[sailordave]

I've finally decided to rig up a preventer for my boat. Whilst I'm pretty adept at sailing deep angles and not backwinding the main (and the accompanying SLAM!) the Admiral isn't, and I really would feel safer so I can sail DDW if need be.
My plan is to have a permanently rigged length of Amsteel from the end of the boom where the Mainsheet attaches led to about where the vang attaches. When needed I would unclip the forward end and attach to one of two NYLON lines (P/S) led fwd to a snatch block and then back to the cockpit.

The question is.... for a 280 sq. ft. mainsail what size Nylon line should I go w/ to get just the right amount of streeettttccchhhhh to cushion the blow w/o putting too much force on the line to permanently deform it, or worse break?

My initial gut feeling is 1/2". Supposedly 3.5% elongation at 10% of avg. strength of 7500 lbs.

 

[christian.hess]

what size is your main sheet? go one size smaller or so

 

[smurphny]

Some things to think about with a preventer are where to attach to the boom and where to the boat. There is a definite danger of bending the boom if the attachment is in the center of the boom length. I have never been able to adequately set up a typical preventer but have opted for a boom brake. The brake exerts enough friction to slow down an unintended jibe but cannot exert enough pressure to bend the boom or create dangerous line breaks, pulling out deck fittings, etc. I've found that when in a steep following sea on a run it is all too easy to get into trouble with a rigidly attached preventer.

 

[Bill-Rangatira]

I usually run my preventer to the same point as my sheet and on deck to the padeye near my shrouds

 

[capta]

I have always run a preventer from the end of the boom, outside everything, to a convenient point farthest forward on the boat, which usually ends up being the bow chock. This is completely independent of a vang.
Preventers have saved us from many a gybe and possible dismasting, so I can't even imagine not using one on a passage or difficult ddw run.
I don't understand why the OP would want to complicate the rig with Amsteel bits and connections, when a single line off a fitting on the end of the boom (or the main sheet shackle?) would suffice.
This is one of the places where the KISS principle seems the most practical.

 

[SVAuspicious]

I have always run a preventer from the end of the boom, outside everything, to a convenient point farthest forward on the boat, which usually ends up being the bow chock. This is completely independent of a vang.

Capta has it exactly right.

Evans Starzinger wrote a great article about preventers and makes an excellent case for minimizing stretch. For what little it's worth I agree with him.

 

[aelkin]

I'm not trying to answer for the OP, but when I read his idea, a little light bulb actually went off in my head...I hate rigging a preventer for the simple reason that you either have to bring the boom inboard to tie it off on the boom, or hang out over the lifelines.

If you had a permanently rigged line that was just shorter (say 3/4 length?) the boom, you could get the sail out where you want it, then unclip the inboard end of the preventer, and hook it to the stretchy, adjustable nylon line. Seems like it might make the whole process a bit safer in rolly seas.

I'm sure those of you who cruise extensively would have other solutions for this problem, but for a guy who races weekly, and only cruises occasionally, this would be an improvement over anything I've got set up.

Cheers!
Andy

 

[mitiempo]

Here is a link to John Harries solution to the problem on Morgan's Cloud. Basically a line on each side of the boom that is hooked near the gooseneck. Easy to use and no leaning over the side.

Rigging a Proper Preventer?Part 2

 

[JonEisberg]

I'm not trying to answer for the OP, but when I read his idea, a little light bulb actually went off in my head...I hate rigging a preventer for the simple reason that you either have to bring the boom inboard to tie it off on the boom, or hang out over the lifelines.

If you had a permanently rigged line that was just shorter (say 3/4 length?) the boom, you could get the sail out where you want it, then unclip the inboard end of the preventer, and hook it to the stretchy, adjustable nylon line. Seems like it might make the whole process a bit safer in rolly seas.

I'm sure those of you who cruise extensively would have other solutions for this problem, but for a guy who races weekly, and only cruises occasionally, this would be an improvement over anything I've got set up.

