Preventer Details
Now that we understand the forces at work when something goes wrong and we get caught aback (unintended jibe), including the fundamental geometry of how a safe preventer should be set up, and we have a calculator so we can select the preventer line and fittings to safely withstand the loads for our specific boat, let’s look at the details of actually rigging a safe and easy-to-use preventer, based on Phyllis and my many years of doing just that at sea, but updated in light of Kurt’s excellent engineering work.
If you have not read the above-linked two articles, please do so now before reading on.
As a reminder to the sleepy dummy, on some boats it might be possible to hang the deckline tail on the mainsheet winch. Removing it before sheeting-on would remind the sleep walker that the preventer is rigged.
Hi PD,
Sure that might work, but it would also be easy to forget, or not understand what the tail meant in the dark. I long ago learned not to underestimate how stupid I can be when overtired and seasick in the middle of the night!
Premade soft shackles intended for industrial lifting or commercial towing applications are available up to extreme load ratings, and from multiple manufacturers (including Samson Rope’s Link-It Plus product). While they don’t typically have all the nice-to-have features of the Harken T-Close soft shackles, is there another reason why you wouldn’t consider using these?
Mark, I had the same reaction. 4WD soft shackles are good for 14000kg and cost +/A$60. Not as pretty as a Harken but plenty strong wnough and a fraction of the price. Ironman 4×4 Soft Shackle – 14000kg 500mm x 12mm Offroad 4WD ISOFSHA001 | 4wheelhouse
Hi John,
Sure, great value, but I would want to dig really deep into the credibility of the load rating before selecting any soft shackle, or for that matter, any connector, hard or soft.
Hi Mark,
Sure, you could do that. I’m a huge fan of the Tylaska shackles particularly because they can’t shake off, but then again they are expensive and there is the issue of them banging up the topsides if we don’t slide a piece of tube over them.
Bottom line, based on tens of thousands of miles using them I just trust the Tylaska shackles and therefore feel better about recommending them for this function, particularly on bigger boats, where someone could get killed if the shackle failed.
Hi Mark,
The Sampson product looks good I agree, but note that they don’t mention overhead lifting. Also note their warning about rope diameter. And further, the instructions call for a specific installation orientation that will be difficult to always achieve in this application: https://www.samsonrope.com/docs/default-source/technical-bulletins/install_instructions_linkit_single_pass.pdf
Add it all up, and I’m still more comfortable with the Tylaska.
Yes, John, calling out the 2.5 D/d requirement for full strength of the Samson soft shackles is very appropriate. Normally, a soft shackle has about twice the breaking strength of a single strand of the base rope from which it is made. The question then becomes how much should the strength be de-rated if the soft shackle is used to link lines that are smaller than the 2.5 D/d ratio?
I won’t try to give a hard answer to that since I simply don’t know for certain, and don’t want to mislead anyone into thinking that I do. I am, however, a huge fan of the HowNOT2 YouTube channel, which contains numerous break tests of soft shackles, as well as a whole lot more related to ropes, knots, splices, rigging, etc. Almost none of the content is specific to sailing, but there’s still a lot to learn from the climbing-oriented content. In particular, highliners often want to join together two highline segments using a soft shackle. Since highlines are made of webbing, this is more like joining together two jacklines than rigging a preventer, but the break tests are still similar enough that I think they are useful. Oversimplified, the results seem to be that the soft shackle breaks at about the single-strand breaking strength instead of double that. Interestingly, the break strength that Harken lists for their soft shackle is also about the single-strand breaking strength instead of double the breaking strength of the base 7mm dyneema, so it looks like they may be de-rating in a similar way.
If the soft shackle was linking together hard, smooth, low-friction elements, then it could be doubled- or tripled-up using smaller diameter base rope to meet the 2.5 D/d requirement, resulting in something more like lashing than a simple shackle. It doesn’t appear, though, that this is a reliable way to multiply the breaking strength of the base rope when the soft shackle is connected to a rope or webbing eye, so I’m going to scrap that idea.
