Hi

one of the jobs that has me out of the cockpit is putting on a preventer each time the boom is out before the wind. Simply a length of rope doubled
from the main sheet attachment at the end of the boom to a strong point forward of the boom. This is on a 40 foot sailing boat, sometimes sailed single handed.
An experienced friend suggests I should rig port and starboard ropes from the vang mounting point on the boom down to blocks each side attached to strong points near the chain plates These are led from the blocks back to the cockpit and held with.a winch or cleat or jammer depending on your set up. When the boom is out wide the relevant guy is hauled In to prevent the boom coming back in an unwanted gybe. The other guy is left slack until needed when the boom is out the other side. Essentially permanently rigged preventers which don't require leaving the cockpit.
Speaking with another chap who sails a Northshore 38 singlehanded, he swears by his boom brake which hangs from his boom at the vang attachment point and has two turns of a rope around its drum, with one end of the rope tied off to a chainplate and the other led through a block on the other chainplate back to a winch in the cockpit. When tightened with the winch the boom brake acts to slow or even prevent the boom swinging across. Does anyone have any good or bad experiences as to the merits and draw backs of boom guys versus a boom brake?

TIA Q

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JonE said..

I was just about to post that one myself and also one on the SV Platino incident but then I went down a rabbit hole checking out NZ maritime regs about safety inspections.

Yep how many spare booms do you have?

second and before everyone gets carried away the gybe was not the cause of death in the Hobart or the Moreton bay death also on a Bene 44.7

and with the Plantino the boat was modified poorly after launch and this modification was the primary failure when auto pilot failed
the report is good reading

the expert NZMaritine used was also an expert in the Moreton Bay death coroner inquest and now the police Hobart investigations from what I hear

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lydia said..
Yep how many spare booms do you have?

second and before everyone gets carried away the gybe was not the cause of death in the Hobart or the Moreton bay death also on a Bene 44.7

and with the Plantino the boat was modified poorly after launch and this modification was the primary failure when auto pilot failed
the report is good reading

the expert NZMaritine used was also an expert in the Moreton Bay death coroner inquest and now the police Hobart investigations from what I hear

I was looking forward to your contribution. I hadn't read that report.

www.orcv.org.au/docman-link/safety/3705-platino-mnz-accident-report-2016/file

Quixotic - what's your take on this? Mine is, that you might have to make 1 trip to the foredeck every few hours but if you rig a Pennant (not something I've done yet) it will at least make it easier. There's no point having a boom brake or a mid-ships preventer if all it does is make you feel good until you really need it, at which point you're screwed.

The report is sobering reading because it's a reminder of how rapidly a steering failure can escalate to a full scale mayday.

No one rigs and spinnaker pole guy/brace to the foredeck for obvious reasons, a preventer rigged midships is basically the same thing.

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JonE said..

lydia said..
Yep how many spare booms do you have?

second and before everyone gets carried away the gybe was not the cause of death in the Hobart or the Moreton bay death also on a Bene 44.7

and with the Plantino the boat was modified poorly after launch and this modification was the primary failure when auto pilot failed
the report is good reading

the expert NZMaritine used was also an expert in the Moreton Bay death coroner inquest and now the police Hobart investigations from what I hear

I was looking forward to your contribution. I hadn't read that report.

www.orcv.org.au/docman-link/safety/3705-platino-mnz-accident-report-2016/file

Quixotic - what's your take on this? Mine is, that you might have to make 1 trip to the foredeck every few hours but if you rig a Pennant (not something I've done yet) it will at least make it easier. There's no point having a boom brake or a mid-ships preventer if all it does is make you feel good until you really need it, at which point you're screwed.

The report is sobering reading because it's a reminder of how rapidly a steering failure can escalate to a full scale mayday.

I posted the query because I figured there'd be some strong views and possibly some interesting anecdotes, Certainly the chap in the video JonE posted has no doubts, though I found his interruptions and editorialising over the original Ministry of Sail video a bit jarring. I'll find their video and watch it as it was intended.

For now I just clip on and go forward. I agree a pennant might be a useful addition. At the moment I head up and haul in to get the end of the boom within reach to string the preventer through the end. Then let the boom back out and go forward to attach the preventer at the strong point forward.

