Preventer rigging


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Philip Heaton
Philip Heaton
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I saw this on Cruisers' Forum and thought it was worth sharing here but suspect vast majority of OCC Members know how to rig a preventer correctly, still if anyone is not sure here it is:
https://www.maritimenz.govt.nz/commercial/safety/accidents-reporting/accident-reports/documents/Platino-mnz-accident-report-2016.pdf

and the relevant diagram:


Philip Heaton
Philip Heaton
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Philip Heaton - 8/2/2019
I saw this on Cruisers' Forum and thought it was worth sharing here but suspect vast majority of OCC Members know how to rig a preventer correctly, still if anyone is not sure here it is:
https://www.maritimenz.govt.nz/commercial/safety/accidents-reporting/accident-reports/documents/Platino-mnz-accident-report-2016.pdf

and the relevant diagram:


I should say that we rig the preventer from the end of the boom and not in the middle.
Dick
Dick
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Philip Heaton - 8/2/2019
Philip Heaton - 8/2/2019
I saw this on Cruisers' Forum and thought it was worth sharing here but suspect vast majority of OCC Members know how to rig a preventer correctly, still if anyone is not sure here it is:
https://www.maritimenz.govt.nz/commercial/safety/accidents-reporting/accident-reports/documents/Platino-mnz-accident-report-2016.pdf

and the relevant diagram:


I should say that we rig the preventer from the end of the boom and not in the middle.

Hi Philip and all,
Thanks for highlighting some design elements of this important, and often neglected, collection of gear. This gear is rarely called into play, but when it does get called upon, could prevent your rig from catastrophic failure or, worse, prevent a head injury or a crew being swept off the boat.
I looked at the url and found a very long paper. The design alone, however, has features that I believe problematic, possibly dangerous.
First and foremost is the green hash-marked line going forward from a point on the boom about 1/3 back from the aft end. This is a recipe for a broken boom. If, when running hard in seas, you dip your pole end in the water, there will be tremendous pressure on that forward preventer line which will be directly transferred to the boom. The boom is likely to break or bend at the point where the preventer line attaches. Much better to bring the attachment to the boom end where loads will be compressing the boom and not bending it.
The rest of my comments are a bit fuzzy as interpreting the diagram was a bit of a challenge. It looks like preventer lines/pennants are being brought to the toe rail and to snatch blocks. In general, snatch blocks are not up the shock loads that a preventer can generate and toe rails certainly are not. Any connections of this sort on the side deck have to be very robust and installed with significant backing plates.
In general, an offshore preventer, to my mind, should go from boom end to the bow and back to the cockpit where it can be controlled, likely by a cleat rather than a brake. If it is called into use, the forces on the boom are primarily compression, a direction the boom should handle well. When the preventer line is more mid boom, the forces try to bend and fold the boom and the forces are much too likely to succeed.
Come back with questions/comments/thoughts.
My best, Dick Stevenson, s/v Alchemy

Dick
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Dick - 8/3/2019
Philip Heaton - 8/2/2019
Philip Heaton - 8/2/2019
I saw this on Cruisers' Forum and thought it was worth sharing here but suspect vast majority of OCC Members know how to rig a preventer correctly, still if anyone is not sure here it is:
https://www.maritimenz.govt.nz/commercial/safety/accidents-reporting/accident-reports/documents/Platino-mnz-accident-report-2016.pdf

and the relevant diagram:


I should say that we rig the preventer from the end of the boom and not in the middle.

