PREVENTERS

[jssailem]

When the weather turns winter foul, do you?

A. Immediately start packing your foulies and head to the boat for some excitement.​

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B. Pause over a cup of java/tea and wonder why you're hesitating. Am I getting too old for this?​

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C. Take one look at the wind and rain hitting the window pane, grab the comforter, pull it over your head, mumbling wake me next summer.​

​

One of the joys of sailing is doing it when the waters are less crowded. If you are feeling the call to get your face sprayed with winter-chilled wind-carrying water as it rises over the bow and you see your bow dip into the next wave, then this thread is for you.

The subject of preventers is focused on holding the boom in place so it does not sweep across the boat wacking crew heads, or carrying your crew off the boat, dumping them into the deep. It seems an easy fix. Just tie the boom to the side of the boat.

When you start to examine the methods and forces involved, many sailors will find the process challenging unless you are an engineer with a deep knowledge of mechanical engineering and a mastery of the sailboat design. One such individual is Kurt Zinsmeyer. His work on the "design of preventers" was recently shared on the Attainable Adventure Cruising website (morganscloud.com).

Kurt developed a spreadsheet that takes the measurements of your boat information, then evaluates the data using a preventer set on the boom end, and the line runs to the bow through a block and back to the cockpit. I found that on my 35-foot boat, the tension on the preventer would be 1040 lbs. I use a bow turning block, and the estimated Total Force at the Preventer Turning Block will be 2,053 lbs. A safety factor of 5-1 would need a line at least 5/16ths diameter with a breaking strength of 8100 lbs.

The calculated results depend on the length of my preventer, the size of my mainsail, and the angles of the preventer to the boat and boom. If I shorten the line, reduce the size, or attach it to a mid-deck cleat, the forces increase exponentially. That is why preventers are not recommended to be secured to the rail or center boom amidships.

Of course, if you are waiting till spring with light breezes and calm seas, this discussion likely is moot.

On the other hand, if you find yourself caught on the water sailing into a squall, using a preventer can be handy.

It is worth discovering the use of this safety sailing tool

 

[shemandr]

I am not an engineer, and didn't sleep at a ..... , but I don't see the need to run the preventer to the bow, from the end of the boom and back to the cockpit.
First of all doubling the line thru the bow block doubles the force on it the block. John's argument then states that the forces would increase exponentially if the line were shortened or run to a mid-deck cleat or rail. What if the line were run from the rail or cleat directly to the mid-boom area, like a vang? And if the line was a little stretchy? Would you need 8100 lbs of breaking strength? I guess I don't understand why forces increase exponentially for a vang type preventer. Like I said, I'm not an engineer.

 

[Foswick]

D) All of the above.

I have rigged various permutations of a preventer on my previous boat (Catalina 27), which I would deploy when sailing wing-on-wing. Was never much of an issue because I would never sail DDW unless winds were pretty modest. Not to say that a gust couldn't ruin my day, but the forces were generally not that great on that rig in those conditions. In fact, usually if the boom made a surprise visit, I would reach up and catch the mainsheet with one hand as it passed over head, to slow it down a bit.

My current boat presents a different picture. She is a ketch and the main boom passes really low over the companionway and the cockpit. If someone just has their head poking up out of the companionway more than a few inches, they are in a really dangerous spot if an accidental jibe is possible. Add to this the fact that the mizzen creates some opportunities for more interesting sail configurations. We could potentially be wing-on-wing-on-wing. (I actually did this in light winds, just to see. Not something I would do, ordinarily, but novel nonetheless.)

The nice thing is that, we we are sailing downwind in any significant wind, we are likely to just drop the main altogether and sail jib-and-jigger. We found this summer (first season on the boat) that this is a really stable and performant configuration. She sails downwind on her own ... I'm not sure I even need to be on the boat ... makes me feel extraneous.

But, this brings another dynamic. If winds were stiff, I would still want some sort of preventer or boom brake. The mizzen is almost like a little toy sail, but there is still enough boom to break something, if we really got nailed. The good news is that rigging a preventer on the mizzen is a little easier. First, the forces are much reduced because of the small sail area and shorter boom. But, the preventer, rigged to a point forward, can still be within reach of the cockpit. That means I can attach it to a cleat or block on the port side. Then, when if I need to jibe or otherwise move the boom to the starboard side, I can just detach it from the deck fitting, walk it around behind the mizzen mast, and hook it up on the starboard side - all without leaving the cockpit.