I am considering installing a set up that does the job and reduces having to leave the cockpit in roly poly weather. I've never used a boom brake, or been on a boat that had one, so have no idea of their merits. If they do what they say on the tin, then it seems to me they're more for helping control a gybe (planned or otherwise) rather than preventing it altogether. That means the forces are less than for a preventer trying to hold the boom despite a backed main. If it is truly backed then having it come across in a more controlled fashion seems like it might be the right outcome.

What I can't grasp from the report is, do I want a static or dynamic rope? I understand that we don't want to pre-tension the preventer because it just takes us closer to breaking point (yet another reason to rig it for'd) but a bit of stretch would be good yes? Or no?

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Quixotic said..

JonE said..

lydia said..
Yep how many spare booms do you have?

second and before everyone gets carried away the gybe was not the cause of death in the Hobart or the Moreton bay death also on a Bene 44.7

and with the Plantino the boat was modified poorly after launch and this modification was the primary failure when auto pilot failed
the report is good reading

the expert NZMaritine used was also an expert in the Moreton Bay death coroner inquest and now the police Hobart investigations from what I hear

I was looking forward to your contribution. I hadn't read that report.

www.orcv.org.au/docman-link/safety/3705-platino-mnz-accident-report-2016/file

Quixotic - what's your take on this? Mine is, that you might have to make 1 trip to the foredeck every few hours but if you rig a Pennant (not something I've done yet) it will at least make it easier. There's no point having a boom brake or a mid-ships preventer if all it does is make you feel good until you really need it, at which point you're screwed.

The report is sobering reading because it's a reminder of how rapidly a steering failure can escalate to a full scale mayday.

I posted the query because I figured there'd be some strong views and possibly some interesting anecdotes, Certainly the chap in the video JonE posted has no doubts, though I found his interruptions and editorialising over the original Ministry of Sail video a bit jarring. I'll find their video and watch it as it was intended.

For now I just clip on and go forward. I agree a pennant might be a useful addition. At the moment I head up and haul in to get the end of the boom within reach to string the preventer through the end. Then let the boom back out and go forward to attach the preventer at the strong point forward.

I am considering installing a set up that does the job and reduces having to leave the cockpit in roly poly weather. I've never used a boom brake, or been on a boat that had one, so have no idea of their merits. If they do what they say on the tin, then it seems to me they're more for helping control a gybe (planned or otherwise) rather than preventing it altogether. That means the forces are less than for a preventer trying to hold the boom despite a backed main. If it is truly backed then having it come across in a more controlled fashion seems like it might be the right outcome.

So lets forget the fashion and look at the physics.
A preventer rigged mid boom pulling downwards is loading the boom in a way never designed for and has the worse physics as while it may stop the boom swinging across there will be no load on the main sheet and all load on the preventer and the loads mostly in tension not compression.
Worst though is that once you do gybe, unless released you have a force aft of the centre of lateral resistance so you end up heading you head too wind which of course if also poled out means the headsail is also completely backed.
Niether the boom or the pole are designed to take these loads.

So if you do want a preventer, take it from the end of the boom to the very bow and have it able to be released immediately. That is best load wise.

As for a boom brake, these are not set and forget and will not prevent the boom changing side. All a boom brake does it what a good main sheet trimmer does and that is control the speed at which the boom comes across.
Bit again the physics comes in to play as the take off will be mid boom and booms are not designed to take that type of load.

Of course, if cruising you can avoid all of this, just sail higher till the sea state drops off and save yourself several thousand dollars in extra stuff on your boat.

If you can't steer you boat downwind that is a different issue.

Lastly most recreational auto pilots are not designed to cope running square in heavy air and most yacht steering system are not up to the job either.
Different if you have the NKE self learning unit like Shaggy had on the pogo 12.

On one well known class of ocean racer which some people had trouble steering in hard conditions when pressing hard, we changed the gear size in the pedestal so when turning the wheel, you got the rudder further off the centreline quicker to avoid a gybe or broach.
The boat was not already mostly gone before the rudder got good effect.
So yep increased load at times but boat was always light on the helm.
Auto pilot then worked heaps better as well.
All worked a treat.
And the new gear was off the shelf and cost $38.00 from memory.
We also replaced the chain with Tsubi chain which was about $100 a metre but you get what you pay for but we made the replacement and 2 spares from the standard 3m length.
So make your boat steer better first and avoid the gybe in the first place.


