Jibe preventer rigging

[wingssail]

Originally Posted by Dockhead

Before doing something like that, I suggest you check the geometry of the preventer lead. You can easilyl magnify the force on the preventer by 10x or 20x with a lead like that. Just do the math and you will see. Rigging the preventer as you suggest is exactly how two guys on Platino got killed.



See also:


https://www.morganscloud.com/2018/10...that-can-kill/


"Amidships Preventers -- A Bad Idea That Can Kill", Morgan's Cloud.


Attachment 242571


Here is the diagram from the Platino accident report:


Attachment 242572


Here's the Platino accident report, which is full of good information about preventers.



So in short, NEVER do this. Preventers should always be rigged to the bow.

I prefer to run the preventer to a point on the rail ahead of the mast, where the angle is good but it avoids the issues I mentioned previously about bein g all the way to the bow.

With regards to the strength of this system, one occasion we had reason to test that:

With the preventer rigged to a point ahead of the mast (yet not all the way to the bow) we did an accidental jibe (through my total screw up) at midnight, in over 26 knots of wind. The mainsail completely backwinded and pressure on it knocked the boat down to windward AND HELD IT THERE.

We were layed down about 75 degrees and by the wind on the back side of the main and the preventer (3/8 polyester line) and held that way until we could release the preventer (quite a ling time). The boat was motionless and on its side and I was aft hanging on the the tiller. Judy came up and released the preventer.

[Dockhead]

Originally Posted by wingssail

I prefer to run the preventer to a point on the rail ahead of the mast, where the angle is good but it avoids the issues I mentioned previously about bein g all the way to the bow.

With regards to the strength of this system, one occasion we had reason to test that:

With the preventer rigged to a point ahead of the mast (yet not all the way to the bow) we did an accidental jibe (through my total screw up) at midnight, in over 26 knots of wind. The mainsail completely backwinded and pressure on it knocked the boat down to windward AND HELD IT THERE.

We were layed down about 75 degrees and by the wind on the back side of the main and the preventer (3/8 polyester line) and held that way until we could release the preventer (quite a ling time). The boat was motionless and on its side and I was aft hanging on the the tiller. Judy came up and released the preventer.

I would certainly check the angle and do the math. I wouldn't do it if it's less than 25 degrees or so.



Another factor in preventing preventer breaks is to keep it tight so that there won't be snatch loads from the boom swinging. If you've got some slack in the preventer, I guess the swinging boom can break whatever preventer rigged however.


You already have some snatch loads from the sail blowing over and that's bad enough.


My preventers are made from racing dyneema, because I don't want any stretch allowing the boom to swing. I use the same lovely ropes as spinnaker sheets.


I have never had any problem rigging a preventer to the bow on any of probably dozens of boats I've done it on. I run my outside the lifelines through the middle of a bow cleat, then back along the deck, using other cleats as leads, and finally to a secondary winch. Very simple and no problems.

[GreenWave]

reading all of this and much makes sense. However, it seems a BRAKE is what is really needed to protect against the accidental gybe. And this could also work for intended gybes.

[marcjsmith]

Originally Posted by Dockhead

So in short, NEVER do this. Preventers should always be rigged to the bow.

NEVER is a strong word



this is kind of what i was talking about.


https://cruisingclub.org/article/per...ffshore-yachts



in lighter winds inshore near shore, IMo mid boom to to rail even using a soft vang on a 4:1 that can be easily removed/released with a snap shackle would be better than nothing.



once the boom start to move off the shrouds, there is no stopping it. even bobbing down the bay with a 10knot breeze. helmsman loose focus or the boat rolls down a small swell from a powerboat the instinct is to grab the mainsheet as its going by to slow it down. and for those that have sailed dinghies, the instinct is hard to ignore.

[Dockhead]

Originally Posted by marcjsmith

NEVER is a strong word

this is kind of what i was talking about.

https://cruisingclub.org/article/per...ffshore-yachts

in lighter winds inshore near shore, IMo mid boom to to rail even using a soft vang on a 4:1 that can be easily removed/released with a snap shackle would be better than nothing.

once the boom start to move off the shrouds, there is no stopping it. even bobbing down the bay with a 10knot breeze. helmsman loose focus or the boat rolls down a small swell from a powerboat the instinct is to grab the mainsheet as its going by to slow it down. and for those that have sailed dinghies, the instinct is hard to ignore.

