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Old 25-07-2019, 09:15   #31
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Re: Preventer Rigging -- Lessons from the Platino Disaster

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Originally Posted by a64pilot View Post
As far as dynamic loads, your getting them and they are quite large, the “looseness” of the sail will impart a significant dynamic load. There is no way that I know of to get around that.
When that sail backwinds and “pops” tight it’s an initial shock load.
I agree; however, to the extent the boom vang exerts downward force on the boom and flattens the sail it will reduce that "pop."
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Old 25-07-2019, 09:16   #32
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Re: Preventer Rigging -- Lessons from the Platino Disaster

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An interesting question – what is the ideal place to rig the preventer to on the boom? My boat’s Selden boom has a specific preventer fitting at the end of the boom, and that is what I use. However, the end of the boom somewhat worsens the angle compared to sheeting somewhat further down. But forces increase further down the boom due to leverage. It should be pretty straightforward to calculate the optimum balance. In my case, the end of the boom is probably ok – visually it look to me like about 30 degrees. But other boats might benefit from moving the preventer down.

I actually did this math a few years ago when looking at my boom vang. Different line, but the same conclusions. I'm an engineer, and questions like this can't go unexplored. The end result is, the farther out the better. Without limit -- the boom vang (and the preventer) are most effective at the very end of the boom. Now, the impact diminishes, but it never reaches a "most effective" position and begins to lose effect.


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Old 25-07-2019, 09:30   #33
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Preventer Rigging -- Lessons from the Platino Disaster

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I agree; however, to the extent the boom vang exerts downward force on the boom and flattens the sail it will reduce that "pop."


Agreed, actually as my brake pulls hard down, I have to have the topping lift to counter that, my Vang is a nitrogen gas charged shock and the brake would collapse it without the topping lift.
Normally I don’t think a topping lift is fitted on a Profurl boom.

Brakes I believe normally are attached to the midship cleat or the aft chainplate, and therefore really pull down on the boom pretty hard.
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Old 25-07-2019, 09:31   #34
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Re: Preventer Rigging -- Lessons from the Platino Disaster

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Dockhead,

Don’t fool yourself into thinking your preventer will hold if your main sail becomes backwinded. The forces are intense, something will break as I witnessed firsthand. The preventer is really intended just to keep the boom in place so its not flopping about due to the swell motion and help prevent a gybe. After the gybe becomes inevitable.... it won’t hold, something will break.
Yes, that is my experience also, and the boom comes across with much more force than without a preventer. That is why I do not rig one in stronger winds.

Maybe a boom brake is a better option.
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Old 25-07-2019, 09:35   #35
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Re: Preventer Rigging -- Lessons from the Platino Disaster

One assumes that if a preventer won’t hold in stronger winds, that it’s an inadequate preventer?
Seems to me that stronger winds is exactly where you need one?
Now I’m a newby sailor, I’m not the guy with millions of miles and circumnavigations under my belt, so I go by intuition, which of course can be wrong.

But surely the experienced guys that fit preventers fit ones that are enormously strong?
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Old 25-07-2019, 09:38   #36
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Re: Preventer Rigging -- Lessons from the Platino Disaster

I just used a mid boom 4 part vang tackle to the toerail area. yes it could be a disaster... but so can just an accidental jibe with no preventer, which can kill someone. You have to be on top of things when sailing no matter what you're doing....
Why does one need a preventer in stronger winds anyway? The wind keeps the sail taught. In seas and light winds is when things flop around.
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Old 25-07-2019, 09:42   #37
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Re: Preventer Rigging -- Lessons from the Platino Disaster

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Originally Posted by a64pilot View Post
Normally I don’t think a topping lift is fitted on a Profurl boom.
I'm not sure about profurl, but our Leisurefurl boom is fitted with a topping lift. Although it isn't needed to support the boom (the hydraulic vang does that), I find it useful for two reasons. First, with the mainsail down it provides tension against which the mainsheet can pull in order to keep the boom centered and reduce swing. Second, it can be used to provide & set boom angle which is important for in-boom furling.
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Old 25-07-2019, 09:45   #38
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Preventer Rigging -- Lessons from the Platino Disaster

I understand why to have a topping lift, my profurl recommended that when the sail is down to attach the main halyard to the rear of the boom to take shock loads off of the Vang, which is of course a sort of topping lift.
But fit a boom brake and in my opinion the topping lift becomes mandatory at least on a nitrogen gas filled Vang, other wise it will be collapsed, and that shock absorption is similar to loose line allowing extra ‘looseness” if or when the sail backwinds.

