Safety on board: preventer setup on Totem

downwind offshore ocean sailing sailboat

Boom preventer. Boom brake. Whatever you do, whatever you call it, having a way to prevent or dampen the force of the boom to prevent accidents while sailing deep downwind is important. A lot of cruising IS downwind, so thinking through a smart setup is critical! I’ll never forget learning about a boat some miles ahead of us on the Pacific crossing where a crew sustained life-threatening injuries after a crash gybe. Even a planned, controlled gybe tends to give me the willies due to the tremendous force involved: a violent, unexpected gybe can cause significant damage.

The sliver of a new moon wil set before we pull up Totem’s anchor tonight. Ahead is a challenging passage, one we’re not sure how far we’ll take: hopefully, all the way to Puerto Rico, if all goes well (follow our progress at our tracking page on PredictWind). Breeze expected is all forward of the beam, so there’s no need for a preventer– but recently someone asked about our setup. Jamie wrote it up, I took a few pictures to illustrate.

whale spout sailboat sailing ocean mountains
Whale-watching as we sail away from South Africa – preventer in place

DSC_7708What works for us will not right for every boat, but is a safe, strong, and reliable method on Totem. and I’m sharing here in case it helps others install or improve their own preventer. We like it because:

  • Simple approach
  • Side decks left uncluttered
  • No specialized/dedicated gear purchase necessary
  • Puts loads at points able to withstand them (mast/vang/midpoint of boom not intended for the shock loading involved- outboard end of boom is much better)
  • Quickly/easily released from the cockpit if necessary

Totem’s setup: Component Parts

  • 1 x boom lanyard – Dyneema single braid, with ¼” (6mm) diameter.
  • 2 x preventer lines – polyester double braid, diameter depends on sail area (Totem uses 1/2”). Polyester gives a little stretch, but not too much. Length depends on preventer block location and center or aft cockpit. Lines should be long enough that preventer set on one side can remain in place through a gybe.
  • 2 x Preventer blocks or low friction rings – we have blocks, but low friction rings are a great choice: they are more robust and lower cost.


A preventer must bear considerable loads; in the worst scenario, shock loads that will cause a weak link to fail. For this reason it’s safer to secure the preventer to the aft end of the boom. A middle boom attachment point is more likely to break the boom in an extreme situation.

End of boom attachment can make setup awkward/hazardous or require fixed preventer lines that will cross the deck and get in the way. This preventer setup splits the preventer line into two sections. One line is the boom lanyard; and the others are the preventer lines (1 on each side of the boat). The lines are out of the way when stowed and easy to deploy.

Boom Lanyard

sailboat boom

The boom lanyard is shown above as the line running below the boom. The aft end is spliced around the boom. The forward end has an eye splice to secure to the boom when not in use, as shown, and to use as an attachment point to the preventer lines. When stowed, it’s important to keep the boom lanyard tight along the boom because a drooping line can catch on something or someone.

Eye splice rope line lanyard
Eye splice at the forward end of the lanyard…and around boom on the aft end

This shows one end of the boom lanyard spliced around the boom and the eye splice in the other end. Boom lanyard length should be set as follows:

  1. Easy to secure to the vang attachment when not in use.
  2. Easily reaches the side deck when the boom is out, so it’s safe tying to the preventer line.
  3. Another use for this line is to secure the boom from swinging back and forth when not sailing.

Preventer Lines

sailboat deck

This view down Totem’s side deck shows one preventer line, stowed and ready for use. Things to note, besides those lovely clear side decks:

  1. One end of the preventer line is secured to the lifeline. The other end leads back to the cockpit and is coiled and ready for use.
  2. Fair leads are important! Note that one side of the preventer line runs outside of the lifelines. The other side runs aft along the deck and is NOT fair in this picture. You’ll see that in a later picture I reran this side to go between shrouds so it doesn’t chafe.
  3. The next picture shows me (Jamie) getting ready to connect the boom lanyard and preventer lines together. Note that I am pulling the boom outward for the picture; normally I would be sitting in an easier and safer position when underway. (Behan: you bet he would, or I’d be unhappy about it!)


Secured to the toe rail with a Dyneema loop is the preventer line turning block. Friction is not an issue with the preventer, so consider a low friction ring instead.


Location matters:

  1. Setting the block too far forward increases preventer line length and is hard to run fair.
  2. Setting the block too far aft makes a bad angle when the boom is all the way out.