"Never" is indeed a strong word, and entirely appropriate to this case, in my opinion. I'm sticking with it.

A preventer rigged the way it's shown in that article is an accident waiting to happen. The engineering is extremely simple -- you can do the math yourself. Measure the angle and look at the table I posted and you see instantly the extent to which that angle magnifies the forces on the preventer.

Just because you are near shore doesn't mean you can afford a severely weakened preventer because of incorrect geometry. In fact, near shore you are more likely, not less to encounter a sudden violent squall, from land-based thermal weather events.

I would really suggest reading the linked Platino accident report. If you do, I can just about guaranty you will never rig a preventer like that again. It's exactly what killed two guys on Platino.

[goboatingnow]

A block and tackle from the mid cleat to the boom end works for me.

[Dockhead]

Originally Posted by goboatingnow

A block and tackle from the mid cleat to the boom end works for me.

Have you checked the angle of the line to the boom IN THE HORIZONTAL PLANE?


Have you read the Platino accident report? It's linked above.

[marcjsmith]

read the report and while you are damning the preventer. when in fact simple maintenance check by the skipper damned the autopilot to failure.


due to a malfunction of the rudder drive unit. This was mainly due to a lack of hydraulic oil due to an undiscovered leak in the system.

Generally, no crewmember was positioned to take immediate control


The owners indicated that they had not checked the level of the fluid in the header tank of Platino’s hydraulic linear drive at any time.


The quadrant was constructed of FRP20 and did not appear to have been designed to accommodate the steering ram brackets as they were installed. The quadrant was not obviously reinforced in the area where the brackets for the rams were mounted and no doubler plates21 were fitted on the upper side.



. The likelihood of the mounting points failing in this manner was increased by the addition of the spacers.


the sealing surface was found to be in substandard condition and this was judged to be the cause of the leak.


Neither the leak, nor the reducing fluid level in the header tank, was noticed by the crew.


. In this environment, even a momentary lapse of attention from an experienced and capable helm person could be unrecoverable and result in an uncontrolled gybe.




looking tat the diagram. if the pennant had been attached to the boom closer to the snatch block , the angle from the boom to the block would have been much better. the way they had it rigged it wouldn't have taken much stretch in the line to start moving. since the angle was effectively pulling along the length of the boom and down. just a crappy way to rig a preventer. the angles of the dangles and all...the preventer had no leverage if they had rigged the preventer at the boom directly above the snatch block (orange dot). they would have had a better angle , but, higher loads (lever arm of the boom and all) than what would have occurred at the end of the boom



You say the the poorly rigged preventer killed the crew. I say the

Captain killed the crew by not checking the oil level in the auto pilot.

[Dockhead]

Originally Posted by marcjsmith

read the report and while you are damning the preventer. when in fact simple maintenance check by the skipper damned the autopilot to failure.

due to a malfunction of the rudder drive unit. This was mainly due to a lack of hydraulic oil due to an undiscovered leak in the system.

Generally, no crewmember was positioned to take immediate control

The owners indicated that they had not checked the level of the fluid in the header tank of Platino’s hydraulic linear drive at any time.

The quadrant was constructed of FRP20 and did not appear to have been designed to accommodate the steering ram brackets as they were installed. The quadrant was not obviously reinforced in the area where the brackets for the rams were mounted and no doubler plates21 were fitted on the upper side.

. The likelihood of the mounting points failing in this manner was increased by the addition of the spacers. the sealing surface was found to be in substandard condition and this was judged to be the cause of the leak.

Neither the leak, nor the reducing fluid level in the header tank, was noticed by the crew.

. In this environment, even a momentary lapse of attention from an experienced and capable helm person could be unrecoverable and result in an uncontrolled gybe.
looking tat the diagram. if the pennant had been attached to the boom closer to the snatch block , the angle from the boom to the block would have been much better. the way they had it rigged it wouldn't have taken much stretch in the line to start moving. since the angle was effectively pulling along the length of the boom and down. just a crappy way to rig a preventer. the angles of the dangles and all...the preventer had no leverage if they had rigged the preventer at the boom directly above the snatch block (orange dot). they would have had a better angle , but, higher loads (lever arm of the boom and all) than what would have occurred at the end of the boom

You say the the poorly rigged preventer killed the crew. I say the

Captain killed the crew by not checking the oil level in the auto pilot.