It’s a sales pitch for a brake, but if you watch, you can imagine how much down force there is on the boom, it’s not insignificant.
https://youtu.be/pGVgpPuQIuY
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Old 25-07-2019, 09:46   #39
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Re: Preventer Rigging -- Lessons from the Platino Disaster

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I just used a mid boom 4 part vang tackle to the toerail area. yes it could be a disaster... but so can just an accidental jibe with no preventer, which can kill someone. You have to be on top of things when sailing no matter what you're doing....
Why does one need a preventer in stronger winds anyway? The wind keeps the sail taught. In seas and light winds is when things flop around.
Two reasons: First, off the wind in anything but calm seas the roll of the hull can backwind the main and the boom will slam about -- not a gybe, but large shock loads and a jarring experience. Second, because an accidental gybe can be catastrophic (as described in Platino report), precautions should be taken to avoid it. Saying, "I'll stay on top of things" is prone to human error and is of limited viability on multi-day offshore passages with multiple crew. I can watch something closely for a few hours -- not for a few days.
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Old 25-07-2019, 09:56   #40
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Preventer Rigging -- Lessons from the Platino Disaster

What happened to me was un forecasted winds and of course seas built to about 30 ish kts mostly from behind us. I was having a grand time as boat speed was exceeding 10 kts and we have never surfed before and the ride wasn’t bad either.
The I think due to the following seas and the required steering back and forth when a wave went under us the autopilot couldn’t keep up and we broached and then things got exciting of course.
I knew we were pushing the limits but as the winds weren’t really behind us I didn’t expect what happened.
I think about 120 off of the Starboard side
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Old 25-07-2019, 10:40   #41
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Re: Preventer Rigging -- Lessons from the Platino Disaster

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This fantastically professional report from the New Zealand Maritime authorities contains a number of important lessons.


https://www.maritimenz.govt.nz/comme...eport-2016.pdf


I guess everyone here heard of the Platino disaster a few years ago -- a 66 foot custom sailing yacht experienced a gybe which broke the preventer and let the immensely heavy (more than 500kg with the sail) in-boom furling boom loose, which broke loose the traveller car, scything through the cockpit and killing one crew outright, knocking another overboard, whom the surviving crew didn’t even try to find, so two dead. The remaining two on board could not get the boom under control, which swung around for the better part of another day and eventually brought the whole rig down. The multi-million dollar yacht was then abandoned.

Yikes!

The proximate cause of the disaster was an improperly rigged preventer. The preventer was rigged from mid-boom to a padeye midships, like this:


Attachment 196548

The grey line marked “pennant” is the actual preventer which failed; the green lines show the way it should have been rigged. The very acute angle obviously magnifies the force on the preventer hugely, and it’s even worse than on this 2D representation because there is also a vertical angle from boom level to rail.



How much was the force magnified? The excellent report actually has a table:


Attachment 196549

11.5x. But the eye of most decent sailors would probably discern that without a table.



I have often worried about this very thing. My preventers are rigged from the boom end to the bow cleats, and the preventer lines are 10mm racing dyneema with spliced-on Wichard shackles. The whole system is strong to about 5 metric tonnes. I have always questioned what kind of loads the system could withstand; I assumed that dipping the boom would break it instantly, but that backwinding the main even in a storm would hold. I have aft swept spreaders which reduces the likelihood of dipping the boom, but which worsens the angle, which I guess (need to measure) is approximately that what is shown in the report for a “properly rigged” preventer, which magnified the force by something like 2.5x, so 1 tonne at the end of my boom. According to the calculations in the report, that ought to be about OK for my boat, whose mainsail must be about half the size of this one.