Our blocks are set about 2 feet forward of the forward lower shrouds, a position that gives a fair lead and good angle to secure the boom.


Above is a snapshot of the boom lanyard and preventer line, tied together and ready to use. Do not use a shackle! The knot is much gentler should it hit something or someone. This is especially true when you do gybe (by choice). Simply ease the preventer line to allow the boom to swing over.

andersen winch

The other end of the preventer line, ideally, goes on a self-tailing cockpit winch: ours goes to one of Totem’s spiffy Andersen secondaries. This approach makes a quick release easy if needed. If you don’t have an open winch here, you can cleat the line. Either way, be sure to coil the end of the line, and keep it clear to run freely in case you need to quickly release the preventer.


Here’s the completed setup, much as it can be from our Bahamian anchorage! Notice how the preventer lead has been moved to run fair between the shrouds.

Boom brakes

For some boats, a brake makes sense. These don’t prevent the boom from crossing over, but dampen the movement. We’re not fans of this on Totem because it would place tripping-hazard lines on the side deck. But for other boats, other layouts, they’re a great option: the setup at our friend’s boat Akira, anchored a few boatlengths away, is a great example of this. Keeping it all on the coachroof means there’s no dangerous deck clutter, and they can handle it right from the cockpit.

green line runs to brake on boom, and clutch in cockpit


I’m looking forward to having a passage that requires setting up the preventer, not this upwind stuff! But for now, will tackle the upwind days ahead by cooking up a storm, checking and re-checking all stowage, and loading books on the kindle from our hometown library.

Another cutthroat game of DogOpoly with crew from the Manta 42, Akira
Another cutthroat game of DogOpoly with kids from the Manta 42, Akira: having a lot of fun with this crew.

Adults in the cockpit, kids in the cabin, paparazzi mama.

With thanks to Bonnie,  for the question and for the kind donation to our cruising kitty!

13 Responses

  1. Hi Jamie,

    Your approach to rigging a preventer is the one I’ve always used. However, John Harries is adamant about leading the preventer all the way to the bow and has load stress based arguments for doing so. Could you discuss your (and my) reasons for leading the attachment to a position a few feet forward of the mast?


  2. Hi Richard,
    I used to lead the preventer to the bow. It made sense because of the angles. More forward means the line is further from parallel to the boom, so carries less load. No mystery there. I quickly found the lead aft to the cockpit is lousy. Preventer line loading is dynamic and the line has stretch, so it suffered chafe against metal bits or caused chafe to canvas or bow stowed dinghy. I started shifting the block aft. For the Pacific crossing I had the block set midway between mast/bow. This worked fine, and chafe was less but the line still rubbed on the dodger. I’d seen no sign of overloading, despite the block being fairly light duty (there is no shackle distortion). So I moved it further aft; and it’s been this way for the Indian and Atlantic Ocean crossings. It’s well tested (especially crossing the Mozambique channel!). The Ronstan blocks show wear (one cheek is slightly bent from hitting the toe rail) and have worked well, but I do want to switch to low friction rings.

    There are a few considerations,foremost is force from shock loading. A lower stretch system with high modulus fiber mainsail or preventer line (Dyneema, Technora, Vectran, Carbon, etc) results in far great peak loads than a system with shock absorption (Dacron main and Polyester preventer). I suspect the block location (preventer angle) and block load rating does make a difference in a stretch-less system. Shock loads break things – I had a Harkan high-load snatch block used for barbour hauler blow apart in just 10 knots of wind – because the headsail collapsed/filled due to sea state. It’s also important that the preventer line not be too stretchy such as with nylon. Lastly, the run between boom and block needs to run clear of the lifelines – sometime I have the preventer on when broad reaching, to prevent a gybe if there are big wind shifts (off Thailand for example, we had big shifts when crossing headlands and bays). If the preventer block is all the way forward and the boom isn’t out so far, the preventer line can foul/chafe on lifelines; and if it really loads up, can damage stanchions.

    There is no single best way that works best for everyone.

    1. Great post Jamie, appreciate the information, still trying to figure out the best setup for my boat.

  3. Hi Jamie,

    In my case the search for a fair lead away from the life lines led to choice of attachment position.