Hi Philip and all,
Thanks for highlighting some design elements of this important, and often neglected, collection of gear. This gear is rarely called into play, but when it does get called upon, could prevent your rig from catastrophic failure or, worse, prevent a head injury or a crew being swept off the boat.
I looked at the url and found a very long paper. The design alone, however, has features that I believe problematic, possibly dangerous.
First and foremost is the green hash-marked line going forward from a point on the boom about 1/3 back from the aft end. This is a recipe for a broken boom. If, when running hard in seas, you dip your pole end in the water, there will be tremendous pressure on that forward preventer line which will be directly transferred to the boom. The boom is likely to break or bend at the point where the preventer line attaches. Much better to bring the attachment to the boom end where loads will be compressing the boom and not bending it.
The rest of my comments are a bit fuzzy as interpreting the diagram was a bit of a challenge. It looks like preventer lines/pennants are being brought to the toe rail and to snatch blocks. In general, snatch blocks are not up the shock loads that a preventer can generate and toe rails certainly are not. Any connections of this sort on the side deck have to be very robust and installed with significant backing plates.
In general, an offshore preventer, to my mind, should go from boom end to the bow and back to the cockpit where it can be controlled, likely by a cleat rather than a brake. If it is called into use, the forces on the boom are primarily compression, a direction the boom should handle well. When the preventer line is more mid boom, the forces try to bend and fold the boom and the forces are much too likely to succeed.
Come back with questions/comments/thoughts.
My best, Dick Stevenson, s/v Alchemy

Hi Philip,
Agree, preventer to boom end. See previous comment. Dick
Bill Balme
Bill Balme
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On Toodle-oo! we rig the preventer whenever we're more than about 110 degrees off the wind. Penants on the boom are connected to a line that goes through a low friction eye attached to the toe rail at the bow and come back to a clutch in the cockpit. We can adjust as that as we trim the mainsheet.

Dick's concerns about the strength of the toerail sound a little alarmist - too much Morgan's Cloud perhaps? - since the tensioning line to the boom lies at such a low angle to the toe rail (not 90 degrees to it) it is certainly well strong enough for the job at hand.

Having a simple to deploy (and therefore always gets deployed) preventer system has been a significant contributor to our safe and enjoyable sailing experience.



Bill Balme
s/v Toodle-oo!

Philip Heaton
Philip Heaton
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Bill Balme - 8/4/2019
On Toodle-oo! we rig the preventer whenever we're more than about 110 degrees off the wind. Penants on the boom are connected to a line that goes through a low friction eye attached to the toe rail at the bow and come back to a clutch in the cockpit. We can adjust as that as we trim the mainsheet.

Dick's concerns about the strength of the toerail sound a little alarmist - too much Morgan's Cloud perhaps? - since the tensioning line to the boom lies at such a low angle to the toe rail (not 90 degrees to it) it is certainly well strong enough for the job at hand.

Having a simple to deploy (and therefore always gets deployed) preventer system has been a significant contributor to our safe and enjoyable sailing experience.


My goodness, having an aluminium boat you take lots of things for granted as pretty much what can be welded is welded. So we have part of the preventer (well two actually, port and starboard) that starts at the end of the boom, runs the length of the boom and has a spliced eye with a Wichard snap shackle that is tied off to a cleat on the boom when not in use. The rest of the preventer is similar with another Wichard snap shackle stowed midships on the guardwires and a long line that goes right forward round a stanchion base and back to the cockpit to a cleat.  So stanchion base is welded to the deck and cannot be ripped out. Cleat at stern is welded and cannot be ripped out. Is there a weak point? Yes the two snap shackles are connected together and could fail but they were seriously expensive and we pray that you get what you pay for - and we have the same set up on both sides of the boat. Has kept us safe so far, and occasionally we do sail by the lee. 
Dick
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Bill Balme - 8/4/2019
On Toodle-oo! we rig the preventer whenever we're more than about 110 degrees off the wind. Penants on the boom are connected to a line that goes through a low friction eye attached to the toe rail at the bow and come back to a clutch in the cockpit. We can adjust as that as we trim the mainsheet.

Dick's concerns about the strength of the toerail sound a little alarmist - too much Morgan's Cloud perhaps? - since the tensioning line to the boom lies at such a low angle to the toe rail (not 90 degrees to it) it is certainly well strong enough for the job at hand.

Having a simple to deploy (and therefore always gets deployed) preventer system has been a significant contributor to our safe and enjoyable sailing experience.



Dick
Dick
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Bill Balme - 8/4/2019
On Toodle-oo! we rig the preventer whenever we're more than about 110 degrees off the wind. Penants on the boom are connected to a line that goes through a low friction eye attached to the toe rail at the bow and come back to a clutch in the cockpit. We can adjust as that as we trim the mainsheet.