An interesting question – what is the ideal place to rig the preventer to on the boom? My boat’s Selden boom has a specific preventer fitting at the end of the boom, and that is what I use. However, the end of the boom somewhat worsens the angle compared to sheeting somewhat further down. But forces increase further down the boom due to leverage. It should be pretty straightforward to calculate the optimum balance. In my case, the end of the boom is probably ok – visually it look to me like about 30 degrees. But other boats might benefit from moving the preventer down.

The story in the report of the scything, out of control boom, is chilling. The snatch loads from a heavy boom smashing into the mainsheet will be colossal. I have never liked the heavy Selden boom on my boat and one reason why boom furlers don’t appeal to me is that these are even much heavier. The boom, it seems to, would be much better being as light as possible, and carbon, even if the mast is alu. In my case, an out of control boom would rip the traveller out of the deck in a twinkle – even more so since I am using a dyneema main sheet without any stretch. With triple purchase to the end of the boom – would be like running the boom into a brick wall.

However, I still can’t get my head around, why the crew couldn’t get the boom under control, quarter ton of it notwithstanding. Maybe I just can’t imagine the massiveness of that boom. On my boat, I’m sure I could get the boom under control -- I would tie a line to a midships cleat, throw it over the boom on the roll, then run the end of the line back through the cleat to a winch, and reel it in. I’m sure this would work on my boat, and I would make the boat heel with the headsail to cut down on the swinging, or motor into the wind. I feel quite sure that this would work. But for some reason, the crew of Platino just couldn’t manage and just waited for hours for the rig to come down.
The preventer bail on my Forespar roller mast boom is about 1/3 of the way back from the gooseneck (end boom sheeting). I don't end up with the severe shallow angles they show in the deck plan view.
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Old 25-07-2019, 10:53   #42
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Re: Preventer Rigging -- Lessons from the Platino Disaster

The old system of securely stowing the boom and employing a tri sail was designed to avoid these situations. Sometimes prevention is the only viable cure.
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Old 25-07-2019, 10:55   #43
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Re: Preventer Rigging -- Lessons from the Platino Disaster

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The old system of securely stowing the boom and employing a tri sail was designed to avoid these situations. Sometimes prevention is the only viable cure.
In a storm situation, sure. This was downwind sailing in force 8.
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Old 25-07-2019, 11:02   #44
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Re: Preventer Rigging -- Lessons from the Platino Disaster

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Originally Posted by a64pilot View Post
It’s a sales pitch for a brake, but if you watch, you can imagine how much down force there is on the boom, it’s not insignificant.
https://youtu.be/pGVgpPuQIuY
No, it wouldn't be. I'm thinking about the forces on the attachment points. With the boom attachment that far forward, I'd be worried about the forces involved on a bigger boat.
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Old 25-07-2019, 11:09   #45
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Re: Preventer Rigging -- Lessons from the Platino Disaster

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Originally Posted by Dockhead View Post
... I still can’t get my head around, why the crew couldn’t get the boom under control, quarter ton of it notwithstanding. Maybe I just can’t imagine the massiveness of that boom. On my boat, I’m sure I could get the boom under control -- I would tie a line to a midships cleat, throw it over the boom on the roll, then run the end of the line back through the cleat to a winch, and reel it in. I’m sure this would work on my boat, and I would make the boat heel with the headsail to cut down on the swinging, or motor into the wind. I feel quite sure that this would work. But for some reason, the crew of Platino just couldn’t manage and just waited for hours for the rig to come down.
I can speak from some experience, with time spent on a 40m modern sloop with a huge boom, even if in this case with normal slab reefing (sounds crazy but just like on a smaller boat, with a good system and lazy jacks it works fine, way less to go wrong, and if it does some type of jury rigged temporary fix is possible).

But I still tend to agree with you. I would also like to think that steps could be taken to get the boom under control. Especially when things are going badly wrong.

In this situation I suspect that it was possibly a question of both seamanship as well as shock. Shock really does impair people's ability to think clearly. I have seen experienced full racing crews start to lose the plot when things go badly wrong.

And seamanship, well, without at all casting doubt on this crew, in general there are a lot of people who have done a lot of miles but this type of seamanship may still not be their strongest point.

We are all tested in these type of extremis situations.

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