Anyway, I may soon be putting together a preventer system that needs to be only slightly smaller than the M&R 56 requirements, and I haven’t yet made up my mind whether to use a soft shackle or the Tylaska. If I do go the soft shackle route, though, I’ll likely use 11mm Amsteel for the soft shackle and will also significantly build up the diameter of the spliced eyes with sheathing and/or anti-chafe material to improve the bend radius and mitigate the sawing of the rope-to-rope connections.
Hi Mark,
Great information that will help anyone trying to make the same call, thanks. I think I would still stick with the Tylaska, but then that’s what I’m used to and know from experience is very reliable. I also still have a nagging worry that a soft shackle, particularly if not properly snugged up, could come undone.
Hi John,
I have attached my preventer to the boom end in much the same way as you suggest.
One advantage of not using padeyes, besides not having rely on fasteners, is that I want the aft end of the boom to be as free as possible of “protuberances” that can exacerbate head injury. A padeye makes the boom end look like a medieval mace. Booms, even well secured with preventer or over the deck with mainsheet, can jump around with force a few inches and any protuberance just makes injury far more likely and severe.
My best, Dick Stevenson, s/v Alchemy
Hi Dick,
I agreed that’s a very nice added benefit.
This is great detail, John. My current form of attainable adventure cruising with my 3 young kids is on a 19’ Mariner, displacing 1200 lbs. Even though it’s a small boat a preventer seems like a good idea with 3 young skippers at the helm. However, it’s a centerboard model. Does a preventer increase the risk of a capsize / death roll if the main backs?
Hi Stephen,
The Mariner is a great boat. Years ago I sailed a Rhodes 19. Based on that, I don’t think I would rig a preventer for just the reasons you state.
I appreciate your reply, John!
Great article John. I have a query about the potential twisting moment on the gooseneck if attaching the boom line to the underside of the boom. I’ve always done it that way because I assumed that the main sheet attachment point was strong enough (it’s close to the end of the boom on my boat). Is the danger of that twisting moment a hunch or are you referencing any external source? I would have thought that booms / goosenecks are designed to withstand exactly that twisting moment as it would presumably be similar with the boom eased out and running downwind. The force on the attachment point isn’t just straight down.
Incidentally I also use J locks. I haven’t experienced them dinging the topsides. Maybe it’s because the boom line ends at the vang tang, rather than the gooseneck, I’m not sure.
Hi Pete,
My concern was based on Kurt’s work. The issue is that the loads in Kurt’s worst case scenario when caught aback with a full main up in 35 knots are prodigious and far higher than normal and probably higher than the mast designer was thinking about. Would a given boom gooseneck or mainsheet bail fail? No way to know without an in-depth engineering study of each boat. Therefore it seems best to recommend against those practices and go with a solution that minimizes the chance of a failure that could kill someone.
Would not a 20 lb-force on a 10 inch handle in a 40:1 winch would put 800 lb-force on the mainsheet leaving the winch and with a 4:1 block and tackle attachment put 3200 lb-force mid boom and thus 1600 lb-force on a preventer at the end of the boom ?
Hi William,
Hum, I think you are right about the load on the mainsheet, although I used 25 lbs on the handle so that’s 4000 lbs. being 40 x 25=1000 lbs on mainsheet leaving winch x 4= 4000lbs
I will change it. Thanks for the correction. I think my mistake was multiplying the power ratio by the tackle advantage.
However I can’t see how the load on the preventer halves. I’m fairly sure since both ends of the boom are fixed in space and assuming the mainsheet is in the exact middle of the boom the tension on the mainsheet, goose neck, and preventer, assuming we ignore the mainsheet vertical and horizontal angles would all be the same at 4000 lbs.
Or to look at it another way the bending force on the middle of the boom is 4000 which is, I think, plenty to break a typical boom on a 40 foot boat. And that’s supported in that I have seen two booms that were broken this way.
