slopping about in a seaway when winds are light. In fact, aboard Al Shaheen, we used our preventer more often for this than running downwind. A sub-set of the first function is to use a preventer system to actually control a gybe.

Any preventer system needs to be effective, and easy and safe to set up in rough weather, preferably without having to go to the bow, and can be controlled from both sides of the boat without having to re-rig after a gybe.
The disadvantage of the "boom end to the bow" arrangement is easily seen from the geometry; the angle between the line and the boom is necessarily acute so the line is not very effective in restraining movement of the boom. Consequently, it is not an effective system for restraining movement of a boom in a seaway.

I suggest a more effective arrangement is as follows if I can explain it adequately without a diagram.

A two-part purchase, on both sides of the boat, positioned just aft of the aft lower shrouds with a line running from a pad-eye on deck through a block with swivel and Wichard hook then back to another block attached to a pad-eye on deck then aft to a rope clutch accessible from the cockpit and positioned so that it can be led to a winch.The block with a Wichard hook is then attached to a boom fitting just aft of the vang. When running downwind with the boom say 70 degrees from the boat's centreline, the attachment point on the boom is nearly vertically above the deck attachment so the boom is restrained very effectively. This system can be set up  hard, especially if led to a winch and it may be attached to the boom (both sides) and left in place even when not sailing downwind, or attached in anticipation of a downwind leg.

Previous posts have commented on shock loads and the strength toe-rails and deck fittings. In this arrangement, when set up properly and properly used, shock loads do not occur. That's not to say that the system should not be designed for shock loads; it should. In my case I had alloy decks of 6mm plate with 10mm inserts welded in high load areas.

I take no credit for this design; it came from the late Mike Pocock, past OCC Commodore formidable designer of ocean cruising boats and was developed from his own experience of a lifetime of ocean cruising.and featured in all his recent ocean cruising boat designs.






Bill Balme
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I knew I should never have helped you onto the forum!!!

You're going to have to draw me a picture, photograph it and email it to me so that I  an upload it and add it to your post. Why? Because, I can't quite picture what you're describing and my preventer system follows what I thought you explained to me back in the Azores in 2013! I followed your advice and now I'm finding out I apparently screwed up!!

From your description, it sounds like a potential boom breaker - so I'd really like to see a picture.

Cheers!




Bill Balme
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Simon Currin
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We have been using a Dutchman for the last 13 years which does all of these things, is permanently rigged, very simple to use and adjustable.

It would, however, be nice to have some downwind sailing to use it more!
Dick
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John Franklin - 9/11/2019
When I recently overcame barriers to access the Forum, with Bill's assistance, he invited me to start making "controversial posts". So, I'll start here!
I am in fundamental disagreement that the best preventer arrangement is a line from boom end to the bow. I know it is a very popular system but I suggest below what I believe is a far superior system.

Looking at fundamentals, the primary purpose of a preventer is, when running downwind, to prevent an uncontrolled gybe. I would suggest that an equally important use is to restrain a heavy boom from slopping about in a seaway when winds are light. In fact, aboard Al Shaheen, we used our preventer more often for this than running downwind. A sub-set of the first function is to use a preventer system to actually control a gybe.

Any preventer system needs to be effective, and easy and safe to set up in rough weather, preferably without having to go to the bow, and can be controlled from both sides of the boat without having to re-rig after a gybe.
The disadvantage of the "boom end to the bow" arrangement is easily seen from the geometry; the angle between the line and the boom is necessarily acute so the line is not very effective in restraining movement of the boom. Consequently, it is not an effective system for restraining movement of a boom in a seaway.

I suggest a more effective arrangement is as follows if I can explain it adequately without a diagram.

A two-part purchase, on both sides of the boat, positioned just aft of the aft lower shrouds with a line running from a pad-eye on deck through a block with swivel and Wichard hook then back to another block attached to a pad-eye on deck then aft to a rope clutch accessible from the cockpit and positioned so that it can be led to a winch.The block with a Wichard hook is then attached to a boom fitting just aft of the vang. When running downwind with the boom say 70 degrees from the boat's centreline, the attachment point on the boom is nearly vertically above the deck attachment so the boom is restrained very effectively. This system can be set up  hard, especially if led to a winch and it may be attached to the boom (both sides) and left in place even when not sailing downwind, or attached in anticipation of a downwind leg.

Previous posts have commented on shock loads and the strength toe-rails and deck fittings. In this arrangement, when set up properly and properly used, shock loads do not occur. That's not to say that the system should not be designed for shock loads; it should. In my case I had alloy decks of 6mm plate with 10mm inserts welded in high load areas.

I take no credit for this design; it came from the late Mike Pocock, past OCC Commodore formidable designer of ocean cruising boats and was developed from his own experience of a lifetime of ocean cruising.and featured in all his recent ocean cruising boat designs.