Even on a large powerboat I still have manual steering, chain gear and wire to the quadrants. The reason his that in really heavy downwind ocean conditions when hand steering you can get the rudders off the centreline quicker than standard hydraulics.
Even the oversized commercial grade autopilot cannot complete with good hand steering.
Sure we could come up with a system that worked but if it is not broke why fix out.
l

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JonE said..
What I can't grasp from the report is, do I want a static or dynamic rope? I understand that we don't want to pre-tension the preventer because it just takes us closer to breaking point (yet another reason to rig it for'd) but a bit of stretch would be good yes? Or no?

I would be going with nylon anchor rope to reduce shock loading.

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cammd said..
No one rigs and spinnaker pole guy/brace to the foredeck for obvious reasons, a preventer rigged midships is basically the same thing.

Actually, on the old Maxis and particularly in the US they would rig a foreguy to the bow to control an alloy pole that 3 of you could not lift.

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lydia said..

JonE said..
What I can't grasp from the report is, do I want a static or dynamic rope? I understand that we don't want to pre-tension the preventer because it just takes us closer to breaking point (yet another reason to rig it for'd) but a bit of stretch would be good yes? Or no?

I would be going with nylon anchor rope to reduce shock loading.

I'd be worried that all nylon would be too stretchy. Generally nylon stretches at least 15 and as much as 30 per cent. If the preventer is running from cockpit to bow and back to boom end then it will be longer than the boat's LOA. On a 40 footer it'll likely be at least 50 feet of rope and possibly a bit more. If nylon stretches 20% then there's 10 foot of give in a 50 foot rope. My boom is about 12 foot long. So using an all nylon preventer wouldn't prevent the boom from gybing, unless I winched most of the stretch out of the nylon - putting a helluva lot of force on the preventer, and if I'm going to take out the stretch, then might as well use less stretchy rope to begin with.

The physics behind the consensus that a midship boom preventer is unseamanlike is all about the load multipler affect.

To arrest a back winded sail you need an equal force in the opposite direction, that means the preventer needs to be perpendicular to the boom running forward to prevent the boom swing aft if it only has to arrest an equal force

however

It is not possible to run the preventer perpendicular to the boom on a yacht, we would need a bow that is wider than midships to do so.

As a consequence the angle the preventer is run forward is less than perpendicular therefore it requires the ability to a arrest a greater force than the one it is opposing.

In other words the load on the preventer is multiplied as the angle it is run from the boom is reduced.

Therefore we need to run the preventer as far forward as we can so that it has the largest angle from the boom possible and thereby minimising the load multiplier.

If we run it to the bow the multiplier may only be times 2 or times 3, if we run it to midships it may be time 10 or 15. Doing the math would provide an accurate number but you get the idea.

The load multiplier affect is further amplified by adding a downward angle by attaching it to a rail close by and it is further amplified by attaching it half way down the boom by the lever arm affect that is added, if that doesn't break the preventer it will probably break the boom.

So you can see the maths paints a very dangerous picture for a midships preventer. In the case of SV Platino the autopilot problem caused a unexpected gybe, the midship preventer failed/broke and the boom was left to swing wildly as the boat continued to gybe due to a autopilot failure. That resulted in the modification to the mainsheet attachment breaking which began to swing wildly off the end of the boom like a wrecking ball.


As for attaching a boom brake to chain plates, I think chain plates are designed to take loads along their vertical axis and a boom brake will provide a close to horizontal load that is multiplied by the force acting on the sail many many times over by the multiplier affect.

This will be my set up soon, running back to a winch on the windward side

I have to give the website a plug, the subscription is the best $40 I have spent, so much good info in it and constantly updated.

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cammd said..
This will be my set up soon, running back to a winch on the windward side

That's very similar to my setup.
I have a line from the end of the boom to the front held with elastic, and some hooks, which is held along the boom when not in use.
I have a line from the cockpit, through a clutch, to a block on the fore deck, back to the mast base when not in use.
I do have to leave the cockpit to attach the two lines, by a hook, to set the preventer but it only takes me less than a minute!!

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lydia said..

So lets forget the fashion and look at the physics.
A preventer rigged mid boom pulling downwards is loading the boom in a way never designed for and has the worse physics as while it may stop the boom swinging across there will be no load on the main sheet and all load on the preventer and the loads mostly in tension not compression.
Worst though is that once you do gybe, unless released you have a force aft of the centre of lateral resistance so you end up heading you head too wind which of course if also poled out means the headsail is also completely backed.
Niether the boom or the pole are designed to take these loads.

So if you do want a preventer, take it from the end of the boom to the very bow and have it able to be released immediately. That is best load wise.