Dick's concerns about the strength of the toerail sound a little alarmist - too much Morgan's Cloud perhaps? - since the tensioning line to the boom lies at such a low angle to the toe rail (not 90 degrees to it) it is certainly well strong enough for the job at hand.

Having a simple to deploy (and therefore always gets deployed) preventer system has been a significant contributor to our safe and enjoyable sailing experience.


Hi Bill,
I wrote my comment, as it was my take that someone following the diagram to put together a preventer might go wrong, perhaps seriously wrong, if following the diagram.
You are right, in my understanding, to take your preventer line to the bow as, among other attributes, it gives by far the most forgiving angle to absorb loads which may be asked of it. The diagram has lines to the bow and to amidships. It sounds like we all agree that the preventer should go to the bow. It also sounds like there is general agreement to take the preventer to the end of the boom. This is in contrast to the diagram where the attachment point is well away from boom end. In my comment I was referring to the diagram and only the diagram, at least as I interpreted it and only to those aspects I considered problematic.
Your defense of using the toe rail may be completely warranted when the toe rail in use is at the bow and the forces are significantly softened by the advantageous angle, as you accurately noted. There is the added benefit of the load being largely in shear to the fasteners, hopefully bolts, that are securing the toe rail.
I, however, was only referring to the use of the toe rail as depicted in the diagram. In the diagram the pennant goes from the attachment on the boom to a snatch block on the toe rail near amidships. (This loads the fasteners at their weakest: eg not in shear.) This amidships position escalates the forces dramatically. The angle of this pennant to the amidships toe rail is quite dis-advantageous compared to the one you refer to when taking the line to the bow: the forces would be similarly quite different. For the vast majority of the time, these forces are pretty minimal and easily handled. However, when the forces are generated by the onset of an uncontrolled gybe that you wish to prevent, (or dipping boom end in the water) the shock load forces will be momentarily huge. Many toe rails are aluminum extrusions and are not designed to take those kinds of loads in that direction. There are few toe rails that can tolerate that shock load and I write, in part, to discourage readers from thinking that they can use their amidships toe rail to stop an uncontrolled gybe.
Similarly, I would discourage any boat from having their preventer go to any location but the end of the boom: I believe a mid-boom attachment of a preventer is a recipe for a broken boom.
Those are the major caveats I have to following the diagram.
And Bill, I very much support your comments that for an offshore boat; that having an easily deployable preventer line is an important safety item and, in my casual observations, is too little used.
Let me also say a couple of words about your aside that I am (possibly) alarmist and overly influenced by the John Harries’ Attainable Adventures Cruising web site. When I write about procedures and gear and design for cruising safely, I am writing for offshore voyaging boats. As such, my focus is on what is in the reasonable realm of possibility in a boat at sea for a couple of weeks or two or longer. I do not want a boat fully prepared at all times for the worst possible scenario, a hurricane for instance, but I do try to write for those possibilities that do occur with some degree of regularity on a passage making boat. For a preventer system, that would include an unplanned gybe in winds of 20-30kn caused either by a passing nighttime squall and wind shift or by helmsman inattentiveness. It is for that degree of safety that I write for, that gear should be designed for and for which I believe every offshore boat should be prepared to deal with at all times
My take is that John, and generally the AAC contributors, write their articles and comments with a similar degree of carefulness in mind. And although I have disagreed with John and AAC at times, I do not find the site alarmist and that when it comes to engineering and assessing systems, such as the loads being discussed above, they are quite accurate.
That said, lets move away from talking about people and judging their motivations, alarmist and otherwise, and look to the specifics of what makes for a solid preventer system.
My best, Dick Stevenson, s/v Alchemy
.

Dick
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Dick - 8/6/2019
Bill Balme - 8/4/2019
On Toodle-oo! we rig the preventer whenever we're more than about 110 degrees off the wind. Penants on the boom are connected to a line that goes through a low friction eye attached to the toe rail at the bow and come back to a clutch in the cockpit. We can adjust as that as we trim the mainsheet.

