The Offshore Voyaging Reference Site

Rigging a Proper Preventer, Part 1


One of the most common ways that people get killed or maimed on a sailboat is by being hit by the boom. So, what can we do about that? Pretty obvious answer: Rig a preventer.

Such a nice simple solution, right? Well, unfortunately, no, it’s not. Rigging a proper preventer can be a pain in the neck. But there are things we can do to make rigging a proper preventer easy and quick.

But before we get into that, have you noticed that I seem to be overusing the word “proper”? That’s no accident. You see, a lot of the gear I see out there being used to restrict the swing of the boom does not constitute a proper preventer.

These set-ups might be boom brakes, like the one that Colin writes about here. Or boom tackles, where a line is taken to the boom, then down to a block near the rail, and then to a winch or tackle. A lot of people call these preventers, but they are not.

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More Articles From Online Book: Sail Handling and Rigging Made Easy:

  1. Six Reasons To Leave The Cockpit Often
  2. Don’t Forget About The Sails
  3. Your Mainsail Is Your Friend
  4. Hoisting the Mainsail Made Easy—Simplicity in Action
  5. Reefs: How Many and How Deep
  6. Reefing Made Easy
  7. Reefing From The Cockpit 2.0—Thinking Things Through
  8. Reefing Questions and Answers
  9. A Dangerous Myth about Reefing
  10. Mainsail Handling Made Easy with Lazyjacks
  11. Topping Lift Tips and a Hack
  12. 12 Reasons The Cutter Is A Great Offshore Voyaging Rig
  13. Cutter Rig—Should You Buy or Convert?
  14. Cutter Rig—Optimizing and/or Converting
  15. Cruising Rigs—Sloop, Cutter, or Solent?
  16. Sailboat Deck Layouts
  17. The Case For Roller-Furling Headsails
  18. UV Protection For Roller Furling Sails
  19. In-Mast, In-Boom, or Slab Reefing—Convenience and Reliability
  20. In-Mast, In-Boom, or Slab Reefing —Performance, Cost and Safety
  21. The Case For Hank On Headsails
  22. Making Life Easier—Roller Reefing/Furling
  23. Making Life Easier—Storm Jib
  24. Gennaker Furlers Come Of Age
  25. Swept-Back Spreaders—We Just Don’t Get It!
  26. Q&A: Staysail Stay: Roller Furling And Fixed Vs Hanks And Removable
  27. Rigid Vangs
  28. Rigging a Proper Preventer, Part 1
  29. Rigging a Proper Preventer—Part 2
  30. Amidships “Preventers”—A Bad Idea That Can Kill
  31. Keeping The Boom Under Control—Boom Brakes
  32. Downwind Sailing, Tips and Tricks
  33. Downwind Sailing—Poling Out The Jib
  34. Setting and Striking a Spinnaker Made Easy and Safe
  35. Ten Tips To Fix Weather Helm
  36. Running Rigging Recommendations—Part 1
  37. Running Rigging Recommendations—Part 2
  38. Two Dangerous Rigging Mistakes
  39. Rig Tuning, Part 1—Preparation
  40. Rig Tuning, Part 2—Understanding Rake and Bend
  41. Rig Tuning, Part 3—6 Steps to a Great Tune
  42. Rig Tuning, Part 4—Mast Blocking, Stay Tension, and Spreaders
  43. Rig Tuning, Part 5—Sailing Tune
  44. 12 Great Rigging Hacks
  45. 9 Tips To Make Unstepping a Sailboat Mast Easier
  46. Cruising Sailboat Spar Inspection
  47. Cruising Sailboat Standing Rigging Inspection
  48. Cruising Sailboat Running Rigging Inspection
  49. Cruising Sailboat Rig Wiring and Lighting Inspection
  50. Cruising Sailboat Roller Furler and Track Inspection
  51. Download Cruising Sailboat Rig Checklist
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Peter Morley

A valuable post and I look forward to seeing the follow up. I definitely agree it’s critical to have the line lead back aft, especially on a cruising boat.
I’d just like to add some thoughts to your statement that one should “always have a proper preventer rigged whenever the boom is let out far enough to make it work”.
I routinely use a boom tackle when sailing anywhere between close-hauled and a reach, when the angle of a preventer rigged forward would be wrong.
While the boom can travel less far in these cases a sudden change of course, or wave-induced roll in light winds, can still send it across the cockpit with dangerous force.
(It also saves me having to go forward to the mast to fiddle with the kicking strap as I can use the combination of mainsheet and boom tackle from the relative safety of the cockpit).