Physics is just maths, and I'm a sucker for maths. Based on my calculations as below, the forces don't get to boom breaking territory at all.

Perhaps I have got the maths wrong at some point.

I've a 12 metre boat with the mast about 5m from the bow and a boom about 4m long. So a preventer rigged from the bow to the boom end with boom at 90 degrees to mast will be angled from the centre line about 39 degrees. Vector-wise 100 kg of forward force on the boom end requires 128kg of tension on the line. This is leaving aside the vertical angle of the rope (given boom isn't level with bow), as at about 11 degrees it doesn't much alter the vector equation (adds about 2 per cent, so 130 kg all up).

The beam at the widest point is 3.7metres, so 1.85 from the centre line. At the mast and chainplates (spreaders are not back swept and shrouds are in line with mast) the beam is 3m, and 2 metres for'ard of the mast is 2m. If I were to rig a preventer from the toe rail 2m for'ard of the mast, the angle of the rope (to the centre line) to the end of the boom would be 56 degrees. And vertical angle would be about 27 degrees. To exert 100 kg of forward force on the boom end would require 200kg of tension on the rope, this time allowing for the both the horizontal and vertical angles as the latter adds about 11 per cent. The preventer would be about 6 metres shorter, as it wouldn't have to travel to the bow and back.

The vector forces can be understood the other way round. That is, if the main backs, the preventer to the bow will experience 1.3 times the load on the rope as the pressure back on the boom measured at the end of the boom. Whereas the preventer rigged to the toe rail 2 metres in front of the shroud will experience 2 times the load on the rope as the pressure back on the boom measured at the end of the boom.

If the wind is 15 degrees ahead of the mainsail, and my main is an unreefed 27 square metres in 20 knots of wind, the initial back pressure on the sail is about 46kg. As the boom is a lever and the centre of effort is about one third of the boom length back from the mast, the force on the end of the boom is about 15kg. So not likely to trouble a preventer rigged to the bow nor one to the toe rail 2m ahead of the chainplate.

This sounds pretty light, and it is. The reason an uncontrolled gybe in 20 knots is more forceful is that as the boom responds to the back wind, the angle of the sail changes, and so the wind pressure on it increases. If the wind is 15 degrees ahead of the beam, by the time the boom has swung to 15 degrees from the centreline the wind force on the main is 175 kilos, and boom will be moving quite fast.

Going back to preventer loads, if the wind is 15 degrees ahead of the mainsail, and my main is a third reefed 10 square metres in 40 knots of wind, the back pressure on the sail is about 68kg. As the boom is a lever and the centre of effort is now perhaps a quarter of the boom length back from the mast (moves forward as reefs put in), the force on the end of the boom is about 16kg. So again not likely to trouble a preventer rigged to the bow nor one to the toe rail 2m ahead of the chainplate (where the force on the rope will be about 32kg).

Let suppose I rig the preventer to the vang point on the boom. Mine's about 1.2 metres from the mast. This changes the angles quite a bit and loses the leverage available from using the end of the boom.

Rigged to the bow, the angle of the preventer from the centre line is about 14 degrees. Vector-wise 100 kilos of forward force at the vang point requires 103 kg of tension on the rope.

Rigged to the toe rail 2m ahead of the chain plate the horizontal angle of the preventer to the centre line is just 6 degrees, as the vang point is only 20cm outboard of the toe rail where the preventer is rigged. The more important angle is the vertical angle of about 26 degrees from the block on the toe rail to the vang point. Together 100 kg of forward force on the vang point would require about 112 kg of tension on the rope.

If the wind is 15 degrees ahead of the mainsail, and my main is an unreefed 27 square metres in 20 knots of wind, the initial back pressure on the sail is about 46kg. As the boom is a lever and the centre of effort is about one third of the boom length back from the mast, the force on the vang point is about 64kg. Still not likely to trouble a preventer rigged to the bow nor one to the toe rail 2m ahead of the chainplate, where the force on the rope will be about 72kg.

If the wind is 15 degrees ahead of the mainsail, and my main is a third reefed 10 square metres in 40 knots of wind, the back pressure on the sail is about 68kg. As the boom is a lever and the centre of effort is perhaps a quarter of the boom length back from the mast (moves forward as reefs put in), the force on the vang point is also about 68kg. Still not likely to trouble a preventer rigged to the bow or to the toe rail 2m ahead of the chainplate (where the force on the rope will be about 76kg).

