Preventer Rigging -- Lessons from the Platino Disaster

[Jim Cate]

My warnings and opinions based on personal experience rather than theory, have been clearly stated.

OK Ken, have a nice day. During that day, you might consider that some of us also have personal experience upon which to base our comments. Some are even engineers who can calculate loads and stuff.

Meanwhile,how are you going to rig your replacement preventer? Or are you not going to replace it, seeing that it will inevitably fail?

Jim

[stevensuf]

Originally Posted by Kenomac

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.

Our rudder snapped mid atlantic, on a deep reach with 20-25knts, we spun round gybing several times before the boat settled, my preventer did not snap, i did use 1 inch line, winched tight, we let the boom and sail into the spreaders tied off to the bow cleat then winched the boom back to get the sail off the spar, i think having the system rock solid with little give, takes away much of possible momenta of a system with some spring in it.


I think its all about the strength of the mounting points and rigidty of the lines.

[El Pinguino]

Originally Posted by Ericson38

https://www.brinwilsonboats.co.nz/ga...ges/photo6.jpg

Not seeing a traveler. I think in this shot it is still under deck fit out.

She had been 'signed off' by the yard when that shot was taken and was in what I would call the 'northern basin' at Gulf Harbour. In the report you will see that the traveler track was about 380mm long so at deck level and not visible.

As stated in the post after yours that is a temporary 'runner and guy' harbour stow.

I do that when laying up so the main sheet can be stowed below away from the WX and the UV.

[Fuss]

Originally Posted by Jim Cate

Ken, I submit that if your preventer failed under such conditions it was woefully under engineered. Even with a full backwind, with only 15-18 AWS it should not be anywhere near failure point.



This does not represent a cascade of events leading to a failure, it is simple undersizing of the gear. I'm surprised that you can't see that. The failure occurred under modest conditions without extenuating circumstances, at least as you have twice described it, so whether it was due to inadequate size of line, a knot or splice weakening it, chafe or sun damage weakening it... it WAS TOO WEAK, or it would not have failed under these conditions.



I agree that no matter how strong it might be, an event that overcomes it can be imagined. This was no such event.



Jim

Exactly, I’m surprised it took so long for someone to say this.

[El Pinguino]

Originally Posted by Tillsbury

Platino didn’t have an arch traveller. That’s the point.

I doubt it would have made any difference one way or t'other..

[El Pinguino]

Originally Posted by Nikki S

I am always debating how best to rig the preventer on our 49ft. There is nothing that points to a preventer ever being rigged before we bought her, third-hand. But someone must have rigged something to get from Sydney to Whitsundays multiple times as per the log book....The options are limited without adding some hardware.... I was taught one way on RYA courses, but my husband prefers advice from experienced friends’ , which differs from what I was taught. It’s such a standard necessity that I can’t understand why it wasn’t an obvious part of the rigging seeing as everything else was left onboard by the previous owner, from harnesses to soft drinks ....

Nobody buying my boat would find any sign of preventers having been rigged.

I'm a bit wary about sharing this as I'm sure I'm doing it wrong and its going to lead to tears... however...

I use my spin pole downhauls........

They lead to points on the deck about 4 feet aft of the stem through substantial blocks on substantial pad eyes ( rebuilt - by Brin Wilson Boatbuilders - from original to something much more robust ) and from there - when used as preventers - to the end of the boom.

I sail very conservatively.... have had a couple of gybes ( don't ask... the story has already been told ) with every thing strapped down... and including a poled out headsail... why is it always on a moonless night? Isn't that a bugger to sort out....

But nothing has - thus far - broken...

No shock loadings may be the secret.... weight taken on preventer so boom is a few inches shy of the lowers... crank in hard on the main sheet.... make sure the vang is under good tension.....

Biggest danger is blowing the main if it is already old and kanakerd.. do not ask how I know that.... but that was in a controled gybe...

[Matt Johnson]

Originally Posted by Jim Cate

OK Ken, have a nice day. During that day, you might consider that some of us also have personal experience upon which to base our comments. Some are even engineers who can calculate loads and stuff.

Meanwhile,how are you going to rig your replacement preventer? Or are you not going to replace it, seeing that it will inevitably fail?

Jim

The condescension and arrogance towards every poster here has gotten really old.

I appreciate the fact that both you and Ann post information from your vast experience without sounding like your way is the only way to do things. A little humility goes a long way towards getting your point across vs the steadfast/pigheaded positions from others.

Matt

[Fuss]

Originally Posted by funjohnson

The condescension and arrogance towards every poster here has gotten really old.

I appreciate the fact that both you and Ann post information from your vast experience without sounding like your way is the only way to do things. A little humility goes a long way towards getting your point across vs the steadfast/pigheaded positions from others.

Matt

It’s got nothing to do with arrogance.
The reason to try to be very black and white when contributing is to keep the information clear and to the point.
Preventers should go from the end of the boom to the bow and be strong enough to be backwinded in much more than 25 kts and this point has to be made clear.
In my opinion

[Dockhead]

Originally Posted by jmh2002

...

Gear 'breaking in an instant' is never good, and it normally means something is not specified correctly to handle such a load, or (as is also often the case) is not being used as expected/designed.

Indeed.

[s/v Jedi]

Originally Posted by Fuss

It’s got nothing to do with arrogance.
The reason to try to be very black and white when contributing is to keep the information clear and to the point.
Preventers should go from the end of the boom to the bow and be strong enough to be backwinded in much more than 25 kts and this point has to be made clear.
In my opinion

Fully agree. Primary systems aboard offshore sailboats are literally a matter of life or death and there is no room for political correctness when that obscures the important factual system and the way it should be setup.
That said, threads like these can do without outright hostility as well.