Andrew Troup

The nastiest stars I’ve seen from a close encounter with a boom were actually close-hauled, in very modest breeze, close inshore; – I was using binos to pick up my leads, head near the boom, because I was steadying my back against the falls of the mainsheet.
Although the sea was far from sloppy, a bunch of things conspired to cause the boat to give a little wriggle, dropping the turn of her bilge to leeward, easing the mainsheet and setting my head a little closer to the boom – and then a sudden lurch combined with the foot of the sail pulling tight (from the counter-roll of the boat, rather than anything to do with the wind) — and suddenly the boom rotated like the cracker of a whip, about its horizontal long axis — not far, and it was not a heavy boom, but if anything, that lack of mass compounded the capacity for cat-like acceleration.
At the same time, the mainsheet snapped tight, propelling my shoulders up and forwards and possibly initiating another inertial counter-rotation whiplash impulse to the inverted pendulum of my head, throwing it back closer to the boom.
However it happened, the oval shape of the boom meant its arc intersected forcefully with my skull. I received no lasting injury, because it was smooth aluminium, although it hurt like hell … but if it there been a cheek block or cleat in the wrong place, I’m not certain I’d have been so sanguine … more like sanguinating, if not leaking cerebral fluid …
and, if I was capable of coherent thought, wishing I had thought to bring a crew.

And the most sobering thought is that I actually had a boom tackle rigged, pulling against the mainsheet, to stabilise the boom. In the light airs I had not set it up fully hard, as I was continually fine trimming as the puffs came through. It was nevertheless snug enough that I’d had no qualms doing what I did.

Dick Stevenson

John, As frequently happens, your analysis cuts right to the quick of things. Keeping aware (and away) of those possibilities where a bit of bad luck could maim you is an important reminder, especially as no systems are in any way foolproof.
Dick Stevenson, s/v Alchemy

Dick Stevenson

Dear John,
I too consider the boom to be one of the most. if not the most, potentially lethal pieces of kit on a boat. Even if close hauled or close reaching where the boom draws straight to the traveller, a momentary burp in the wind can cause the boom end to jump a few inches with impressive force and velocity. On Alchemy we try to have the boom triangulated at all times fixing the boom in place.
I have used what you call a boom tackle and I call a boom vang/preventer for well over 40,000 miles and am quite content with its performance, both as a vang and as a preventer. We also have an end-of-boom set-up for ugly and/or unpredictable conditions. A few comments;
I know some do it, but I have never seen a toe rail up to the task of a vang/preventer. The side deck attachment should be the equivalent of a running back stay attachment. One should not condemn a side deck preventer because some use the toe rail. (Boom breakage is also possible, but you can spread the load and know your equipment.)
In the vast majority of cases, mainsails becoming back-winded (gybing) generate quite mild pressures, especially if the boom is fixed and has no opportunity to run out and generate momentum. These occasional gybes are not uncommon and usually result from my pushing the limits a bit too much (waiting too long to volitionally do the work of gybing the main and jib when the wind is backing or veering) and is easily brought to back into line by a course change getting the wind to fill the correct side of the main.
It is largely downwind that this is an issue, much less so when on a broad reach, but maybe there as well. When there is enough wind (and seas) to be anxious in this area, there is likely enough wind so the mainsail can be put to bed. The boat likes it better to have its power pull from the front. This is easier on the autopilot/vane as gusts will not twist the boat around as much. My experience is I go almost as fast, with more comfort and much less stress by dousing the main early and not worrying about a preventer, side decks or end of boom.
On cruising boats, I consider the scenario you describe of being wildly out of control as generally unlikely. For sure,you have to know your boat and its behaviour in various conditions. Alchemy tracks well and I try to keep her sailing at less than 75-80% of her capacity. It is that last 20% percent or so where people get hurt and damage to the boat occurs. I feel I can make a judgment when to go to our end of boom preventer. In the meantime, my side decks vang/preventer, controlled from the cockpit, is always keeping the boom in check, wild sailing or the much more common everyday sailing.
I worry that your condemnation a side decks preventer will leave an opportunity for injury to take place. I believe that the boom should be controlled by triangulation at all times especially the vast majority of sailing that is regular everyday sailing where guard is down. Side decks prevention is basically a no brainer/no hassle method whereas every end of boom system has more, often much more, hassle (at least that I know of as yet). I can see some boats experiencing an end of boom preventer as just too much hassle for crews to use all the time and not avail themselves of a side deck preventer because of your critique leaving them unprotected from the everyday boom dangers.
My best and looking forward to the next segment,
Dick Stevenson, s/v Alchemy