Lets suppose I'm racing and so only have two reefs in at 40 knots, for about 16 square metres of main. If the wind is 15 degrees ahead of the main, then the force on the sail will be 110kg and maybe 130kg at my vang point if the centre of effort is a bit more than 1.2 metres aft/outboard of the luff. Force on the toe rail mounted preventer might be about 145kg, and perhaps 135kg for the bow mounted preventer.

None of this equates to broken booms. Perhaps if to the 2 reef/40 knot scenario, we add another 20 degrees as the boat broaches a bit on a wave, then the wind force backwinding the sail gets up to about 240kg, and at the vang point about the same, with the load on the toe rail mounted preventer getting to 270kg. Still no drama for 12mm double braid rope. But perhaps some booms might break with the weight of 3 large people concentrated at the vang point. I've got my doubts though, since my vang has a five to one purchase and then is run to a winch, I'd be mighty surprised if it doesn't routinely exceed 300kg of force and more when winched tight.

To be clear, if the main is somehow 90 degrees to a fifty knot gust, with just 2 reefs in, the pressure on the main is well over half a tonne at 650 kg. But that's an unlikely scenario, unless the boat has lost steering and somehow ended up with the bow to the wind with the preventer holding the boom backwinded at 90 degrees to the wind. Or perhaps if there's a 180 degree wind change accompanying a squall. Or...?

Have I stuffed up the calculations somewhere?

The report from Maritime NZ in the link that JonE posted has all the math in it complete graphs and graphics. The multiplier effect increased the loads enormously on the preventer when it attached midship.

Multiplier is times 11.5 when the angle is 5 degrees, stated in the report

You know the loads are high.

Why would you rig for any less mechanical advantage than the maximum?

The whole thing is meant to prevent fatality in the instance when you have necessarily lost control of the boat.

But most booms break more in the vertical plane not horizontal plane.
The load exerted by the mainsail leech trying to lift the boom is at the end of very long lever if the boom is tied down at the vang fitting by the preventer.
So you need to calculate the loads in the other plane with for example of Cammd's boat a say 4.0m lever
Using the end of the boom negates the length of the lever.
So you need to calculate the loads on the clew in a gybe then apply that to lever.

Then you have the loads on the deck fittings.

Next most booms are designed with bigger sidewalls then tops/bottoms so even any lateral load is been exerted on the weakest part of the boom in any event.

I just watched this and am a little confused.
I get that he angles should be as broad as possible,
Incidentally , I have a heavy duty rubber loop that I fit to the end of the boom with a soft shackle. A line is shackled to it that leads to my forward mooring cleat. The line cleats off there. The rubber allows for some give which I hope will save my boom.
Not perfect, and I am rigging up a gybe brake system. Will let you know how that goes.


Right , rig from a solid fixture at the boom end with stretchable (weak) polyester or nylon 3 strand line. (29:00 in video)
That weak line is fixed to an indestructible Dyneema line with eye slices and proper shackles. that will hold 15,000 or 17,000 pounds (29:30 in video)
This line goes forward and then back to the cockpit to be secured.
You use Dyneema because you don't want stretch.
You use nylon because you want stretch?
He says that the nylon line will stretch, which is what he wants, (17:22).
This stretch is the nylon line will allow for a "controlled"gybe.

My point here is if you want super strong line that will not break and splices instead of knots that will not give way, why introduce weak line that can break?
If you want a slow controlled gybe, why not just fit a gybe brake?

He says "You cannot rig a midship preventer and call your self a serious sailor".(38:00)
Sorry Pip. That rules you out.

Love this comment from another site!
"I like it when the boom breaks cleanly in the middle. Easier to sleave."

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Quixotic said..

lydia said..

So lets forget the fashion and look at the physics.
A preventer rigged mid boom pulling downwards is loading the boom in a way never designed for and has the worse physics as while it may stop the boom swinging across there will be no load on the main sheet and all load on the preventer and the loads mostly in tension not compression.
Worst though is that once you do gybe, unless released you have a force aft of the centre of lateral resistance so you end up heading you head too wind which of course if also poled out means the headsail is also completely backed.
Niether the boom or the pole are designed to take these loads.

So if you do want a preventer, take it from the end of the boom to the very bow and have it able to be released immediately. That is best load wise.

Physics is just maths, and I'm a sucker for maths. Based on my calculations as below, the forces don't get to boom breaking territory at all.

Perhaps I have got the maths wrong at some point.

