Preventer Rigging -- Lessons from the Platino Disaster

[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:




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:




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.

[El Pinguino]

The massiveness of the boom indeed..... and the area between cabin and the helm was all being swept by the mainsheet and traveler combo..... so not as simple as tossing a line over the boom although that was attempted....and then reeling it in...... and I don't think the three of them just sat around waiting for the rig to come down....


From the report...

The boom was 8.6 metres long and weighed 678kg with the mainsail furled inside it. A length of the mainsheet was hanging from the end of the boom and the traveler car (which had broken free of its track) was still attached to the end of the mainsheet. The traveler car was 350mm long, weighed just over two kg and the crew described it as acting like a wrecking ball as it quickly caused substantial damage to the rear of the yacht.
After making their way to the helm station, the Owner and the surviving crewmember watched the motion of the boom and lay on the deck each time it swung past. The damage that occurred in the initial few minutes included the following, which happened in close proximity to the two men:
 The helm console (where the hydraulic sail handling controls were mounted) was smashed off of its pedestal while the Owner was at the wheel. This happened before he was able to finish furling all of the mainsail or any of the headsail (the headsail remained half furled as it had been set). The surviving crewmember soon realised that the broken helm console was interfering with the steering wheel, making it even more difficult to turn. He used his legs to push the console away until he was later able to tie it back from the wheel.
 The bimini was smashed off while the two men were beneath it. This was left hanging by one webbing strap which the surviving crewmember cut to allow it to fall overboard and out of the way.
 The cockpit table was torn from the deck and propelled over the side.
 The steering wheel was buckled and one of its spokes was broken, adding to the difficulty in
turning the wheel.
 The life lines at the rear of the yacht were knocked down on the starboard side.

Platino’s crew were unable to regain control of the boom following the failure of the preventer and mainsheet traveler. The surviving crewmember did attempt to lasso the boom by throwing a line over it as it swung across the rear of the yacht. However, he was unsure what he would tie the line to, if he had been successful. The two men took turns at steering the yacht and were able to subdue the boom’s motion somewhat at times. This was achieved by adjusting the yacht’s heading in an attempt to both restrict the rolling motion and keep the boom in contact with the water on one side or the other.
Despite the crew’s efforts, the boom continued to swing from side to side throughout the day. The surviving crewmember stated that the force was sufficient to cause the entire boat to shake as the boom repeatedly impacted on the shrouds.

The actual cause of the initial gybe was not down to a rogue wave as the owner thought but a leak on the hydraulic steering.....

PS... the weight of boom and sail was 678kg....nearer to 3/4 of a tonne than a 1/4 of a tonne... about the weight of a small car....

[Kenomac]

Bad things can happen even when you do everything correctly. Our preventer on our 53 broke three weeks ago even though it was rigged the correct way... from the end of the boom, the preventer run all the way forward to the bow cleat. All it took was a 15 knot wind from dead astern, then punching in a goto heading on the chartplotter. The boat suddenly and without warning turned itself 60 degrees to starboard due to an auto pilot compass malfunction, then before I could get behind the helm and hit standby... snap! The preventer broke.

Fortunately, nothing else broke, nobody was hurt and only a forward stanchion was slightly bent. But stuff happens, and that’s one great feature of a center cockpit boat... it places you in a very safe location when things do go wrong. The entire gybe occured in less than maybe 5-10 seconds.

I’m convinced now after the fact, that it was better that the preventer broke as it slowed the boom coming across, then to have held and possibly broken the boom or ripped the sail in half.

[a64pilot]

So question on the Platino accident.
It broke with the sail furled in the boom? Surely it wasn’t a preventer then, but a line that was used or secure the boom when the sail was furled?
I make that distinction because if it was used as a preventer then it was obviously very inadequate, if it broke from just the forces of the weight of the sail and boom.

[accomplice]

Thirty years ago while crewing on a C&C 44 an unexpected gybe caught me in the side of the head. I was knocked unconscious and had a convulsive seizure. (I haven't raced since!) Now whenever I sail any deeper than about 120 AWA, we rig a preventer.

A few years ago, our autopilot hiccuped and the main backwinded. The end-boom attached 1/2" double braid dacron preventer parted without a second thought. Fortunately, no injury or damage resulted. Now our preventer is 1/2" spectra core.

The report wisely points out that sheeting angle and position matters.

I am still trying to figure out a good way to rig our preventers so they can be set from the cockpit.