Anyway, I will change the force number, thanks.
Do the engineers have any wisdom on this? Matt, Eric, Kurt?
Hi Again William,
Wait, now I think about it more (and get more caffeinated!), I think you are right. The force on the preventer and gooseneck, assuming mainsheet attachment in the middle, are half the force on the mainsheet or 2000 lbs each. Duh, of course the forces must zero out…I think. That said, I don’t think that changes whether or not the boom will break because what matters is the force in the middle of the boom and the distance from that to the two fixed points, since the longer we make the span the more likely it is we break the boom in the same way that it’s easier to break a 10′ 2×4 by standing on the middle with each end supported off the ground than it is a 2′ 2×4.
Anyway, thanks for the correction and making me think more deeply about this.
Hi John and William,
As William says, if we assume all of the loads are perpendicular to the boom and the mainsheet is mounted at the mid point, the load of the mainsheet will be split between the preventer and the gooseneck. This is actually making 1 more assumption which is that there are no other forces and of course there is the force of the sail. The actual angles are not terribly close to perpendicular so they need to be taken into account to get accurate numbers.
I think the overall thing to keep in mind is that mainsheet purchases and winches are sized for heavy air close hauled sailing where the sheet load on the boom is mostly vertical. Booms are typically much taller than they are wide if we pretend we are looking at a box section, we should be comparing the square of these numbers. Roughly speaking, the boom on our boat is about 8X stronger for the vertical sheet scenario than the horizontal one. With our 340 ft^2 main and 5:1 mid-boom sheeting, I have literally never used the winch below a close reach whereas I routinely have to use it when close hauled and sometimes apply at least John’s 25lbs (only a 16:1 power ratio). So I think the issue is simply that the sleepy or careless crew member can put close-hauled type sheet tensions on the boom in a direction that it is only designed for running type sheet tensions which are enormously lower.
Eric
Hi Eric,
Great explanation. I should have pointed out that booms are much stronger in the vertical, so thanks for adding that. Anyway, your explanation really reinforces the point that it would be very easy to break a boom with a preventer separated from the mainsheet, something I think people are sceptical about, but is all too true.
Hi Willam,
With the benefit of yet more caffeine I think I was kinda-right the first time, except I used the wrong words “bending force” instead of “bending moment”.
Bending moment, in its simplest form equals the force at the mainsheet times the distance to the fixed points.
Anyway, I’m going to leave it as it is with the changes because calculating actual bending moment is past my pay grade, although I did learn quite a bit from this page: https://www.structuralbasics.com/bending-moment/
Not knowing the actual geometry, I assumed that all three forces (the mainsheet block attachment at mid-boom, the gooseneck at one end of the boom, and the preventer at the other end of the boom) were in the same plane and perpendicular to the boom. I ignored the force of the sail on the boom and the loads it imposes on the preventer, the mainsheet block, and on the gooseneck.
My interest in preventers comes from a surprise gybe sailing downwind on our Pacific Seacraft 34 south of the Florida Keys from Fort Jefferson to Miami. Off Islamorada in sloppy seas and 20 knot wind, we gybed breaking two of the Schaefer, swivel shackle, single bocks in the mainsheet. Thankfully, their breaking absorbed most of the shock, and no injury or other damage was done.
The preventers we have used since that incident are much like the ones this article proposes except… they are much less strong and much more elastic. I now think they are inadequate and will be replacing them with polyester sheathed UHMWPE rope.
My mainsheet winch is under the dodger restricting visibility raising the possibility of my unintentionally over tensioning the mainsheet and bending the boom, damaging the traveler, breaking the mainsheet bails or —–. Because I don’t see a way of my doing any useful calculations, I plan to resort to experimentation. At the dock I’ll remove the mainsail (or just move the bundled sail out of the way), set up one of the preventers, run a tight string between the clew and tack attachment points on the boom, then start cranking away on the mainsheet winch. I’ll stop when I can turn the winch no more, when I lose my nerve, when something distorts, or when the boom bends more than 1 inch in 100 as measured from the boom to the string.