I have some on-topic points as well:

- in almost every case I see (unqualified?) people make changes to primary systems, it leads to unsafe setups. An engineer was involved designing that primary system and an engineer is required to modify/improve it.

- we just had a rough passage from Panama to Jamaica and our jib sheet broke. It just parted when there wasn’t even a big gust, just 25kts wind. This was our light weather sheet but still 1/2” diameter double braid and has worked flawlessly in much higher winds for a decade. The problem was that it was a decade old and deteriorated. We donated the good parts to the Jamaica Defense Force who tied a boat to the dock with it and the light surge broke it again. Good systems require maintenance and parts that wear out need to be replaced in time.

[jmh2002]

Originally Posted by s/v Jedi

- in almost every case I see (unqualified?) people make changes to primary systems, it leads to unsafe setups. An engineer was involved designing that primary system and an engineer is required to modify/improve it.

I agree with this point too, and it somewhat also relates to my previous post about gear also often not being used as expected/designed.

However a point that I want to add, somewhat opposite to this, is also to not always put full blind faith in the original design / factory setup.

On Jedi's Dashew Sundeer 64, well that is a well designed boat purposely built for long distance ocean sailing.

But unfortunately the same cannot be said for many other boats, even if they say they are 'bluewater' or 'ocean going' or whatever.

Some of them are just plain rubbish, especially in relation to these high load important systems.

Note an earlier post just above for example:

Originally Posted by stevensuf

Our rudder snapped mid atlantic, on a deep reach with 20-25knts,

I mean, rudder snapped? What the hell... (at least in this case the owner installed preventer didn't break though!)

Unfortunately there is not much substitute for experience when trying to judge these things, but as other members have noted some basic calculations can (and should!) be done, safe working load of gear should be checked and noted, etc.

Plus, do a thorough inspection of the boat, and deck and rig fittings, etc. Is that padeye just screwed to the deck with self tappers... (we have seen it, haven't we!), or is it through bolted to a reinforced pad, or perhaps even connected to a rod or chainplate type arrangement passing down inside the boat to a structural point? Better

Also an important note here which often gets missed. Padeyes, shackles, etc, and even blocks are designed to take load in a certain orientation. The safe working load can reduce substantially at other orientations, or the gear can even fail totally at a much lower load than expected if used in an incorrect orientation.

This problem is getting better now that boats are moving to soft shackles, etc but the concept still needs to be correctly understood.

[Ericson38]

Originally Posted by Dockhead

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 problem with the recommended preventer geometry in the above deck plan view is that with the boom attachment towards the end of the boom, the acute running angle the preventer makes with the boom towards the bow of the boat still results in high goose neck side loads and overall twisting of the boom under each surge.

http://www.cruisersforum.com/forums/...hmentid=196548

In my view, there is a better location, being the base of the after lower main stays. Preventer lines are shorter, less connections, easier to move tack to tack, and there is no 180 deg direction change at the bow.
Plus, the horizontal component of the resistance force is close to 90 deg, and the vertical uploads are countered by the main itself (along the foot) as the boom is forced down by the preventer and boom vang.

[RaymondR]

Originally Posted by Ericson38

The problem with the recommended preventer geometry in the above deck plan view is that with the boom attachment towards the end of the boom, the acute running angle the preventer makes with the boom towards the bow of the boat still results in high goose neck side loads and overall twisting of the boom under each surge.

http://www.cruisersforum.com/forums/...hmentid=196548

In my view, there is a better location, being the base of the after lower main stays. Preventer lines are shorter, less connections, easier to move tack to tack, and there is no 180 deg direction change at the bow.
Plus, the horizontal component of the resistance force is close to 90 deg, and the vertical uploads are countered by the main itself (along the foot) as the boom is forced down by the preventer and boom vang.

If you look at the diagram provided in the report I think you will find that the arrangement you propose is similar to the one which failed. I have not confirmed the numbers in the report but suspect that the trigonometry was carried out correctly and that the boom-end-to-bow model generates far less loads on the preventer.

[CassidyNZ]

The preventer rigged in the diagram on Post#1 still is a one-line setup. Where it connects of course has a direct effect on efficiency but still - it’s just one line.

We all use multiple-purchase setups on our main sheets to manage - at best - a moderate load. Most boats that don’t have solid boom supports use a multi-purchase setup on the vang to keep the boom down - another comparatively low load element. And yet, many sailors will rig a preventer using just one line. If that is what you’re doing, all I can say is good luck.

I use the same multi-purchase setup on my preventer as I have on the vang and main sheet, connected to the toe rail. If anything is going to fail it will be the boom or the toe rail, not the preventer. Both of those elements are massively stronger than any one-line product. Up until now, in the uncommon event that my boat has accidentally gybed in inclement weather, neither element has broken and the boat was easily brought under control with the boom securely pinned by three different multi purchase systems. And I’ve employed that setup on more than one boat over the years.

As I said, good luck with single line preventers, however they are set up.

[Ericson38]

Originally Posted by RaymondR

If you look at the diagram provided in the report I think you will find that the arrangement you propose is similar to the one which failed. I have not confirmed the numbers in the report but suspect that the trigonometry was carried out correctly and that the boom-end-to-bow model generates far less loads on the preventer.

Figure 6 in the report shows a grey line with their preventer. It connects about 3/4ths the way to the end of the boom (OK), but then the hull side is connected just aft the chain plates. So the angle between them is a very acute 4-8 degrees (almost nothing).

What I'm recommending is that the boom end of the preventer be secured about 1/3rd the way out from the gooseneck, so, top down in the horizontal plane, the angle it makes with the boom is about 90 degrees.