Andrew Troup

There *is* a way, I’ve discovered, to safeguard the boom from the potentially boom-breaking loads which vanging the boom to the side-deck can generate.
I’ve become a big fan in recent years for a ‘progressive fuse’, connecting the side-deck block to the pad-eye or deck tang. By progressive, I mean a connection which fails in stages, except for the last stage, which matches the strength of the vang line. The first failure acts like a “warning shot”, and each successive failure (if the load is sustained) bleeds off some more energy, and allows the boom a little closer to the point where it comes in over the gunwhale, and starts smacking heads.
The final stage (if it ever makes it that far) pulls the boom up short, at the point where a person on the lee sidedeck is close to being at risk, but the boom is no longer square to the wind . Importantly, the boat will have been provided with a bit of time to both roll and yaw with the ‘punch’ of the wind, (not necessarily, on a voyaging sailboat, assisted by the helm) through angles whose aggregate effect has proved in practice (at least up to boats in the mid-40′ range) sufficient to further reduce the loads.
Caveat: I have not tried this setup, because I am not convinced it would be entirely suitable, on heavy, narrow, traditional vessels which do not quickly recoil from being caught catastrophically aback, (and, incidentally, where the narrow beam at the mast station means the attachment point is a long way forrard on the boom, which is typically long and often slender)
In fact, it is not suited to any boats where the slenderness of the boom relative to potential loads is already a limiting consideration.

The failure element in the form I initially tested was very simple, consisting of a bundled collection of separate lashings, freestanding at the time of their creation, perhaps starting with two turns (or ‘laps’ of the endless circuit) for stage one, three for stage 2, four for stage three, and so on: all the same length, contained in an offcut of tubular webbing, and collected at each end by the pin of a suitable shackle.
The final stage should be maybe 30% stronger than the penultimate stage, being calculated to have sufficient turns to match the breaking strain of the vang line. The latter is preferably a material and layup which provides a SMALL amount of additional shock absorption, to cope with pulling up against the non-failing final stage.

This has proven to be a good system, very accurate and easy to calibrate, and the quality control and verifiability is unmatched. However (even when there are many stages) it is unavoidably somewhat jerky in operation, giving the boom and fittings a somewhat harder time than optimal … so once I had established that this was a valuable concept, I progressed to a more sophisticated energy absorption element, with a closer approximation to an analogue resistance curve, rather than the rigorously digital response of the multiple lashing setup. The system as described should, however, be kept aboard in case the ‘smarter’ element does what it is ultimately designed to do. And for long trips, plenty of cord of the same exact specification would also be a desirable and reserved item.

The improved element comprises a sandwich of webbing, sewed together in such a way that it rips open in a zig-zag sequence, making an interesting noise, and progressively putting up more of a fight.

In recognition of John Harries “sleepy dummy” challenge, and because sewing the sandwich is laborious, it MAY pay to rig a lighter, slightly shorter lashing which bridges the space occupied by this webbing element. The twofold purposes are to protect the latter from ‘seeing’ any initial load, and to fail noisily, if the mainsheet is naughtily ground in.
Provided the mainsheet and/or vang lines are not nylon, the strain energy in the mainsheet is fully released by a short swing. Hence this ‘warning’ failure does not need to apply a nasty load to the actual sandwich, which remains unaffected.
My experience would suggest this to be a necessary enhancement only when the mainsheet is taken to a winch. Even some quite large modern boats will accordingly not need this extra safety fuse.
I’ve since done some research, and found that this webbing stack is a recognised technology (eg, Google “Yates Screamer”). It seems to be used mainly to arrest falling humans, either linemen and the like, as when working in elevated personnel buckets, or on the leisure side most commonly it is issued to inexperienced climbers using “via ferrata” > metal cable ‘fixed lines’, set up in places (in parts of the European Alps) where challenging tramping routes feature easy but not trivial climbing pitches.

They are used because they are reliable, self tending, durable, consistent in their failure loads, and provide a very smooth, soft arresting force with lots of energy absorption. In the boat situation, they only fail as far as they need to, protecting both the crew (crucially) and the boom (importantly). At the same time, this setup permits the simplified user interface of side-deck vangs, which keeps the crew in the cockpit, AND aft of the “death alley” zone of the mainsheet throughout gybes.
The midspan of the windward vang tackle can pass through a ring or S-hook, enabling it be lifted towards another ring seized to the aft lower, above the heads of (perhaps crouching) crew travelling forrard on the windward jackline. On the lee side it is necessary to do the twin-tether shuffle to safely cross the vang tackle when beating or close reaching, but when running there is not a problem.

As a practical example: On a 40′ cruiser-racer with a tall three spreader rig and a heavy Leisurefurl boom, this allowed a 2:1 vang on either side to be safely used to triangulate against the topping lift and mainsheet on all points of sail, offering effectively a full-width “boom gallows” for all purposes on demand, even in conditions where the boom might dip or catch a high sea when surfing at speed (although with modern, beamy hulls, moderate length booms, and swept spreaders, this is admittedly seldom seen).
It worked well for controlling gybes without the need to shorten the mainsheet, right up to the hairy edge as might be chanced when offshore racing with a strong crew in a gale, the vang tail being taken to a spare secondary winch on either cockpit coaming. The characteristics conferred by the geometry would be particularly valuable when an inline rig with running backstays requires the boom to be ‘middled’ for some time while the runner guys do their baton-pass routine. I say this because the achilles heel of the “proper” traditional preventer is that, once the boom is near amiships, the geometry approaches that of a toggle, where the axial compression loads in the boom, fed through the gooseneck into the mast, escalate very steeply indeed because the angle of the preventer has become so acutely close to the boom. If an unrehearsed stuffup means the boom, having crossed the midline, must be returned to the original gybe, in big seas, perhaps coinciding with a squall from a new quarter, the ‘user interface’ of the side-deck tackles is much less challenging, offering much speedier, safely controlled reorientation of the boom in either direction at any instant.