Cheers!
Andy

I think you've got it exactly right, that's definitely the way to go... I'm amazed at how many boats I deliver, that have no proper preventer setup in place, and one has to cobble together something using a dockline, or whatever...

Good articles from John Harries, as well:

Rigging a Proper Preventer, Part 1

Whether you want stretch in the line or not, I think is somewhat of a function of the aspect of your rig, and whether your boom is longer than average... With a taut preventer taken forward, the biggest shock load will not likely be a backwinding/jibe of the main, but rather dipping the end of the boom in a deep roll... So, for that reason, I'm inclined to go with a rope that will absorb such a shock, and I use a climbing rope for that application...

But, those like capta have it right, the way to go is from the end of the boom, to as far forward as is possible to maintain a fair lead outside of your stanchions... Anything less, especially taking something from mid boom down to the rail, is a recipe for breaking something...

Oops, I see mitiempo beat me to the link from MORGAN'S CLOUD

 

[GeorgeB]

Annapolis' Navy44 program uses a variation of this type of preventer. They do a combination of line and bungie cord along side the boom so the preventer returns to the boom once the bow line portion is slackened. That way, a crewmember doen't have to lean outside the life lines to detach the preventer from it's bow line component. Pretty slick.

 

[aelkin]

Hey guys;
Thanks for the link to the John Harries site.
I just spent 40 minutes over there and almost forgot that I was reading something here!

Hope the OP gets as much from this thread as I have!

Cheers!
Andy

 

[JonEisberg]

Hey guys;
Thanks for the link to the John Harries site.
I just spent 40 minutes over there and almost forgot that I was reading something here!

Cheers!
Andy

Glad you liked it, morganscloud is a wealth of information, some extremely knowledgeable and experienced people contributing there... I have as much respect for John and Phyllis as anyone in the world of cruising, it's amazing what they have done...

Some of the stuff on their site is only viewable to subscribers, but the modest annual fee is well worth it, in my opinion...

I've never had the pleasure of meeting them, but I spoke to John on the VHF one night a few years ago after a crossing of the Gulf of Maine. They were hove to about 10 miles out of the Cape Cod Canal, waiting for daylight... Very proper and professional, I felt like I was talking to the Captain of the QE II, it was obvious the guy is the Real Deal...

I was impressed, and a bit surprised, frankly, by their caution in waiting for daylight... the CCC is a pretty straightforward approach, after all, and it was a crystal clear night, should have been a piece of cake for folks as experienced as them, and they were gonna be missing catching a fair current thru the canal and down Buzzards Bay...

Myself, on the other hand, was singlehanding, hadn't slept in 24 hours, was drinking coffee and hot chocolate by the gallon, and pushing to catch the tide and make it to Montauk by that evening... In comparison, looks like I'd rather be lucky, than good...

 

[sailordave]

Geez guys, did you read my first post? What I was describing is exactly what most of you posted. I AM going to attach to the end of the boom and the AMSTEEL is a just a short section that stays attached to the boom and stored there and is long enough that even if the boom is all the way out I can easily reach the forward end and clip it on to the preventer line which is rigged exactly as everyone was telling me it should be. I know this.

What I want to know is sizing the nylon preventer line. I don't want to put a big line on that won't stretch at all for the loads it will encounter but I don't want some wimpy line that will fail if by some chance I have a full on flying gybe in heavy air. Probably the best answer I've received was to go one size down from my mainsheet.

AELKIN had it right in his first post to this thread.

 

[jimrafford]

I had 3\8" double braid line on our on our 36t.
Jim

 

[JonEisberg]

What I want to know is sizing the nylon preventer line. I don't want to put a big line on that won't stretch at all for the loads it will encounter but I don't want some wimpy line that will fail if by some chance I have a full on flying gybe in heavy air. Probably the best answer I've received was to go one size down from my mainsheet.

Jim's recommendation of 3/8 sounds about right to me... The high aspect main on my 30-footer is considerably smaller than yours, I'm using 8mm dynamic climbing rope of a Kernmantle construction for the connection to the low stretch portion that lives on the boom, and it seems about right, to me...