That's pretty much like saying that it wasn't the inoperable bilge pumps and unserviced life raft which killed the crew. It was the captain who didn't check the weather. By this logic, whey rig a preventer at all? Just don't have an accidental jibe. Keep your pilot oil up, watch the luff.



But bottom line -- don't bother rigging a preventer to midships. It will give you a false sense of security. Just take your jibe like a man, and don't forget the oil in your pilot.



If you want a preventer which will actually work in an emergency, however, and not break and slash across the cockpit, beheading someone -- rig it to the bow.

[wingssail]

Originally Posted by Dockhead

I would certainly check the angle and do the math. I wouldn't do it if it's less than 25 degrees or so.

There is math and there is judgement. 25 degrees or so is not hard to achieve. How strong your preventer arrangement is, and is it strong enough, is a opportunity for some judgement, if yours is sound enough.

As I have demonstrated the rig I use has the strength to hold the boat layed over 75 degrees. In my judgement it's enough.

On the other hand, with the boom eased out for a close reach (meaning not even out to the lifeline) the angle from the end of the boom to the bow is not enough to hold the boom steady, which is just as often what we're trying to accomplish. In that case the preventer should go to the widest part of the boat.

NEVER is a pretty strong word. Rigging for various sailing situations requires some flexibility and judgement.

[wingssail]

Originally Posted by GreenWave

reading all of this and much makes sense. However, it seems a BRAKE is what is really needed to protect against the accidental gybe. And this could also work for intended gybes.

The issues I have with the boom brake is that it is a lot of falderal hanging from the middle of the boom over the cabin top. and there are permanent lines to each railing. And possibly some adjustment for it to work properly.

AND I don't want it to slowly ease the boom over, I want it to PREVENT to boom from coming over at all.

But most of all I don't want that crap hanging from the boom all the time, it's going to be in the way 100% of the time.

[goboatingnow]

Originally Posted by Dockhead

Have you checked the angle of the line to the boom IN THE HORIZONTAL PLANE?


Have you read the Platino accident report? It's linked above.

Yes I have good angle as the mid ships cleat is quite far forward. I’m comfortable enough with the horizontal angle.

[marcjsmith]

Originally Posted by Dockhead

That's pretty much like saying that it wasn't the inoperable bilge pumps and unserviced life raft which killed the crew. It was the captain who didn't check the weather. By this logic, whey rig a preventer at all? Just don't have an accidental jibe. Keep your pilot oil up, watch the luff

Deck, the way they rigged from the end of the boom to the toe rail resulting in a 6* angle, the difference in forces between that and a 30* is HUGE. Nothing good was going to come from that.

Bear with me as I open up my statics textbook from college
we are making the assumption that everything is In a single plane. For simplicities sake

Let’s figure a force of 490 Newton’s hanging on a 5 meter bar 90* to a wall and then a wire from the end of a bar to the wall at 30*
sine 30*=490/force on wire Algebra gives us a force of 980 newtons on the wire.

Now let’s use the same formula for a 6* wire from the end of the bar That gives us a force of 4687 newton on thewire. That’s nearly 10 times more than the original 490 Newton’s and 5 times more than the 30* wire.

Now let’s look at the 5 meter bar with 490N hanging on one end, the other end is fixed, but we are going to support it one meter from the fixed end. Essentially creating a fulcrum that’s going to bear the load. The formula is F2=(F1 x load distance)/effort distance
Load distance is 5m
Effort distance is 1m
Force 1 is 490n
F2 is unknown
2450N=(490 x 5)/1.

We end up with 2450 Newton’s of force at that point so that nearly 3 times the force of the 30* wire, but half as much as the 6* wire.

The skipper admitted that he didn’t perform what should have been a routine check. It resulted Ina catastrophic failure. Some of the ways the rigged the autopilot also contributed to a catastrophic failure. The crew failed to properly rig a preventer. The crew placed all of their faith in “Otto” and the preventer. Two where sacrificed .