I've a 12 metre boat with the mast about 5m from the bow and a boom about 4m long. So a preventer rigged from the bow to the boom end with boom at 90 degrees to mast will be angled from the centre line about 39 degrees. Vector-wise 100 kg of forward force on the boom end requires 128kg of tension on the line. This is leaving aside the vertical angle of the rope (given boom isn't level with bow), as at about 11 degrees it doesn't much alter the vector equation (adds about 2 per cent, so 130 kg all up).

The beam at the widest point is 3.7metres, so 1.85 from the centre line. At the mast and chainplates (spreaders are not back swept and shrouds are in line with mast) the beam is 3m, and 2 metres for'ard of the mast is 2m. If I were to rig a preventer from the toe rail 2m for'ard of the mast, the angle of the rope (to the centre line) to the end of the boom would be 56 degrees. And vertical angle would be about 27 degrees. To exert 100 kg of forward force on the boom end would require 200kg of tension on the rope, this time allowing for the both the horizontal and vertical angles as the latter adds about 11 per cent. The preventer would be about 6 metres shorter, as it wouldn't have to travel to the bow and back.

The vector forces can be understood the other way round. That is, if the main backs, the preventer to the bow will experience 1.3 times the load on the rope as the pressure back on the boom measured at the end of the boom. Whereas the preventer rigged to the toe rail 2 metres in front of the shroud will experience 2 times the load on the rope as the pressure back on the boom measured at the end of the boom.

If the wind is 15 degrees ahead of the mainsail, and my main is an unreefed 27 square metres in 20 knots of wind, the initial back pressure on the sail is about 46kg. As the boom is a lever and the centre of effort is about one third of the boom length back from the mast, the force on the end of the boom is about 15kg. So not likely to trouble a preventer rigged to the bow nor one to the toe rail 2m ahead of the chainplate.

This sounds pretty light, and it is. The reason an uncontrolled gybe in 20 knots is more forceful is that as the boom responds to the back wind, the angle of the sail changes, and so the wind pressure on it increases. If the wind is 15 degrees ahead of the beam, by the time the boom has swung to 15 degrees from the centreline the wind force on the main is 175 kilos, and boom will be moving quite fast.

Going back to preventer loads, if the wind is 15 degrees ahead of the mainsail, and my main is a third reefed 10 square metres in 40 knots of wind, the back pressure on the sail is about 68kg. As the boom is a lever and the centre of effort is now perhaps a quarter of the boom length back from the mast (moves forward as reefs put in), the force on the end of the boom is about 16kg. So again not likely to trouble a preventer rigged to the bow nor one to the toe rail 2m ahead of the chainplate (where the force on the rope will be about 32kg).

Let suppose I rig the preventer to the vang point on the boom. Mine's about 1.2 metres from the mast. This changes the angles quite a bit and loses the leverage available from using the end of the boom.

Rigged to the bow, the angle of the preventer from the centre line is about 14 degrees. Vector-wise 100 kilos of forward force at the vang point requires 103 kg of tension on the rope.

Rigged to the toe rail 2m ahead of the chain plate the horizontal angle of the preventer to the centre line is just 6 degrees, as the vang point is only 20cm outboard of the toe rail where the preventer is rigged. The more important angle is the vertical angle of about 26 degrees from the block on the toe rail to the vang point. Together 100 kg of forward force on the vang point would require about 112 kg of tension on the rope.

If the wind is 15 degrees ahead of the mainsail, and my main is an unreefed 27 square metres in 20 knots of wind, the initial back pressure on the sail is about 46kg. As the boom is a lever and the centre of effort is about one third of the boom length back from the mast, the force on the vang point is about 64kg. Still not likely to trouble a preventer rigged to the bow nor one to the toe rail 2m ahead of the chainplate, where the force on the rope will be about 72kg.

If the wind is 15 degrees ahead of the mainsail, and my main is a third reefed 10 square metres in 40 knots of wind, the back pressure on the sail is about 68kg. As the boom is a lever and the centre of effort is perhaps a quarter of the boom length back from the mast (moves forward as reefs put in), the force on the vang point is also about 68kg. Still not likely to trouble a preventer rigged to the bow or to the toe rail 2m ahead of the chainplate (where the force on the rope will be about 76kg).

Lets suppose I'm racing and so only have two reefs in at 40 knots, for about 16 square metres of main. If the wind is 15 degrees ahead of the main, then the force on the sail will be 110kg and maybe 130kg at my vang point if the centre of effort is a bit more than 1.2 metres aft/outboard of the luff. Force on the toe rail mounted preventer might be about 145kg, and perhaps 135kg for the bow mounted preventer.