Hi John,
If I understand you correctly, your preventer is attached to the boom just aft of the vang (perhaps a third of the way aft of the gooseneck) and goes to the side deck just aft of the shrouds and then back to the cockpit for adjustment with a b&t arrangement between boom and side deck to a winch for increased purchase. It sounds like there is only one attachment point to the boom.
Your use of this arrangement is to control the heavy boom from slopping around in light air and leftover swell. I believe this is a good idea, works well, and we do the same on Alchemy. But that use, to my mind, is more accurately described as a side deck boom vang than as a preventer. Although it can function as both, its main purpose is to keep the boom down and immovable. It can stop unintentional gybes, but with the lever arm so close to the gooseneck, a violent un-controlled gybe might damage the boom.
And, yes, a preventer is to prevent uncontrolled gybes, but it is also in place to prevent damage to the boat/boom if the boom tip gets dipped into the sea: perhaps when the roll of the boat coincides with a wave top. This is an admittedly rare, but with enough ocean time, also a predictable event.
This is a potent way shock loads might occur. The boom end would immediately be dragged aft and, in the arrangement you describe, the load would go directly to the point-loaded part of the boom just aft of the vang attachment. This, as you mentioned, is set up hard and is almost vertical for very little give. I suspect, the boom would likely just fold up like cardboard (or the attachment to the boom would give way). It is this scenario that a preventer to the boom end is designed to prevent in addition to an uncontrolled gybe.
I believe this why preventers are almost always recommended to boom end. Yes, it causes some extra work when gybing the boat, but in ocean passages, this will not often occur and going to the bow when in downwind conditions is not so fearsome.
My best, Dick Stevenson, s/v Alchemy



Dick
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Simon Currin - 9/12/2019
We have been using a Dutchman for the last 13 years which does all of these things, is permanently rigged, very simple to use and adjustable.It would, however, be nice to have some downwind sailing to use it more!

Hi Simon,
Would the Dutchman release enough and quickly enough to prevent boom damage were you to dip the boom end into the sea at speed? 
Thanks, Dick
John Franklin
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Dick - 9/12/2019
John Franklin - 9/11/2019
When I recently overcame barriers to access the Forum, with Bill's assistance, he invited me to start making "controversial posts". So, I'll start here!
I am in fundamental disagreement that the best preventer arrangement is a line from boom end to the bow. I know it is a very popular system but I suggest below what I believe is a far superior system.

Looking at fundamentals, the primary purpose of a preventer is, when running downwind, to prevent an uncontrolled gybe. I would suggest that an equally important use is to restrain a heavy boom from slopping about in a seaway when winds are light. In fact, aboard Al Shaheen, we used our preventer more often for this than running downwind. A sub-set of the first function is to use a preventer system to actually control a gybe.

Any preventer system needs to be effective, and easy and safe to set up in rough weather, preferably without having to go to the bow, and can be controlled from both sides of the boat without having to re-rig after a gybe.
The disadvantage of the "boom end to the bow" arrangement is easily seen from the geometry; the angle between the line and the boom is necessarily acute so the line is not very effective in restraining movement of the boom. Consequently, it is not an effective system for restraining movement of a boom in a seaway.

I suggest a more effective arrangement is as follows if I can explain it adequately without a diagram.

A two-part purchase, on both sides of the boat, positioned just aft of the aft lower shrouds with a line running from a pad-eye on deck through a block with swivel and Wichard hook then back to another block attached to a pad-eye on deck then aft to a rope clutch accessible from the cockpit and positioned so that it can be led to a winch.The block with a Wichard hook is then attached to a boom fitting just aft of the vang. When running downwind with the boom say 70 degrees from the boat's centreline, the attachment point on the boom is nearly vertically above the deck attachment so the boom is restrained very effectively. This system can be set up  hard, especially if led to a winch and it may be attached to the boom (both sides) and left in place even when not sailing downwind, or attached in anticipation of a downwind leg.

Previous posts have commented on shock loads and the strength toe-rails and deck fittings. In this arrangement, when set up properly and properly used, shock loads do not occur. That's not to say that the system should not be designed for shock loads; it should. In my case I had alloy decks of 6mm plate with 10mm inserts welded in high load areas.

I take no credit for this design; it came from the late Mike Pocock, past OCC Commodore formidable designer of ocean cruising boats and was developed from his own experience of a lifetime of ocean cruising.and featured in all his recent ocean cruising boat designs.