Haven't done this yet. It's still a thought experiment. But, that's my plan.

The new boat has a diesel furnace, so the intent is to exercise a bit more of option A, above, this winter.

 

[jssailem]

What if the line were run from the rail or cleat directly to the mid-boom area, like a vang? And if the line was a little stretchy? Would you need 8100 lbs of breaking strength? I guess I don't understand why forces increase exponentially for a vang type preventer.

Ok..clarity may be needed. The forces that are applied by a 252sq ft sail in 30 plus knots of wind when backed are calculated to be such that if you tie a line mid-boom to the rail, the boom will bend and collapse at the point it is tied unless the rope fails. In a rope failure, the boom will sweep across the boat. At the end of the sweep are shrouds to help stop the boom, but at what cost?

A short, stretchy line, maybe like nylon. How stretchy? What happens to the line that is stretched? Nylon breaks suddenly.

Tying the line to the end of the boom protects it from damage. Like an anchor snubber, the long line provides a product that can stretch to absorb the forces on the boom while keeping the boom in place.

Preventers are not meant for conditions when you frequently adjust the sail, like round-the-can races. I use them when sailing on a tack that is off the wind for more than 30 minutes, when I'm on a broad reach, or when I'm running downwind.

I want to let the sail out and yet prevent the boom from swinging across the boat should a swell unbalance the trim and the backwind the sail. This usually means the vang is eased, as I want the sail to be full.

The preventer lets me fix the boom in place as I tension it between the preventer and the main sheet.

 

[Foswick]

I think it's worth noting that, like me, John sails in an area where sudden wind shifts are the rule, rather than the exception. Around here, the winds twist around several islands and, particularly when combined with all kinds of weird currents, can really spring surprises on you. It is not unheard of (or really that uncommon) to have 10 kts from one direction and, within just a few seconds, 10 kts from nearly 180 degrees. These sudden shifts are even more common when winds are sub-10 kts and we are more inclined to sail on a broader reach or DDW. Also, weird tidal convergences can turn your boat 30 degrees or more in just a second or two.

 

[Rich Stidger]

I use a Dutchman Boom Brake, model BB500 on my h40.5. This boom brake utilizes a line that is tied from toe-rail to toe-rail. This brake line is threaded through the brake mechanism and it provides the friction to control the boom movement athwart-ships. The location of the toe-rail attachments is determined so that the boom brake traverses a constant radius arc from side to side. This brake mechanism has an adjustable friction device that increases the friction on the brake line and controls how fast the boom is allowed to traverse from side to side.

Additionally, there is a single control line that attaches to the boom brake and is run back to a cockpit winch. It's purpose is to pull on the brake mechanism and tension the brake line in the brake mechanism.

This system offers two functions. First and foremost it controls the boom in a gybe situation and slows the boom crossing to a very slow safe rate. And second, if the boom is out on one side , like in a downwind configuration, the brake can be tightened to the point that it will not allow the boom to move at all- essentially providing a preventer function.

All of this is controlled by a single line lead to the cockpit. The friction device on the brake is generally set once and does not need to be re-adjusted for different situations. See the attached pictures -

 

[dLj]

I am not an engineer, and didn't sleep at a ..... , but I don't see the need to run the preventer to the bow, from the end of the boom and back to the cockpit.
First of all doubling the line thru the bow block doubles the force on it the block. John's argument then states that the forces would increase exponentially if the line were shortened or run to a mid-deck cleat or rail. What if the line were run from the rail or cleat directly to the mid-boom area, like a vang? And if the line was a little stretchy? Would you need 8100 lbs of breaking strength? I guess I don't understand why forces increase exponentially for a vang type preventer. Like I said, I'm not an engineer.

The forces involved are related to the angles the line makes with the boom and point of attachment. This is essentially vector related math.

The strength needed can be calculated using a vector that is running 90 degrees from point of attachment to the mast but you can't achieve that so you are looking at where you attach the preventer to the boat to get as close to that ideal 90 degree connection. As that angle gets smaller, the forces increase rapidly. Kurt's analysis went through the forces involved as that angle gets smaller. It's a very eye opening analysis.

dj

 

[Charles Erwin]

How about a boom brake as an option? That way no need to release the preventer to gybe the boat, but no crash gybes either.

 

[dLj]

How about a boom brake as an option? That way no need to release the preventer to gybe the boat, but no crash gybes either.

Kurt's analysis was prompted by some disasters that lead to broken booms and such. I don't recall if he looked at boom brakes.