None of this equates to broken booms. Perhaps if to the 2 reef/40 knot scenario, we add another 20 degrees as the boat broaches a bit on a wave, then the wind force backwinding the sail gets up to about 240kg, and at the vang point about the same, with the load on the toe rail mounted preventer getting to 270kg. Still no drama for 12mm double braid rope. But perhaps some booms might break with the weight of 3 large people concentrated at the vang point. I've got my doubts though, since my vang has a five to one purchase and then is run to a winch, I'd be mighty surprised if it doesn't routinely exceed 300kg of force and more when winched tight.

To be clear, if the main is somehow 90 degrees to a fifty knot gust, with just 2 reefs in, the pressure on the main is well over half a tonne at 650 kg. But that's an unlikely scenario, unless the boat has lost steering and somehow ended up with the bow to the wind with the preventer holding the boom backwinded at 90 degrees to the wind. Or perhaps if there's a 180 degree wind change accompanying a squall. Or...?

Have I stuffed up the calculations somewhere?

These loads seem more realistic.
SPEC GUIDE: CALCULATED BOOM VANG LOADS





BRAKE MODEL WALDER 250/250B WALDER 500/500B WALDER 750/750B

BOAT SIZE 20-32 feet 30-40 feet 42-60 feet MAINSAIL SIZE (FT2) 250 sq. feet 500 sq. feet 750 sq. feet LINE SIZE 1/4 - 5/16 in. 3/8 - 7/16 in. 1/2 - 3/4 in. LINE BREAKING STRENGTH 2,350 - 3,850 lbs. 5,100 - 7,000 lbs. 10,100 - 11,700 lbs. LINE SAFE WORKING LOAD 470 - 770 lbs. 1,020 - 1,400 lbs. 2,020 - 2,340 lbs. HARDWARE SAFE WORKING LOAD 1,600 lbs. 2,700 lbs. 4,100 lbs. CALCULATED BOOM-END MAINSHEET LOAD AT REEFING LIMIT (HARKEN CALCULATOR) 650 lbs. 1,400 lbs. 3,500 lbs. CALCULATED BOOM-END MAINSHEET LOAD WITH 40% IMPACT (CRASH JIBE) 910 lbs. 1,960 lbs. 4,900 lbs. CALCULATED BOOM-END MAINSHEET LOAD (RESTRAINED BY LOCKED BRAKE) 352 lbs. 759 lbs. 1,897 lbs. BOOM BRAKE LOAD IF APPLIED AT RECOMMENDED BRAKE SADDLE LOCATION (25%) 1,409 lbs. 3,035 lbs. 7,587 lbs. PERCENT OF WORKING LOAD LIMIT (WLL) OF THE BRAKE HARDWARE 88 percent 112 percent 185 percent PERCENT OF WLL OF ROPE (MAXIMUM SPECIFIED DIAMETER) 183 percent 217 percent 324 percent PERCENT OF BREAKING STRENGTH LINE (MAX LINE SIZE)* 52 percent 62 percent 93 percent Note: All manufacturers of jib taming devices have similar specifications. *Includes 30 percent loss due to constriction in jammer.

So on a 42 footer, 1600kg on the main sheet with end boom sheeting in a high wind gybe
About 3500 kg at the boom brake in recommended position.

The other really stupid thing and I mean really stupid is that often these brakes are hung off the vang fitting it worse a block hanger on the track on the underside of the boom.
so firstly where does the vang sleeve end as often it looks like the attachment point is aft of the end of the sleeve
more importantly these fitting are designed to have a vertical load not a horizontal load.
so where does the sideways load ultimately bare?
on the gooseneck which again is not designed to have a twisting load.
My 20 cents so think it through first every boat is different
l

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lydia said..
So on a 42 footer, 1600kg on the main sheet with end boom sheeting in a high wind gybe
About 3500 kg at the boom brake in recommended position.