Dick's concerns about the strength of the toerail sound a little alarmist - too much Morgan's Cloud perhaps? - since the tensioning line to the boom lies at such a low angle to the toe rail (not 90 degrees to it) it is certainly well strong enough for the job at hand.

Having a simple to deploy (and therefore always gets deployed) preventer system has been a significant contributor to our safe and enjoyable sailing experience.


Hi Bill,
I wrote my comment, as it was my take that someone following the diagram to put together a preventer might go wrong, perhaps seriously wrong, if following the diagram.
You are right, in my understanding, to take your preventer line to the bow as, among other attributes, it gives by far the most forgiving angle to absorb loads which may be asked of it. The diagram has lines to the bow and to amidships. It sounds like we all agree that the preventer should go to the bow. It also sounds like there is general agreement to take the preventer to the end of the boom. This is in contrast to the diagram where the attachment point is well away from boom end. In my comment I was referring to the diagram and only the diagram, at least as I interpreted it and only to those aspects I considered problematic.
Your defense of using the toe rail may be completely warranted when the toe rail in use is at the bow and the forces are significantly softened by the advantageous angle, as you accurately noted. There is the added benefit of the load being largely in shear to the fasteners, hopefully bolts, that are securing the toe rail.
I, however, was only referring to the use of the toe rail as depicted in the diagram. In the diagram the pennant goes from the attachment on the boom to a snatch block on the toe rail near amidships. (This loads the fasteners at their weakest: eg not in shear.) This amidships position escalates the forces dramatically. The angle of this pennant to the amidships toe rail is quite dis-advantageous compared to the one you refer to when taking the line to the bow: the forces would be similarly quite different. For the vast majority of the time, these forces are pretty minimal and easily handled. However, when the forces are generated by the onset of an uncontrolled gybe that you wish to prevent, (or dipping boom end in the water) the shock load forces will be momentarily huge. Many toe rails are aluminum extrusions and are not designed to take those kinds of loads in that direction. There are few toe rails that can tolerate that shock load and I write, in part, to discourage readers from thinking that they can use their amidships toe rail to stop an uncontrolled gybe.
Similarly, I would discourage any boat from having their preventer go to any location but the end of the boom: I believe a mid-boom attachment of a preventer is a recipe for a broken boom.
Those are the major caveats I have to following the diagram.
And Bill, I very much support your comments that for an offshore boat; that having an easily deployable preventer line is an important safety item and, in my casual observations, is too little used.
Let me also say a couple of words about your aside that I am (possibly) alarmist and overly influenced by the John Harries’ Attainable Adventures Cruising web site. When I write about procedures and gear and design for cruising safely, I am writing for offshore voyaging boats. As such, my focus is on what is in the reasonable realm of possibility in a boat at sea for a couple of weeks or two or longer. I do not want a boat fully prepared at all times for the worst possible scenario, a hurricane for instance, but I do try to write for those possibilities that do occur with some degree of regularity on a passage making boat. For a preventer system, that would include an unplanned gybe in winds of 20-30kn caused either by a passing nighttime squall and wind shift or by helmsman inattentiveness. It is for that degree of safety that I write for, that gear should be designed for and for which I believe every offshore boat should be prepared to deal with at all times
My take is that John, and generally the AAC contributors, write their articles and comments with a similar degree of carefulness in mind. And although I have disagreed with John and AAC at times, I do not find the site alarmist and that when it comes to engineering and assessing systems, such as the loads being discussed above, they are quite accurate.
That said, lets move away from talking about people and judging their motivations, alarmist and otherwise, and look to the specifics of what makes for a solid preventer system.
My best, Dick Stevenson, s/v Alchemy
.


neilm
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What kind and size of line for the preventer ?????
I am puzzled as to how springy a preventer should be.  Nylon ?  Dyneema/Spectra or something in between.
We have gybed with Dacron and with nylon, and both avoided a big bang, although the nylon let the boom go a long way across.

What are members thoughts?

What size line?  We have an 8mm Spectra pennant at the moment and joined to a 14 mm three-strand nylon line, so we are intermediate in elasticity.​
47 foot boat with 550 sq ft main.​​​​​​
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