In very light winds, the mainsheet can be left slack, and the triangulation of the two vangs and the topping lift be used to position the boom. This requires minimal tensions, because the force vectors form a balanced star. The topping lift directly sets the leech curve, and does not need changing unless the wind builds. During a tack, the boom will stay close to where it needs to end up, so there is little change needed. Compare this with having to haul the main traveller right across the cockpit to the new windward extreme, and keep juggling the traveller and mainsheet to re-establish the leech tension whenever the angle of attack changes.

If you like the sound of this, I strongly recommend engaging an offshore sailing type with broad knowledge of boat rigs, and preferably some background as an engineer, ideally in such fields as industrial heavy lift practices (a broader field like this seems desirable to me, if this project represents a departure from their established norms in the sailing arena), to design and specify the oddly angled tangs which should be fabricated for the sides of the boom, along with their positions (they should generally go aft as far as the lifelines permit when the boom is squared outboard, and quite high up the sides of the boom) and the necessary backing plates and, conceivably, doublers to locally strengthen the boom and spread the loads into the surrounding metal.

In the Leisurefurl case mentioned above, it has proven unnecessary to go to the extreme of doublers. Backing plates of 1/4″ SS, somewhat larger than the generously sized baseplates of the tangs, were tapped for multiple machine screws, and bedded on /insulated by heavy mylar smeared with non-setting compound/corrosion inhibitor. These fittings have served with no signs of the extrusion cracking, or indeed of undue stress concentrations at any location, for ten years in challenging waters. Obviously corner radii need to be as large as practicable.

Initially we fitted the strongest ratchet blocks available to the tangs on the boom (Lewmar 100mm Ocean) but the very first inadvertant crash gybe on a loose vang in a rising gale by an inexperienced and disoriented helmsman at 0300 on the first leg of the maiden circumnavigation liberally strewed the decks with acetal bearing balls. The strongest plain blocks in the same size from Lewmar have coped perfectly well ever since, and in practice there has always been a spare winch, so the putative advantages of ratchet blocks were illusory. They do however work well on smaller installations, allowing a 2:1 to easily be handled without recourse to a winch on, say a tall masthead-rigged 25 footer.

Stedem Wood

An eye opener, for sure.

I’m one of those who thought they had it right by rigging a boom tackle. Certainly, it worked better than a vang for sail control as the wind goes aft. I was always able to control the boom in an offshore gybe with careful use of heading, the sheet, boom tackles (one permanent on each side) and sometimes the topping lift. (Not racing, rarely a cruising chute.)

I just never got caught in the examples you’ve given.

But you should have seen my expression while reading your post, probably a combination of horror and surprise as what you said sank in. I’m guessing you may have saved someone’s rig, or maybe someone’s life if others are equally affected by what you’ve written. It’s that strong a point.

Looking forward to your follow-up.

Thank you,


Dick Stevenson

Dear John,
A couple of re-joinders:
Rounding up to hoist the main in the conditions we are talking about would be a lousy thing to have to do. However, I agree with your previous recommendation that slippery mainsail track (ours is Antal) is a huge safety feature. Not only can we douse the main in a gale going downwind, but we can put a 3rd reef in without rounding up.
Generally, I much prefer to keep some main up for just the sail balance reasons you describe. That said, on my cutter (mast amidships or there-abouts), having any main up downwind in heavier, changeable, gusty conditions makes for a quite erratic course (we are usually on electric autopilot or vane, but hand steering would not do much better in the long run). (I would not strike the main except deep into downwind sailing.) Downwind, the boat corkscrews around the main/keel which, in gusts, loads up the turning momentum more than a headsail. With the sail area on the bow (or the staysail) we sashay around much less. I am also always surprised at how little sail area is needed to maintain good boat and steering speed downwind.
I absolutely concur that an end of boom preventer is the safest most seaman-like way to control the boom. In the interest of overall safety, everyday sailing as well as the much more rare wild conditions, I would want the readers to consider what their probable behaviour will actually be. A safety device used is superior to one put off or resisted. I suspect the CCA article is written in that spirit.
I believe you are correct that a hybrid system may be best. Your tentative (at this juncture) suggestion is a boom brake/end of boom preventer (with a side decks boom control at other points of sail). This solution has 3 pieces of kit and you are already resistant to the boom brake part of that solution. You are correct that my use of a side deck preventer is dependent on judgment as to when to move on to the end of boom preventer. Probably, like reefing and the myriad of other judgment calls we all make daily on the water, it is best done when first thought of.
My best,
Dick Stevenson, s/v Alchemy