Hi John,
If I understand you correctly, your preventer is attached to the boom just aft of the vang (perhaps a third of the way aft of the gooseneck) and goes to the side deck just aft of the shrouds and then back to the cockpit for adjustment with a b&t arrangement between boom and side deck to a winch for increased purchase. It sounds like there is only one attachment point to the boom.
Your use of this arrangement is to control the heavy boom from slopping around in light air and leftover swell. I believe this is a good idea, works well, and we do the same on Alchemy. But that use, to my mind, is more accurately described as a side deck boom vang than as a preventer. Although it can function as both, its main purpose is to keep the boom down and immovable. It can stop unintentional gybes, but with the lever arm so close to the gooseneck, a violent un-controlled gybe might damage the boom.
And, yes, a preventer is to prevent uncontrolled gybes, but it is also in place to prevent damage to the boat/boom if the boom tip gets dipped into the sea: perhaps when the roll of the boat coincides with a wave top. This is an admittedly rare, but with enough ocean time, also a predictable event.
This is a potent way shock loads might occur. The boom end would immediately be dragged aft and, in the arrangement you describe, the load would go directly to the point-loaded part of the boom just aft of the vang attachment. This, as you mentioned, is set up hard and is almost vertical for very little give. I suspect, the boom would likely just fold up like cardboard (or the attachment to the boom would give way). It is this scenario that a preventer to the boom end is designed to prevent in addition to an uncontrolled gybe.
I believe this why preventers are almost always recommended to boom end. Yes, it causes some extra work when gybing the boat, but in ocean passages, this will not often occur and going to the bow when in downwind conditions is not so fearsome.
My best, Dick Stevenson, s/v Alchemy



Dick
You have correctly interpreted my description of the "Pocock" design of preventer system as fitted to many of his ocean cruising boat designs notably Blackjack (the late Mike Pocock), Troubadour (the late Stuart Ingram) , Sadco (the late Noel Marshall), as well as Al Shaheen. In the case of Taonui (Tony Gooch) now renamed Moli (Randall Reeves) Mike Pocock redesigned the rig after Tony's dismasting in the south Atlantic. All these OCC members have put in collectively hundreds of thousands of ocean miles in rugged ocean passages without, to my knowledge experiencing problems arising from "boom in the water". I am a conservative ocean sailor and in circumstances where boom-in-the-water was likely I would reduce sail, itself quite easy downwind with the Pocock reefing system.
You commented " It can stop unintentional gybes, but with the lever arm so close to the gooseneck, a violent un-controlled gybe might damage the boom". Not can, it does stop unintentional gybes! With the boom held almost immobile there is very little energy in the gybe so boom damage is very unlikely.
In my 60 year sailing career I have experienced many boats with "boom end to bow " preventer systems and can confidently say that I am a complete convert to the system I described which I believe to be infinitely superior. Were I to be commissioning a new boat I would undoubtedly specify it again.

Dick
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John Franklin - 9/12/2019
Dick - 9/12/2019
John Franklin - 9/11/2019
When I recently overcame barriers to access the Forum, with Bill's assistance, he invited me to start making "controversial posts". So, I'll start here!
I am in fundamental disagreement that the best preventer arrangement is a line from boom end to the bow. I know it is a very popular system but I suggest below what I believe is a far superior system.

Looking at fundamentals, the primary purpose of a preventer is, when running downwind, to prevent an uncontrolled gybe. I would suggest that an equally important use is to restrain a heavy boom from slopping about in a seaway when winds are light. In fact, aboard Al Shaheen, we used our preventer more often for this than running downwind. A sub-set of the first function is to use a preventer system to actually control a gybe.

Any preventer system needs to be effective, and easy and safe to set up in rough weather, preferably without having to go to the bow, and can be controlled from both sides of the boat without having to re-rig after a gybe.
The disadvantage of the "boom end to the bow" arrangement is easily seen from the geometry; the angle between the line and the boom is necessarily acute so the line is not very effective in restraining movement of the boom. Consequently, it is not an effective system for restraining movement of a boom in a seaway.

I suggest a more effective arrangement is as follows if I can explain it adequately without a diagram.

A two-part purchase, on both sides of the boat, positioned just aft of the aft lower shrouds with a line running from a pad-eye on deck through a block with swivel and Wichard hook then back to another block attached to a pad-eye on deck then aft to a rope clutch accessible from the cockpit and positioned so that it can be led to a winch.The block with a Wichard hook is then attached to a boom fitting just aft of the vang. When running downwind with the boom say 70 degrees from the boat's centreline, the attachment point on the boom is nearly vertically above the deck attachment so the boom is restrained very effectively. This system can be set up  hard, especially if led to a winch and it may be attached to the boom (both sides) and left in place even when not sailing downwind, or attached in anticipation of a downwind leg.

Previous posts have commented on shock loads and the strength toe-rails and deck fittings. In this arrangement, when set up properly and properly used, shock loads do not occur. That's not to say that the system should not be designed for shock loads; it should. In my case I had alloy decks of 6mm plate with 10mm inserts welded in high load areas.