What is the working load of the turning block at the bow that you're using? Is it clipped to an aluminum toerail? Chances are if the setup is highly stressed, that component might be the first one to fail, especially if you're using a snatch block, which are not known as the most reliable nor recommended for use as a high load turning block...

 

[christian.hess]

i.e: one size smaller than main sheet...jeje

 

[tjvanginkel]

If you go to the previously linked site, in part 2 on preventers John describes how he chose to size his line. It is based on breaking strength not size of line and will depend on the size of your mainsail.
I used the link in his article to calculate breaking strength for our preventer line.
Regards, Tanya

 

[outbound]

+1 one on following set up referred to previously.
Two no stretch lines attached to back end of boom on either side left cleated to boom near gooseneck and ending in trigger snap block. Another two lines lend through a clutch on either side of cockpit lead all the way forward through a low friction swivel block attached to bow sprit, anchor set up or as extremely forward you can get in the bow also ending in snap block ( use long lanyards to make it easy to deploy).
At any time other than feeding line outside shrouds you and snapping the two together there is no need to lean off the boat. I use one whole set up as the preventer on one side and then just the line from cockpit to bow as the fore guy on the other side when DDW. I don't use nylon but low/no stretch lines. I understand the nylon may take up some shock loading better but prefer to be able to get the preventor nice and tight so the boom doesn't rise and I also don't have to re tension it over and over.

 

[smurphny]

Depending on the angle from end of the boom to the bow attachment point, it may be necessary to use no-stretch line like Amsteel. Line with even a small amount of stretch on a narrow hull, creating an acute angle in the line, will allow the boom to swing back over and create a real problem. My decision to go with a ladder type brake was based on two things: 1. My narrow hull and the geometry of a typical preventer as described above and 2. the idea that the purpose of a preventer is exactly that-to prevent a disastrous unintentional jibe that can do damage to people and gear. The brake does that. I fabricated a LONG plate for the boom attachment which distributes the stress over a long section of the boom. The setting of the ladder provides plenty of drag to slow down any jibe but not enough to break lines, pull out deck attachments, or create VERY DANGEROUS projectiles. If the boom swings across at a slow rate, it is not going to hurt anyone, if a fitting snaps from the enormous forces created as the end of a boom submerges, not only will the boom then be free but the boom AND MAST may well come down. Creating the possibility of these kinds of forces IMO, makes a boom-end preventer a dangerous item.

 

[svzephyr44]

Depending on the angle from end of the boom to the bow attachment point, it may be necessary to use no-stretch line like Amsteel. Line with even a small amount of stretch on a narrow hull, creating an acute angle in the line, will allow the boom to swing back over and create a real problem. My decision to go with a ladder type brake was based on two things: 1. My narrow hull and the geometry of a typical preventer as described above and 2. the idea that the purpose of a preventer is exactly that-to prevent a disastrous unintentional jibe that can do damage to people and gear. The brake does that. I fabricated a LONG plate for the boom attachment which distributes the stress over a long section of the boom. The setting of the ladder provides plenty of drag to slow down any jibe but not enough to break lines, pull out deck attachments, or create VERY DANGEROUS projectiles. If the boom swings across at a slow rate, it is not going to hurt anyone, if a fitting snaps from the enormous forces created as the end of a boom submerges, not only will the boom then be free but the boom AND MAST may well come down. Creating the possibility of these kinds of forces IMO, makes a boom-end preventer a dangerous item.

I am curious as to how the brake works in rolling seas. I have traditional preventers. They are used in light winds and beam seas when the boom tends to wiggle back and forth. The wind isn't shifting so much as the roll of the boat causes the boom to swing. I would not think that a boom brake would be optimal in these circumstances but I might be wrong.

If the winds is strong enough that I would have to be concerned about a "crash gybe" my main is down and I am sailing on my 150% jib. I can do boat speed and it is much safer as a single hander. I can trip and/or furl a lot faster than I can reef.

Fair winds and following seas.