    Thanks for the detailed description of your reasoning


  4. Another great post – thanks. So interesting to see different set ups, and I agree it is about finding out what works for you and your boat. We used a similar system but led the control line right up to the bow. This was the easiest way for our set up and a way one person could control the mainsheet and the boom “break”.
    Our boom was lethal – head height through the cockpit!
    I’ve drawn a rather basic diagram on our set up – that I’ve shared with many…. but if anyone wants to see it, it is on one of my FB pages called Of Foreign Build (recent post) and/or Cruisers’ AA (scroll down a bit)… 🙂 Or contact me and I’ll send it on.
    Thanks Behan – once again I’ll be reading all comments with interest.

  5. Behan and Jamie, We agree with your preventer approach. We’ve used a similar (but simpler) system on Wings for many years. Polyester line, (5/`16″), led from end of boom to the side deck abeam the mast, or farther aft when close reaching just to stabilize the boom, then back to a cleat in the cockpit. One line will do, shift it to the other side after a jibe. No blocks or fairleads, just tie it to the boom and run it through a stanchion base. When going upwind we stow it on the boom forward for easy deployment.

    It has saved accidental jibes on more than one occasion. Getting it un-cleated was a problem once and the back winded main held us down for a while, so leading to your winch might be a good idea.

    Fred and Judy

  6. Hi Fred & Judy

    Long time since you used to run down the docks at Shilshole shedding business suits along the way to sail Wings in the Wednesday night beer can races!

  7. Great post!

    On a course which is gybe-prone we rig a preventer similar to your setup and if practical also alter course to bring the apparent wind direction average a bit more on the quarter. This cuts the probability gybing down some and increases through the water speed a bit. Cost is a longer over-the-ground track. Generally not a good tactic for racing but for cruising where the mission is to arrive safely, enjoyably and with no busted heads or hardware tacking downwind has us served well.

  8. Great post as always. I particularly loved the brief shout out to libraries at the end!

    Too few folks realize they can access a huge selection of ebooks, e-audio books, and downloadable videos through their library from ANYWHERE in the world. Well, anywhere with moderately decent internet, that is.

    Titles can be borrowed for periods ranging from 7 to 30 days, depending on the library and service. Most services have apps that streamline the borrowing process. Most also limit the number be of titles borrowed by a card each month.

    The most popular services are:
    Overdrive – for ebooks and eaudio books (and limited video). Best for browsing rather than searching for specific titles since, like physical books, these titles are one checkout at a time.

    Hoopla – ebooks, audiobooks, music, tv series and movies. Video selection is similar to netflix, but not as comprehensive. Titles are always available, regardless of how any others have it checked out.

    Flipster and Zinio both offer magazines. Titles are chosen by the library, but if you suggest a title or topic, they’re likely to give it serious consideration the next time they update their subscriptions. (We want our services used and if we know a patron will read it, we’ll get it!)

    Not all libraries have each of these services, and some have others not listed. Regardless, it’s worthwhile for cruisers (and vicarious cruisers like me) to check out what is available.

    And if you’re somewhere on the Seven Seas with no library card? Well, that’s what friends and family back home (and with library cards) are for, right?

    Alex (the Librarian)

  9. What do you have the big eye splice around the boom going through to keep it from sliding forward?

    1. Hi Greg – there is a pad eye on each side of the boom. They’re offset – with roughly a foot, fore/aft, between them. Then a Dyneema lashing, over the boom links the pad eye together, and keep the preventer (and the main sheet strop) captive. Could be setup to go through a single pad eye, but it will chafe some.

  10. Another great post. Thanks Jamie and Behan.

    On your advice a few years ago before the Pacific crossing we changed our preventor system to a “Totem” one. We have a padeye two feet forward of the fwd lower shroud so that makes a nice turning point. We have also moved to using low friction rings. So easy (and cheap). Two small changes we have made is to use soft shackles to attach the dynmea line from the boom to the 1/2″ dacron double braid. I have stitched them to the dynmea eye so that they do not blow away when not attached (but the stitching is not loaded in use when attached to the dacron line). The other simple change was to attach the dynmea line with a small snap gate carabiner to a loop of shock cord that runs from the gooseneck. That keeps it easy to keep the lines out of the way when not in use.

    Thanks for all the great support over the last five (!) years.

    We are in Vanuatu now and contemplating heading back to BC after next cyclone season. Latest crazy thoughts/intentions/plans are continue to liveaboard there and cruise locally.


    SV Fluenta

  11. Hello ,

    Very Nice Blog, Great post as always. Great tip over suggested on the blog. well explained especially through the images. can you please post blog relayed to stream or boat maintenance also !


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