I take no credit for this design; it came from the late Mike Pocock, past OCC Commodore formidable designer of ocean cruising boats and was developed from his own experience of a lifetime of ocean cruising.and featured in all his recent ocean cruising boat designs.






Hi John,
If I understand you correctly, your preventer is attached to the boom just aft of the vang (perhaps a third of the way aft of the gooseneck) and goes to the side deck just aft of the shrouds and then back to the cockpit for adjustment with a b&t arrangement between boom and side deck to a winch for increased purchase. It sounds like there is only one attachment point to the boom.
Your use of this arrangement is to control the heavy boom from slopping around in light air and leftover swell. I believe this is a good idea, works well, and we do the same on Alchemy. But that use, to my mind, is more accurately described as a side deck boom vang than as a preventer. Although it can function as both, its main purpose is to keep the boom down and immovable. It can stop unintentional gybes, but with the lever arm so close to the gooseneck, a violent un-controlled gybe might damage the boom.
And, yes, a preventer is to prevent uncontrolled gybes, but it is also in place to prevent damage to the boat/boom if the boom tip gets dipped into the sea: perhaps when the roll of the boat coincides with a wave top. This is an admittedly rare, but with enough ocean time, also a predictable event.
This is a potent way shock loads might occur. The boom end would immediately be dragged aft and, in the arrangement you describe, the load would go directly to the point-loaded part of the boom just aft of the vang attachment. This, as you mentioned, is set up hard and is almost vertical for very little give. I suspect, the boom would likely just fold up like cardboard (or the attachment to the boom would give way). It is this scenario that a preventer to the boom end is designed to prevent in addition to an uncontrolled gybe.
I believe this why preventers are almost always recommended to boom end. Yes, it causes some extra work when gybing the boat, but in ocean passages, this will not often occur and going to the bow when in downwind conditions is not so fearsome.
My best, Dick Stevenson, s/v Alchemy



Dick
You have correctly interpreted my description of the "Pocock" design of preventer system as fitted to many of his ocean cruising boat designs notably Blackjack (the late Mike Pocock), Troubadour (the late Stuart Ingram) , Sadco (the late Noel Marshall), as well as Al Shaheen. In the case of Taonui (Tony Gooch) now renamed Moli (Randall Reeves) Mike Pocock redesigned the rig after Tony's dismasting in the south Atlantic. All these OCC members have put in collectively hundreds of thousands of ocean miles in rugged ocean passages without, to my knowledge experiencing problems arising from "boom in the water". I am a conservative ocean sailor and in circumstances where boom-in-the-water was likely I would reduce sail, itself quite easy downwind with the Pocock reefing system.
You commented " It can stop unintentional gybes, but with the lever arm so close to the gooseneck, a violent un-controlled gybe might damage the boom". Not can, it does stop unintentional gybes! With the boom held almost immobile there is very little energy in the gybe so boom damage is very unlikely.
In my 60 year sailing career I have experienced many boats with "boom end to bow " preventer systems and can confidently say that I am a complete convert to the system I described which I believe to be infinitely superior. Were I to be commissioning a new boat I would undoubtedly specify it again.

Hi John,
Perhaps we might have to agree to disagree on this on.
It may be boat size: mine is smaller, I believe, than most you refer to and this makes me closer to the water. I also know that when I have my sails built, I have it done so that reefs lift the boom, which also makes it less likely to dip end in water when in heavier weather.
I like what you describe as it always has the boom triangulated and under control as I consider the boom as one of the more dangerous objects on a boat and keeping its movement predictable is an important safety feature. It is what we do on Alchemy at all times as well.
And I agree that a side decks preventer can prevent gybes as there is little energy with the boom not moving to a bit of back-winding if course is regained relatively quickly. It also happens on Alchemy occasionally. That said, there could be a lot of energy in a serious backwinding if an autopilot fails or a big wave slews the stern around.
All in all, I very much advocate an end of boom preventer line when running hard in high seas.
My best, Dick Stevenson, s/v Alchemy

Simon Currin
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Dick
Absolutely yes it would. It is infinitely adjustable SI can be a boom brake or a boom lock. We use ours as a brake and only lock it to secure the boom when the sail is not up. Bill McLaren managed to break his boom by forgetting to release a conventional preventer before gybing (planned). That wouldn’t have happened with a Dutchman.
Simon

Dick - 9/12/2019
Simon Currin - 9/12/2019
We have been using a Dutchman for the last 13 years which does all of these things, is permanently rigged, very simple to use and adjustable.It would, however, be nice to have some downwind sailing to use it more!

Hi Simon,
Would the Dutchman release enough and quickly enough to prevent boom damage were you to dip the boom end into the sea at speed? 
Thanks, Dick



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