Is that a reasonable plan?
Hi William,
Experimenting is an interesting approach I had not thought of. My guess is that it should be OK, as long as you are really careful to monitor boom bend since I think breaking the boom is by far the most likely fail point. The other option with that set up is to just take Eric’s recommendation and place the preventer at the mainsheet bails, which pretty much gets rid of the sleepy dummy danger. In this case you will want to increase the loads yielded by Kurt’s calculator by about 1/3.
I tried to break my boom…
My mainsheet is attached mid-boom with three equally spaced attachment points. I have the heavier of the two LeFiell booms that I have seen on Pacific Seacraft 34 sailboats.
I ran a 6mm Chineema rope from the port side pad eye at the aft end of my boom to one of the port bow cleats. I set the traveler and boom fully to port. With the mainsail stowed below for winter, I ran a tight small cord from the boom tack pin to the clew outhaul car (14′-9″). I put a 8″ winch handle in my Lewmar 16 ST mainsheet winch (in a tight place under the dodger where a 10″ won’t work and not the best place to use the winch) and while checking the bend gave it my best effort.
The boom bent about 1.5″ in its middle. I’m not too worried about accidently breaking the boom with the mainsheet while sailing downwind .
I did notice that two of the sheaves in my Schaefer traveler were cracked from center to edge. Under the heavy load the cracks opened and became quite noticeable. So, it looks like I’m in the market for new set of traveler sheaves. Sun, bleach, and time seem to have done their evil work.
Hi William,
Interesting experiment. Glad it went well. That said, I would caution others not to extrapolate too far from this test because I think I’m right in saying that the Lewmar 16 is single speed. I think the experiment could have ended badly if it were two speed, as most mainsheet winches are. This highlights a point I have often noticed: in some ways smaller boats are less fragile than larger ones because the gear is less powerful.
I have a Freedom 39 Cat Ketch, a bit of an oddball boat, and have been thinking about the subject a lot. The main mast is about three feet aft of the bow, with the windlass and a short bowsprit forward of that. The bowsprit as built would certainly not take the required side loading, and probably isn’t long enough to provide an adequate angle anyway. Any thoughts on a way to rig a preventer line? My other thought is a gybe would sweep the boom over the foredeck, so maybe it’s less of a crew safety issue, though it could cause massive damage.
Isn’t the Freedom 39 unstayed? So the boom would actually be forward of the beam when on a run?
We worry about preventers because, on a sloop or cutter, you can only let the boom out to maybe 60° or 70° off centre before the sail is rubbing on the shrouds and spreaders. Maybe less if the shrouds are B&R fashion. So the margin between “dead run” and “caught aback, crash jibe” is pretty narrow.
If your boom can be let out to 100° off centre then, while there may be more mainsheet out and waiting to cause trouble, it’s also an awful lot harder to accidentally crash-jibe.
AFAIK, there’s no way to rig a preventer on a catboat’s or cat-ketch’s mainsail. The Nonsuch has the same issue; there are dozens of them around here and I have never once seen a preventer, nor do I see any safe way to rig one.
Hi Stephen,
Hum, I fear that Matt (and you) are right. I can’t see any way to rig a preventer either. Like Matt we have a bunch of Nonsuchs around here, and I have sailed on one a couple of years ago. Based on that, the only thing I can suggest is to always reef early so that if anything does go wrong the loads are less dramatic. Sorry, not that useful.
Hi John and
I am not really familiar with them, but might this be a good place for one of the “boom brake” designs?
My best, Dick Stevenson, s/v Alchemy
Well, I’m afraid that a boom brake would not be able to work with the boom being forward of the beam?
As far as I understood a bbrake works with a boom swaying port/stbd, and most certainly wouldn’t allow to let the boom go forward. And if it would, it would become slack as soon as the boom starts to swing back?