Let’s not argue that going to sea is safe. It’s not. We can make it safer, but in the end If we wanted to be safe, we’d sit on a sofa watching judge Judy eating our ho-ho’s and still die of a heart attack.

I hope one day to take my sailboat beyond the waters of the Chesapeake bay. And experience blue water under my keel. On the bay will I rig a preventer from the end of the boom to the bow. Probably not... once I nose out past the bridge/tunnel and see what real wind and waves are likes I’m sure my thoughts will change.

Fair winds

[Dockhead]

Originally Posted by marcjsmith

Deck, the way they rigged from the end of the boom . . .

no, from mid-boom

Originally Posted by marcjsmith

to the toe rail resulting in a 6* angle, the difference in forces between that and a 30* is HUGE. Nothing good was going to come from that.

Indeed. And another factor against midships preventers is the angle in the VERTICAL plane, which is much greater when you run the preventer to midships. This is beyond my engineering knowledge, but wouldn't you MULTIPLY the force again by this factor?


So you've got the following situation:


Force on a backwinded sail WITHOUT snatch loads -- say 1400kgf for a 50m2 sail in a 40 knot gust (using cD of 1).


You rig to mid-boom -- 2800kgf (if in a vang position 1/3 of the way from the gooseneck, as recommended in that egregious Cruising Club article, then 4200kgf).



10 degrees angle in the horizontal plane (you're doing better than Platino because your cleats are further forward -- 5x, so 14 000kgf


45 degree angle in the vertical plane -- 1.4x, so 19 560kgf.


Breaking load of 3/8 StaSet? 2000kgf. Of 1/2"? 4454kgf.



A block and tackle will be stronger than a single line, but it's obvious if you do the math that this is BAD ENGINEERING. Again, "never" is not too strong a word for this.

Originally Posted by marcjsmith

. . . Let’s not argue that going to sea is safe. It’s not. We can make it safer, but in the end If we wanted to be safe, we’d sit on a sofa watching judge Judy eating our ho-ho’s and still die of a heart attack. . .

And what's the point here? That we shouldn't try to make going to sea safer? Then why rig a preventer at all? It's all down to the autopilot?

[Dockhead]

Originally Posted by wingssail

There is math and there is judgement. 25 degrees or so is not hard to achieve. How strong your preventer arrangement is, and is it strong enough, is a opportunity for some judgement, if yours is sound enough.

And there is judgement, and there is just guessing. Your judgement is not indeed sound, ipso facto, if you are just guessing whether something is strong enough, which is rigged in a way which hugely compromises its strength, rather than taking a few minutes to do the rather simple math.

Originally Posted by wingssail

As I have demonstrated the rig I use has the strength to hold the boat layed over 75 degrees. In my judgement it's enough.

In 26 knots, I think you said? Is that the strongest squall you've ever been in? I can't count the number of times I've been whacked with 40+ knots in a sudden squall. And those squalls often -- usually -- pack a dramatic wind shift, the kind of thing which could really catch you out when running with the boom out. It's never happened to me -- in some decades of this, I have always managed to get sail down in time, but does that mean I'm so good I'll never get caught out? Nope, it means I've had a large helping of good luck on top of being careful, and it could happen otherwise at any time, so better be prepared.

Originally Posted by wingssail

. . . On the other hand, with the boom eased out for a close reach (meaning not even out to the lifeline) the angle from the end of the boom to the bow is not enough to hold the boom steady, which is just as often what we're trying to accomplish. In that case the preventer should go to the widest part of the boat.

Here I agree with you (obviously). Rigged like that you can steady the boom, but don't kid yourself that it will hold if you somehow get backwinded. Fortunately it's much, much less likely on a close reach.

Another issue, by the way, is aft-swept spreaders. If you have straight spreaders, then you have a much better chance to get a good angle on the preventer. I do not. The further the boom is squared forward, the safer you are altogether, but you can't do much with aft-swept spreaders. Platino also had aft-swept spreaders.

Originally Posted by wingssail

NEVER is a pretty strong word. Rigging for various sailing situations requires some flexibility and judgement.

Good judgement means knowing for sure whether it's strong enough, not guessing at it. If I would back away from "never", it would be on the condition that the math has been done and that the way the preventer has been rigged has been demonstrated to be strong enough.