Horatio Marteleira

Hi John,
Excellent post, especially because it clearly points out the risks of setting up an unsuitable preventer and may save somebody’s life.
In my naïve days I began rigging my preventer line directly through a hole in the toerail. The toerail is massive and through-bolted at about 8-inch intervals. Guess what? In my first gybe, the hefty line was cut like butter at the not-too-smooth inner edge of the toerail hole. That’s why people need to read your posts.
Anyway, I smartened up and now run my line 2/3 back on the boom and run it through the smooth midship cleat (yes, you guessed it, it’s ridiculously massive too). This gives me a tight angle on the line that runs back to dedicated winches, one for each side. I pretty much use it all the time, even sailing upwind – that way I never forget that it’s there. I keep the line snug to eliminate any momentum-induced forces. Works like a charm, super easy to set up and I treat it like my underwear – never leave home without it.
But this is on MY BOAT, probably not suitable to most modern sailboats.

David Head

Sound thinking from all participants. I am readying a steel vessel for a long trip and have erred away from a solid preventer toward a variable friction device. I am also sourcing some head protection perhaps as worn by canoeists or climbers. Normal head protection, as worn on site, is not really satisfactory. I may cope with a smack in the ear but a fractured skull is a real no no!

Colin Speediee

Hi David

we use a Wichard Gyb’Easy on our Ovni at all times, and it’s a great piece of equipment, simple and effective. It really takes the sting out of intentional gybes in up to strong winds, and from that we take it that it should work really well in the event of an uncontrolled gybe, when it might just be a life saver. It’s also really useful when the boom is squared away, when it becomes a nice springy downhaul, keeping the main from lifting and flapping in a swell.

But it is emphatically not a preventer. The point of attachment on the boom is too far forward, unlike a preventer which should be attached as close to the outer end of the boom as possible. Whilst we’d trust the Gyb’Easy set up on it’s greatest friction setting for a short downwind run, on anything longer we rig a preventer, led from a forward cleat to the boom end.

In short, both have their place, and we wouldn’t be without them, but if we could only have one – it would be the proper preventer.

Best wishes


Michael Kornfeld

Hi Colin
A great thread on preventers. I have a question on the Wichard Gybe Easy boom brake that you have extensive experience with: the specs on it recommends a mainsail up to 40 sq m. My catamaran mainsail is about 100 sq m. And a proper preventer sounds essential, but would the Wichard Gybe Easy be useful in its highest friction setting and perhaps stationed closer to the end of the boom to increase the sheet length that would run through it? Cheers. Michael


The one problem we saw when racing 3/4 tonners with the preventer turned at the bow and run back to a secondary winch was if the bow turning block let go, the shrapnel was aimed aft. For this reason we went with over-sized blocks and a strop through the bight of the preventer. We also sheeted a starboard preventer to a port winch and vice versa to keep their bights ahead of the mast so we wouldn’t get the broken line scything aft.

Second benefit. A sister ship dipped her boom and snapped it. The preventer, rigged this way minimized the flail.

Victor Raymond

John, Dick, David, et al,
All interesting information. Several points:
John, what do you think about the single mainsail control lines on the Boreals ie one continuous control line without vangs, toe rail control nor traveller? (Maybe you haven’t seen it and Colin can comment here. )
Next on Rajah Laut we use a mid boom tackle to massive toe rail with 3:1 purchase and use it to control the exact position of the boom. Port and starboard lines go to their respective winches and as one is eased the other is tightened. There is complete control of the boom at all times. It may be to the detriment of the sailing efficiency at any one moment but the boom in conjunction with the boom end mainsheet is very controlled. (We have no traveller. )
I had this set up thoroughly reviewed by Brion Toss and he had no thoughts to improve upon it although we did review how a traveller could be added if desired.
For downwind sailing this boat prefers both head sails on poles ie genoa and Solent with no main, highly reefed or trysail (separate track.) in addition the keel is lifted to be only slightly trailing to preclude a broach.
Finally each boat is different so it is hard to generalize but you have done an excellent job of making us aware of the tremendous loads imposed by the sails and resultant dangers to life and limb. Thank you.

Colin Speediee

Hi Victor, John

The mainsheet arrangement on the Boreals cannot be classed as a preventer in any sense, and I’m sure they’d agree.

I’m with John 100% on the need for a properly rigged preventer, and although I like boom brakes for many other reason, they are not a preventer, which to me is the only way to go.