Hi Dick,
As it happens I’m just working on an article about boom brakes and as part of that looking at loads, which are scary enough on a sloop, due to unfavourable geometry, but would be off the charts on a cat boat since the main is so big and the geometry even worse.
Hi John,
Interesting and I see that. I look forward to the article. Dick
Thank you for all the thoughts. Matt – my boat has traditional booms, so the gooseneck won’t allow the boom to go farther than 90 degrees on a run (more like 85 in reality) or the stress is too much. But you’re right that there are no shrouds/stays, so nothing else to prevent it. And a boom brake would definitely not work, since any attachment point on the boom would be well over the side of the boat when the wind is aft of the beam, so the geometry is wrong. Ok – this at least gives me more to think about how to maximize the safety. Everything is a compromise – the rig simplicity sometimes makes other aspects more difficult.
Stephen, please let me know if you have any luck solving the problem of the forward boom. Seems like np for the main which for me (PH39) is the big one but at a loss for the mizzen (forward) one. I had set a goal of solving this problem over this winter since I got tired of being scared downwind. Thanks
When is the boom brake article coming out? I have been thinking about how to add the boom brake to this system to allow a controlled jibe after the sail backs.
Hi Amanda,
All written and in final edit. A few days, to a week or so.
One additional benefit of this arrangement that I haven’t seen mentioned is that the windward side forward preventer line can also be purposed as your forward downhaul on your whisker pole when you have your headsail poled out to weather. The leeward one is of course used for your boom preventer.
Also LOVE the idea of the soft plastic tube around the J-lock. This would also be really handy on the quick release fittings for the gates in the lifelines, which seem to come apart very easily as well.
Thanks once again for a really well thought out article John, in addition to the two preceding.
Hi Paul,
What a great idea on using the windward line for the pole down haul…why the heck did I never think of that…duh!
And good thought on adding the soft tube for lifeline gates. I never thought of that either…double duh!
John, given your rethinking to favor attaching the preventer at the mainsheet attachment point (for boats with the traveler fore of the dodger), 1. How would one modify Kurt’s calculations for this configuration? 2. Does this also change your thinking about boom brakes?
Thanks
Hi Ken,
Eric Klem has calculated that moving the preventer to the mainsheet bails increases the load on it by about a third from Kurt’s calculator, which is I think acceptable in exchange for getting rid of the sleepy dummy danger.
But none of that changes the calculations I did on boom brakes where the load tension increase is far higher, and to me at least, totally unacceptable.
John when using a J-Lock or other metal shackle, would you put a thimble in the eye you’re attaching to or attach to the bare eye? Thanks,
Hi Ken,
Good question.
I would use a soft eye. The bearing area on the J-lock is pretty good so I don’t think a thimble will add much, although I would allow for some strength degradation of the rope. Guess 25%.
If using metal shackles, how do you get the preventer line through whatever blocks you are using at the bow?
Hi Ken,
There is no need to pass the shackle since the preventer line is in two parts and the aft end of the deck line can be threaded through the forward block when initially setting up.
Ah. I was actually planning to use a single preventer line, with shackles at each end. One end would be attached to the short line at the boom, and the other would attach to an eye in a line on the opposite side, which runs aft to a clutch and winch. There would be an aft line for each side, but I would switch out the preventer from port to starboard when gybing.
Hi Ken,
While I’m generally big into simplification, I would advise against that, since it sounds awkward to me, and go with our tried and proven system as detailed above. That said, I’m not really clear on what you are planning, and I could always be wrong, so try it and see, you can always revert to our system.
Hi, for connecting the both lines, the boom line and the preventer is safe to use a snap shackle (Wichard) ? I saw this and want your opinion , the photo explain better :
Hi Jorge,
I prefer the Tylaska because it locks. Also I note that in the photo the line is knotted, which is a no-no for high modulus line.
When not is usung
I use eye splices on each line. I had Ian Weedman, who now runs Brion Toss’s shop, make me soft shackles from dyneema. The shackles are a special design by Brion,before he passed away, so I trust it.