Preventer Rigging -- Lessons from the Platino Disaster

[sailorboy1]

Originally Posted by Cpt Pat

May I ask a stupid question? Why use a preventer at all instead of using a boom brake, like a Dutchman brake?

I'm thinking the same thing and wonder why a boom broke can't be rigged pretty much the same as a preventer.

[Breaking Waves]

Originally Posted by Kenomac

oversized snap shackle sized for a spinnaker halyard for a 60ft yacht break five weeks ago under just a 280 pound shock load.

hmmm . . . a tylaska T20, sized for "40-70' sailboats' has a static breaking strength of 20,000lbs. I can tell you that will hold a 280lb dynamic load unless it has been damaged or significantly worn beforehand.

I don't know what's happening on your boat with preventers breaking in 20 apparent and shackles breaking at lowish dynamic loads . . . but it is odd. It is like your gear is worn, or been miss-used or undersized.

Stainless snap shackles are not terrific for shock loads; but actually, neither are Dyneema soft shackles, which are high modulus with dynamic loading vulnerability in the stopper knot. Actually, a properly sized (eg pretty damn big) polyester soft shackle might be (I'd want to do some drop tests of various options before fully committing) a better solution for this particular application.

Jedi is correct that adding some energy absorption to the system design is excellent for dealing with peak dynamic loads - there are several ways to do it, but none are 'standard' technique, none used widely by riggers as it is not normally needed - conventional design (as described in the report) can handle boat's this size just fine.

DH - jfyi - if you had a ketch, you probably would not have been flying the mizzen in those conditions (angles and strength). The main would be smaller, but still big enough on these size boats to create large loads. I would have preferred to have just headsails even/also with a ketch in this particular situation. net net - I'm not sure a ketch makes much difference to this particular case.

[Tillsbury]

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.

The point of the report and the engineering within it is to avoid anyone’s “view” being used. Better to use the right way, which is explained in the report.

[Ericson38]

Originally Posted by Tillsbury

The point of the report and the engineering within it is to avoid anyone’s “view” being used. Better to use the right way, which is explained in the report.

But why is a 27 deg angle better than a 90 degree angle, if you can arrange it ?

No one would be flying a full main in winds that they have trouble keeping in check, as they were concerned that they were headed off course from the report. So a 1/2 main seems right, and a stay sail. This then would put the sweet spot for the preventer about 1/3 the way back from the gooseneck; you don't need it as far back aft on the boom if you are reefed down.

[Tillsbury]

I’m not sure where in the report you find anything about a “sweet spot”. The fitting for the preventer is at the end of the boom. It usually says “preventer” on the fitting, even. The discussion about where the other end of the preventer goes is clear in the report.

[RaymondR]

Originally Posted by Ericson38

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.

The diagram and table provided is a little bit misleading in that their model only considers the loads on a single X and Y axis plane whereas in reality the system is configured in a three axis model. If you redo the model with a Z axis for the system you propose you need to add another vector and the resultant comes out considerably higher than the numbers given in the table and on the graph.

The system you propose is exactly what I use as a boom brake on my boat except I rig it on both sides using a single line and use a halyard in the middle where it passes over the coach roof to pull it tight. I don't use blocks down at the toerail but friction fittings made using large thimbles. If I want to prevent the boom from moving if the main is back winded I run a line from the boom end to the most forward cleat as the report recommends.

However at 2.0 metres (6'7") tall I live in constant fear of being killed or knocked overboard by a swinging boom so very rarely sail downwind using the main in anything but light breezes and I keep the DIY brake rigged all the time and generally have the boom brake tightened even when on a reach.

In the situation those folks were in I would only use the genoa which I can pole out and jibe without leaving the cockpit with the double pole and outhaul arrangement I have.

The person I bought my present boat from got "boomed" whilst sailing up the Atlantic and was lucky to regain consciousness lying on the cockpit floor with a very bad headache about fourteen hours after the incident. He was very lucky that the weather was very good with only light winds at the time.

[Tillsbury]

The report explains clearly the 2d model and makes a pretty good stab at estimating how the 3d situation is different, certainly a better attempt than most out there and very honest about how difficult it is to get the correct figures in 3d. The diagram shown above is only a small part of the depth the report gives.

More importantly, it clarifies that the 2d model is the one in which the shock load of a gybe applies.

[Ericson38]

Originally Posted by Tillsbury

I’m not sure where in the report you find anything about a “sweet spot”. The fitting for the preventer is at the end of the boom. It usually says “preventer” on the fitting, even. The discussion about where the other end of the preventer goes is clear in the report.

Sweet spot means that the attachment point for the preventer is directly under the foot of the reefed mainsail, which is the at the center of the wind's side force on the boom, the best balance point to keep additional side forces from working loose the gooseneck.