And I’m particularly taken with a couple of Chris’s ideas above, which goes to show that there’s always things to learn, even with something as seemingly straightforward as a preventer!

Best wishes


Peter Morley

Dear John et al,

it seems to me that there isn’t a single perfect solution – an end-of-boom preventer rigged forward is clearly safest with the wind well aft, but impractical close to the wind – so as several people have said a hybrid, or switching between the two, is the logical conclusion.

In fact I tend to use a single set of tackle, with a line lead to the cockpit from the boom, to switch between the two roles, albeit with careful timing when switching over.

I should state I am talking about a 24ft boat so while the loads are still significant they are rather less than on many boats, and also I rarely carry much mainsail downwind except in light airs, at least when I can set that course for a while.

I think it’s important to consider the different demands of offshore and coastal sailing there. Obviously this site is dedicated to offshore cruising, but (as John mentions with turning into channels etc.) the more frequent course changes required close to land do need to be considered. Dick is quite right that a system must be implemented to be effective.


On the face of it I don’t understand how the Boreal mainsheet could act as a preventer at all – I did a quick google looking for a picture or diagram but no luck. Does anyone have one?

Colin Speedie

Hi Pete

The scenario you mention (a hybrid system) already exists in the form of a boom brake. Our Wichard is simplicity itself, is permanently rigged and can be adjusted in an instant – no need to go and attach or remove anything, just adjust it from either end/side of the boat. I had never owned one prior to our current boat, but am now a complete convert – but it’s not a preventer….

Best wishes


Tom Witzel

Of the two deaths aboard displacement monohulls in the upper Chesapeake Bay fleet that I am aware of in the past 25 years, both were caused by an unpreventered main boom striking a crewmember in the head. In both cases the booms of the boats had vangs but were not rigged with a preventer as defined by John. Another view of preventer technique can be found at after clicking on “videos” and scrolling down to the sections showing Dorade in the 1931 Trans-Atlantic race, narrated by one of the crew. In parts 2 and 3 of these videos can be seen not only one but two main boom preventers rigged simultaneously. Dorade is such a downwind roller it appears Rod Stephens rigged both end- and mid-boom preventers to avoid breaking Dorade’s boom if she rolled it in deep. When I discussed Dorade’s preventer set up during the 2013 TransPac (she finished 1st in class & 1st overall) with her boat captain after their return to CA in August last year, he said Dorade only used one preventer rigged forward in the TransPac. He added that in the delivery south to LA before the race Dorade’s (wooden) main boom was broken while running in strong wind, and showed me where it had been spliced back together about 3′ aft of the gooseneck. Whether the break was due to compression or torque is hard for me to say but it appears Rod anticipated a problem.

Eric Klem

Hi John,

It is my opinion that the preventer should attach at the same point that the mainsheet does. By doing this, you avoid adding additional bending stress to the boom as the force of the preventer only increases the tension in the sheet and the compression in the boom. If something happens where you fill the sail from the other side, your preventer and sheet switch roles but your don’t increase your bending moment.

On a boat like Dorade, the foot of the sail applied a significant, distributed force to the entire length of the boom. If only end boom sheeting was used, the boom would have an enormous amount of bending stress which would make the boom look like a banana. To help deal with this, the sheet typically had parts mid boom and at the end of the boom. In the case where a single sheet is to be used, the bending forces on the boom are lowest if the sheet attachment is pulled in off the end somewhat (you need the actual force distribution to figure out how much but it is significant). This does mean that your sheet loads go up but you can use a lighter boom.

Many modern boats have loose footed sails which do not apply that distributed load along the boom. If the sheet is placed right at the clew, you end up with no bending moment in the boom and only a compressive force. However, if you have a sheet or preventer rigged in the middle of the boom with a loose footed sail, you have a pretty bad bending moment which could potentially break the boom. One thing that makes me nervous is when deeply reefing with these systems as the reef blocks are typically not close to the mainsheet attachment so the bending loads go up a lot. A well designed system will have no problems with this but many owners do things that the designer may or may not have accounted for.


Dick Stevenson

Eric, A really thoughtful analysis and good food for thought. One caveat for those who might jump out there and attach a side force attachment to the mainsheet attachment. Many of these attachments like to be pulled straight down away from the boom. A side force not only pulls from a not optimal direction, but imposes a teetter-totter exchange of forces that is likely to stress metal attachments to the breaking point.
Dick Stevenson, s/v Alchemy

Dick Stevenson

Hi John,
Our end of boom preventer attachment (a short pennant) is not done with padeyes, but rather is around the boom end. It is prevented from migrating forward by running it through the clew attachment on the boom (which just keeps it in place at the end of the boom, but does not take force from the preventer pennant).
Our mid boom attachment is a lashing using bigger lines and going around the mast a few times. It is kept from migrating forward by a random bolt-head (left over from a bale which broke in just the way I described) quite nicely over the years. When pressure is on, it does not take much to hold the lashing in place. The lashing also spreads the load out a bit. There 2 lashings, port & starboard so they do not need to “spin” on the boom to face different directions.
I intend to approach a sailmaker I consider creative with ideas I have about a “saddle” attachment that would spread the loads out further.
Dick Stevenson, s/v Alchemy