When I watched his original presentation a number of months ago, i was focusing on the parts relevant to my situation - I don't have a boom brake, I use a preventer so if he addressed boom brakes, i don't recall.

Are you a subscriber to John's Attainable Adventures web page? I highly recommend it if you haven't . I haven't read what was printed there - you'd have to go see.

dj

 

[Rich Stidger]

How about a boom brake as an option? That way no need to release the preventer to gybe the boat, but no crash gybes either.

+1. See my post #6.

 

[dlochner]

How about a boom brake as an option? That way no need to release the preventer to gybe the boat, but no crash gybes either.

We have a Wichard Gyb'Easy. It is not a preventer, it simply slows down the boom as it crosses over. Works well, I can easily jibe the boat by myself with the autopilot.

It is attached to the boom with a ¼" dyneema strop with a continuous line running from port side of the cockpit to the starboard side. Two blocks attached to the chainplates guide the line back.

Gyb'easy: boom brake | Wichard Marine

The article Gyb'easy: boom brake is on the Wichard Marine website! Discover all our Frein de bôme and all our product ranges.

marine.wichard.com

 

[jssailem]

These wet, cold days, I'm not on the boat, so I'm rereading old magazine articles and drinking cups of coffee. I just came across this on Preventers. It tells the story about rigging one and explains why a preventer run to the bow is safer than a mid-boom rig.

Disagreement is expected as the bow rig is more difficult to rig and few sailors experience the backwinding sudden jybe of the mainsail.

Here it is from the work of Practical Sailor..

A preventer can eliminate the hazard linked to the boom crossing the deck during an accidental jibe. But it can also leave a crew, engaged in the middle of a mainsail reefing effort, caught in a tricky back-winded fire drill. In essence, a jibe with a preventer set puts a sailboat into a pinned down position with the backwinded jib and mainsail held to windward by the preventer. This is when the crew realizes that the preventer now handles as much load as the mainsheet did a few minutes ago, and it’s doing so at a 1-to-1 ratio rather than the usual 4-to-1 reduction that a mainsheet creates. It’s also when the crew fully appreciates having secured the preventer to as large a winch as possible (see “The Best Prevention is a Preventer,” see PS July 2017.

Experienced racing crews learn to ease the boom and reef under sail while the boat is close reaching and the jib is sightly overtrimmed.
A preventer can keep an unintentional jibe from becoming a lethal threat to the crew, but only if it includes an effective means of handling a backwinded mainsail and a boat pinned over on its rail. In such circumstances, the best preventer option is one that has been led from the boom end to a block at the bow and returned to a big winch in the cockpit. This single part line now holds the same force once handled by a multipart mainsheet. By securing the preventer tail to a large cockpit winch, the preventer can be safely eased and the vessel brought back under control.

Other options, such as attaching a mid-boom, cam cleat equipped, block and tackle to the rail may work to keep the boom from careening across the deck. But the real challenge is easing the back winded mainsail and getting the boat back on its feet.

 

[BAD ORCA]

I wonder if it would be possible to make a self contained gear type configuration that absorbs and slows down forces of boom movement, similar to how a car or truck downshifts on a highway to create some geared drag, that could attach directly to the mast/gooseneck at the mast. It could be engaged/disengaged/ set like an autopilot remotely.

Under normal, controlled sailing it could be completely disengaged. When sailing downwind it could be turned on and set depending on wind conditions. I suppose it could even tap into the boat instrument network to get wind data and gyro data to automatically adjust for them. Ultimately, i suppose it possibly could even be used like a traveller and be used to adjust the angle of the boom in or out slightly through the geared mechanism sort of like an electric geared winch, but maybe this is pushing the envelope of stresses to high. Not sure im no engineer.

If possible could be designed to absorb the forces of an accidental jibe incrementally through gear reduction/changes that happen on the fly through settings on a remote console or manually. Think of an inertia type of gear that engages under rapid quick movement like a flywheel or a seatbelt, but instead of abruptly stopping the motion, it slows it down and absorbs the motion through gearing.

Oh. And my answer would most definitely be A

 

[garymalmgren]

When the weather turns winter foul, do you?


A. Immediately start packing your foulies and head to the boat for some excitement.

B. Pause over a cup of java/tea and wonder why you're hesitating. Am I getting too old for this?

C. Take one look at the wind and rain hitting the window pane, grab the comforter, pull it over your head, mumbling wake me next summer.

However if you have promised a mate that you will go out, you are bound by your (foolish) words.