[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.

[a64pilot]

Originally Posted by Dockhead

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.

Couple of things, first is the psychological aspect. This thing has killed, you witnessed it and there is a corpse laying there maybe mutilated in a pool of blood just in case you weren’t sure the thing was deadly. Seeing a friend die is a destabilizing experience.
Then it’s causing mayhem, but the boat is still floating, and your in shock, you just saw a friend die. Call it a lack of courage if you will, but it’s certainly understandable.

I had an accidental Gybe that caused me to fit the boom brake, however the sail was up, yes it tore the car off of the traveler so the boom was free, but it immediately slammed into the stays and stayed pinned there as we were downwind. I was able to furl the Genoa and crank the engine and turn into the wind. The sail of course is what kept the boom under control somewhat , and while the car was off of the traveller the sheet was still connected to the boom, so I had some control, just had lost the mechanical advantage and the sheeve on deck pulley broke.

I assume if they had the sail up, it wouldn’t have been as bad?
Plus of course the sea state was likely pretty sporty, don’t forget that when we wonder why they may not have done this or that.

Two men died pretty much immediately, two survived, maybe the two that did survive did the right thing? Is saving the boat worth possibility of getting yourself killed?

[a64pilot]

That diagram makes no sense, if the line was meant to secure the boom and not be a preventer why would anyone tie it with the boom at an angle that wasn’t centered?
Surely it wasn’t meant to be a preventer. Any reasonably mechanically inclined person could see that angle wasn’t going to work.

I’d bet that normally there would be two lines one to each midship cleat and the boom centered, that decreases the angle significantly.

[Kenomac]

Originally Posted by a64pilot

So question on the Platino accident.
It broke with the sail furled in the boom? Surely it wasn’t a preventer then, but a line that was used or secure the boom when the sail was furled?
I make that distinction because if it was used as a preventer then it was obviously very inadequate, if it broke from just the forces of the weight of the sail and boom.

You need to read the report more carefully before commenting. The owner was able to furl in most of the mainsail into the boom while it was smashing about, then with the added weight, the situation became worse, when the traveler car smashed the hydraulic control console.

Please try again.

[a64pilot]

I was wondering when we would start condemning in boom due to weight, I know nothing about this one, but my much smaller one is not much heavier, I can throw it over my shoulder and carry it, it’s the sail that is heavy.

[El Pinguino]

Originally Posted by a64pilot

So question on the Platino accident.
It broke with the sail furled in the boom? Surely it wasn’t a preventer then, but a line that was used or secure the boom when the sail was furled?
I make that distinction because if it was used as a preventer then it was obviously very inadequate, if it broke from just the forces of the weight of the sail and boom.

No... they were sailing -with the main- when she gybed... managed to furl most of the main after the event.

5 crew, 2 lost, 3 survivors... gets a bit confusing because there was the owner, the skipper ( who were a couple) , and three crew...

[a64pilot]

Originally Posted by El Pinguino

No... they were sailing -with the main- when she gybed... managed to furl most of the main after the event.



5 crew, 2 lost, 3 survivors... gets a bit confusing because there was the owner, the skipper ( who were a couple) , and three crew...

I don’t understand then why they furled the sail before they had some way to control the boom.
I know my boat is a much smaller boat and the forces etc are less, but with the engine running and her nose pointed into 30+ kts of wind, the sail just luffed and the boom was pretty much controllable, swinging yes of course, but not slamming into the shrouds. taking the sail down of course reduces the dampening of the sail, and increases the mass of the boom, both you don’t want.

[Kenomac]

Originally Posted by a64pilot

I don’t understand then why they furled the sail before they had some way to control the boom.
I know my boat is a much smaller boat and the forces etc are less, but with the engine running and her nose pointed into 30+ kts of wind, the sail just luffed and the boom was pretty much controllable, swinging yes of course, but not slamming into the shrouds. taking the sail down of course reduces the dampening of the sail, and increases the mass of the boom, both you don’t want.

There’s a BIG difference between a IP 38 and the size boat involved in the incident regarding the forces in play... I’m surprised you can’t see that.

[a64pilot]

Don’t believe me, try it, point directly into the wind under motor and loosen the sheet and see what happens.

[a64pilot]

For that matter if they had stayed on a somewhat downwind course with the sail up. It would have stayed pinned against the shroud, but it wouldn’t have been swinging around