Dick Stevenson

Dear Andrew,
Nice report of an accident that too many (my take) take precautions around, especially good that there was no lasting damage. Two things (at least) emerge from your report:
The first is to have the boom triangulated at all times. When the boom is over the deck such as you describe this is best done by a boom tackle or (what I use/call) a vang/preventer.
The second is for all to be careful that they are not turning their boom end into a mace. Those protuberances, as you noted, could have ruined your day (or a longer span of time). Many times it might be tempting to install a padeye etc at the boom end. Generally, alternatives are possible for these potentially lethal metal attachments, either with a re-design or by using lashings, especially with webbing which is flat. It would not be unreasonable to remove/rethink those additions at boom end that interfere with it being a smooth round surface, one less likely to do damage.
My best, Dick Stevenson, s/v Alchemy

Andrew Troup

Great observations, and I do indeed now triangulate almost obsessively, certainly before anyone approaches the boom.
Even back then, I was in that habit, and in fact at the moment of impact the boom was snugged up enough NOT able to slew sideways.

However in those days I did not realise that even a triangulated boom can still generally rotate to a certain extent about its long axis, and that was what almost scuppered me. The boom did NOT swing sideways, in the usual way.
This more subtle and insidious problem, evident only in highly unusual conditions, can only be eliminated by a careful rethinking of the vector intersections, generally corrected by raising the tang locations on the boom. This way, when the foot of the sail suddenly fills, the tendency to ‘capsize’ the boom is instantly arrested by the windward vang.
In the case which caught me unawares, the topping lift made things worse, being connecting unusually low, essentially in line with the axis of rotation at the gooseneck, via a swivelling trunnion (to permit one person to easily and neatly harbour-stow the sail by rolling it around the boom)

In the years since, on boats whose sidedeck vang tangs I have engineered, this problem has not resurfaced.

Dick Stevenson

Ps. A third thought. With respect to Andrew’s report, my take is that a boom brake (I have little experience with them) might have softened the blow but would not have prevented the blow. My guess is that they would not have even softened the blow as my understanding is that they would not take effect in the few inches of boom jump that Andrew described. Would those with boom brake experience chime in?
Thanks, Dick

Bob Miller

We rig our preventer this way and think it is a sound way.

However, found the fly in the ointment though…human error.
Sailing south of Bermuda on our way to the Eastern Caribbean, we had a squall line open on us at 0345. Just a double reefed main up, using the engine. Let the boom out a bit to relieve helm pressure and forgot to tell the deck hand to take up the new slack on the preventer control line. Next thing we knew we encountered severe wind shear and the boom popped towards the other side of the boat, cracking at the traveler connection. Boom remained under control and no one was injured, but it was a lesson learned. Lots of stuff to remember when the wind is howling at 40+, lightning is snaking all around and you’ve just woken up.

We were glad the rig stayed up.


Erik de Jong

Hi John,

We use an end boom preventer on “Bagheera” as well. We have a length of dyneema permanently spliced to the eye on the boom for the main sheet, just shorter than the boom, and when not in use, it is attached to the goose neck. This way, we can loop-in an extension rope regardless of the booms position and height, and it can be used both over SB and PS tack. Very easy to use and very low budget.

I do however, always have a healthy concern when sailing fully reefed, and use an end boom preventer combined with a main sheet at the end of the boom as well. With a fully reefed main, the reef clew is somewhere half way the boom. This way one gives a significant mechanical advantage to both the preventer and the main sheet. When not paying close attention, the strain on the leech of the mainsail becomes enormous, especially since the boom usually goes up higher when fully reefed, therefore making both the sheet and the preventer pulling down more than sideways. These loads are not always visible and can become clear in an unpleasant way.
It has cost me a mainsail once (not on “Bagheera” though, but on a 30′ one-off). That was an expensive lesson learned, and a fear for destruction that does not go away when years and miles go by. Always watch what you’re doing!

Frank DeShaw

Good discussion and very sound practice. I now use end boom control after an incident I had coming from Bermuda one year.

I had a preventer rigged to center of the boom and to chain plate area. the wind piped up some and the boat yawed down the face of a wave and suddenly there was a bang and two violent gybes first one side and then the other.I rounded up and pulled the main down and continued on. I was very lucky not to have lost the rig or broken the boom. the vang I was using had let go because the cheeks of the block had bent and the cam cleats let go….talk about force!

The next day I noticed broken wire in both lower shrouds..What luck!