(Link to article: www.practical-sailor.com/sails-rigging-deckgear/be-careful-with-boom-brakes)

This is the maximum load generated during a crash gybe with a boom brake, My calculations were of the forces that occur on a preventer when the wind backs the main by 15 degrees. If the preventer works, then the boom doesn't swing and so the forces do not increase as the wind angle becomes less shallow to the mainsail, nor do they increase due to changes in the geometry. In the article both of these changes occur. This is acknowledged in the article by the statement: "The fundamental problem, which no constant-friction device can get around, is that a jibe begins with very little force, but then requires massive braking force as the main jibes and opens to the full force of the wind." [emphasis added]

As per my earlier post, the initial force due to a 15 degree backwind is not great. If the preventer doesn't stop the boom from swinging, then the loads will escalate rapidly as both the angle of the preventer becomes closer to parallel with the boom, and the wind acts more directly on the sail. This will happen with a boom brake that doesn't prevent the gybe - since by definition the boom swings through. But a preventer that doesn't allow the boom to swing will not face the escalated forces that occur when the boom does swing.

If the preventer is too stretchy or loose, such that the initial backwind causes the boom to swing 25 degrees, then the angle of the wind to the sail goes from 15 degrees to 40 degrees, and the force on 16 sqm of reefed sail from 40 knots of wind escalates from 110kg to 270kg and the vector force of the boom on the preventer also increases, The amount the preventer has to stretch for this to happen is just 1.2m. Assuming a 17metre length from cockpit to bow and back to the boom, that's 7 per cent stretch - entirely possible with double braid, (tho' probably not with HMPE). With a preventer to the bow from the end of the boom, the angle without the boom moving was 39 degrees. If the boom swings 25 degrees, then the angle from the centreline of the preventer to the boom end is reduced to 28 degrees. To resist the now 270kg wind force, if I calculate correctly, the preventer has to take 567kg, which is perhaps 75 per cent of the working load limit of 12mm double braid. If that load extracts another bit of stretch from the line (only another 0.4m), such that the boom moves another 10 degrees, then the wind load reaches 320kg, and by my calculation the preventer needs to exert 780kg of force which is just over the working load limit, though only perhaps 20 per cent of the breaking strain - before allowing for knots and sheath slippage etc. I'm doing these calculations on the fly, so I may have stuffed them up somewhere, but they seem consistent with the general warning about rapidly escalating forces as a gybe occurs. At this point the boom is still 55 degrees from the centreline. From that point on, if the boom continues to move, the forces go through the roof.

The forces escalate because the boom moves, increasing the wind force and narrowing the angle of the preventer to the boom end. It's also the case that as the boom gets close to the centreline, the end of boom mounted preventer will put pretty high compression loads on the boom, pushing the gooseneck forward into the mast. I'm not sure when that becomes a risk to the mast.

If the preventer keeps boom movement to very little, the forces don't approach those levels.

I agree with Gary, the youtube video seems to contradict itself, advocating for low stretch HMPE, which would minimise boom movement, but then wanting to add a stretchy element presumably to cushion shock loads. But if the preventer assembly stretches, the loads escalate rapidly.

I haven't redone the calculations on the forces on a preventer mounted to the toe rail from the vang point, but the general principle of, if the boom moves, the forces escalate rapidly, will also apply to it. And as they climb above the several hundred kg range, which they will if the boom swings quite a bit , then I'd expect boom breakage would definitely be on the cards.

Another risk with any preventer is if the end of your boom (and worse, your sail) end up in the water, the forces can dwarf those exerted by the wind and something will break unless the preventer is let go, even more so if the preventer is rigged mid-boom.

Lots to think about.

Just read part of the CYCA incident report on the 2024 s2h. I don't understand the section on preventers, in the report they are considered by many as relics from the IOR era that are not needed on modern racing yachts as crash gybes are less likely on boats with bow sprits running assymetrical spinnakers.

I am a little gobsmacked to read that given the fatal consequences in that race as a result of 2 crash gybes.

Clearly modern yachts are at risk of crash gybes with the consequences being no less catastrophic than they were to older designs. It doesn't make sense why preventers are so readily dismissed as a way to diminish the risks.

I must be a Philistine. We use ordinary stretchy Ocky straps. A selection kept below and put the chosen ones on for the conditions.

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Trek said..
I must be a Philistine. We use ordinary stretchy Ocky straps. A selection kept below and put the chosen ones on for the conditions.

The report talks about a Farr 50 goosewinging in 300 knots

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JonE said..

Trek said..
I must be a Philistine. We use ordinary stretchy Ocky straps. A selection kept below and put the chosen ones on for the conditions.

The report talks about a Farr 50 goosewinging in 300 knots

300 knots!? Let me read again

The way I read it wind speeds of 30 to 48 knots. I dont understand your point?

Don't overthink it.