Larry Green

Great video, and I agree with others who have asked for more video as in “a picture is worth a thousand words”. The video allowed me to see your preventer system in action and it raises a question. One of the issues I have struggled with on our boat (a Bob Perry designed Tayana 52) is the proper rigging of a preventer. We have a Leisurefurl in boom furling system which significantly increases the weight of the boom making a preventer necessary even in light air when not far off the wind. My setup is similar to MC in that I have a similar boom line attached to the end of the boom and tied forward on the boom to a small cleat and a deck line with a carabineer type attachment to hook to the boom line and the tail runs to a clutch forward of a turning block that allows me to use a winch for tension. The big difference, which I will change is that my deck line goes to a standup block attached to the deck just aft of the forward lower shrouds. In the video and your chapter on rigging a preventer the photographs show the preventer outside of all standing and running rigging which is where my question comes in; when by boom is eased out just a bit, I need to run the preventer inside my standing rigging, as I ease the boom further out it necessitates another trip forward to move the preventer outside the running rigging and ultimately a third trip to run the preventer under the top lifeline when the boom is fully out. Do you have the same issue and if so how do you handle it? Another bit of information I got from reading and watching was your point about the increased loads that can occur when the preventer is attached to the end of the boom and the main sheet attachment is further forward. The mainsheet attachment point on my boom is about 5 feet forward of the end of an approximately 20 foot boom. I could easily move the bail on the end of the boom forward if this would help. One other issue is that once I rig the preventer on one side I routinely set up the other side which makes tacking or jibing easier for my wife and myself as we simply ease one while taking up the strain on the other side as needed. I would appreciate any feedback.
Larry Green
S/V Cailin Lomhara


I saw one mention in this thread about protective headwear, but so few sailors use it that it makes one wonder about the feasibility of fitting high density foam padding to strategic areas of the boom end that are most likely to make contact with heads. I am not knocking boom preventers by any means, but on my old wooden 30 footer there is no such thing, or even a vang/kicking strap of any sort, so injury prevention is dependent on good habits and ducking abilities up to now. Being always reluctant to complicate things, and mostly single-handed, I’d be interested if anybody else has considered padding the boom end as a precaution in case of contact.

Drew Frye

I have a catamaran, which gives me a slightly different perspective:
* Most cats do not have vangs and don’t want them. The wide traveler serves to hold the boom down through most of the range, and a vang could slow main sheet release in a gust. Yes, cruisers can reef, but sometimes when trying to power up wind you need all the sail you can manage.
* A catamaran can be capsized if the main is caught aback in enough wind. Though I’ve never heard of this on a cruising cat, I’ve experienced it on a beach cat when the traveler jammed in a heavy air jibe. I certainly don’t want to sit pinned down while I sort out the other sails.
* We generally do not square-off the boom completely, because there is a shroud in the way, and also to reduce chafe.

I look at a preventer as an extension of the traveler and a special purpose vang. I use it to hold the boom down when going deep. Because I have a lot of beam, I can rig the boom-end (yes, the only place) preventer to the mid-ships cleat (a block would help, but the friction is much less since it is only a 90 degree turn), running back to a winch much as you have described. This answers 2 additional requirements that have not been touched upon:
* It holds the boom down. If you have a vang, this may not matter.
* I can ease it quickly, completely, and under control from the cockpit in the event of an accidental jibe. This is a vital feature for me when running wing-and-wing since any small auto pilot steering error can have me sailing by the lee. (Why would a cat sail wing-and -wing, wouldn’t reaching be faster? Maybe just, but wing-and-wing is way smoother when it is rough).

Not dissimilar to what you use, just adapted to a catamaran.

Michael Kornfeld

Hi Drew, have you ever considered using the Wichard Gybe Easy as a bit of a brake, mentioned by Colin, even though you may have a larger mainsail area?

Geir Ove

We have a Lagoon 380, and we use a mooring (12mm,) line with a rubber pice inside, to take the shock, and use this at the end of the boom, and down to the cleat on stb or port side, on downwind sailing. works fine, we had gybes, up in 40 knots of wind, and al went well, the boom hardly moves only the fathead mainsail, turns over.

Pierre Askmo

Great post and I really like the solution. In your opinion can the stanchion base rod reinforcements serve as a bail for the blocks on fore deck? Or will I need a separate installation for those blocks?

Mike Evans

Hi John, We’re in Praia da Vitória in the Azores, just rigging your preventer. We can get 6mm dyneema for the boom lines and 12mm Liros Hercules yacht braid for the deck line. Would you say those are about right for our Najad 405 ? Nothing else available here.

David Watson

My Cabo Rico cutter, 38 has the main sheet bales attached about midway on the boom. Where would you suggest the attachment point on the boom for the preventer. I believe I saw in your Q & A that is should be placed about halfway from the bale and the end of the boom.