[Ericson38]

Originally Posted by RaymondR

The diagram and table provided is a little bit misleading in that their model only considers the loads on a single X and Y axis plane whereas in reality the system is configured in a three axis model. If you redo the model with a Z axis for the system you propose you need to add another vector and the resultant comes out considerably higher than the numbers given in the table and on the graph.

The system you propose is exactly what I use as a boom brake on my boat except I rig it on both sides using a single line and use a halyard in the middle where it passes over the coach roof to pull it tight. I don't use blocks down at the toerail but friction fittings made using large thimbles. If I want to prevent the boom from moving if the main is back winded I run a line from the boom end to the most forward cleat as the report recommends.

However at 2.0 metres (6'7") tall I live in constant fear of being killed or knocked overboard by a swinging boom so very rarely sail downwind using the main in anything but light breezes and I keep the DIY brake rigged all the time and generally have the boom brake tightened even when on a reach.

In the situation those folks were in I would only use the genoa which I can pole out and jibe without leaving the cockpit with the double pole and outhaul arrangement I have.

The person I bought my present boat from got "boomed" whilst sailing up the Atlantic and was lucky to regain consciousness lying on the cockpit floor with a very bad headache about fourteen hours after the incident. He was very lucky that the weather was very good with only light winds at the time.

I agree that the horizontal plane (x-y) forces are estimated for various attachment points (boom to deck), but the vertical Z plane forces aren't. From that perspective, having the preventer's attachment on the bow to the boom would keep the vertical forces (downward) to a minimum during the jibe. But a stout main sail should be able to resist such a downforce. When you load up the mainsail sheet to keep the sail flat, there are several 1000 lbs being resisted already in the Z direction.

[mikedefieslife]

Originally Posted by Kenomac

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.

Exactly. So the lessons from this aren't really relevant to 98% of the people on this board with their 'small' boats.

For example my mainsail is less than 14m2. In most winds I can hold it myself. It's never going to break a 10mm line let alone the 12mm I often use. It might cause other damage, but breaking a preventer isn't one of them.

[MartinR]

I have had 2 incidents with preventers. Both while racing big gaff rigged sailing vessels.

In the first instance, while running with full main in 35-40 knot winds, a moment of inattentive steering caused the main to backwind. There was no holding the boat on course with the main backwinded, and the more she turned, the more the apparent wind increased. In the end the force overcame the breaking strength of the preventer, and this was a heavy tackle, rigged from the boom end. The boom came over with a vengeance and broke, nobody was hurt.

The second instance was somewhat similar but in lighter winds. The preventer held, but it was somehow jammed, I can not remember exactly why, so the boat was actually making sternway after a while. Could have been a disaster for the steering gear. Was very hard to get the ship back under control.

On gaff rigged ships, preventers fill a double function, as a preventer and a vang, so they are essential.

But there are two sides to this. They increase safety but also present a hazard under certain circumstances.

Correct rigging is paramount of course.

For modern rigs a boom brake is a better solution. Or just a good helmsman

In Platinos case, the preventer was rigged incorrectly of course and in my opinion, having sailed a lot with heavy gear, getting the boom under control did present a major problem, which they obviously were not able to solve. Impossible to say if somebody else would have been able to.

[MartinR]

And, my usual comment

Please note that the vessel was steered by autopilot. In my opinion, this is a major factor in most cases of serious accidents today. Comfort wins over safety.

In reasonably bad conditions like they had, the boat should have been hand steered, and the jibe would in all likelihood have been prevented.

[waterman46]

Stupid question : did they immediately lower the mainsail after the gybe? And then if that was done, why not lower aft end of boom to the deck, which would have controlled it, and lash it down, continuing to sail on headsails?

Also, it was stated by some that there might be an optimum point on the boom to minimize preventer stress. In fact the stress will always be least when preventer is rigged to end of boom. And the bow is best place to run it from.

Now why did the sheet exert such extreme force when the swinging boom fetched up on the opposite side? Dyneema line was used for mainsheet. If just a little stretch had been available in the sheet it would greatly reduce peak load.

[Ericson38]

Originally Posted by MartinR

And, my usual comment

Please note that the vessel was steered by autopilot. In my opinion, this is a major factor in most cases of serious accidents today. Comfort wins over safety.

In reasonably bad conditions like they had, the boat should have been hand steered, and the jibe would in all likelihood have been prevented.

In their case the autopilot fluid ran low. Another thing I was wondering about is when they shut off the engine since they were making such good time, did they run down the battery powering the AP? Depending on the battery bank they were on and the AP loads, they could have eventually ran the battery down, which will trip the AP too. That one can sneak up on you.

[Hardhead]

I'm sure this is also a silly question - but couldn't an attempt been made to bring the boom under control, after the preventer failed, by drawing in on the mainsheet? Or was it that the hydraulic sail controls were smashed, and therefore, with the loads of that large boat, the main sheet could not be brought in otherwise? I know the mainsheet & traveler were pulled out eventually by the free slamming, but I thought maybe an attempt would have been made to draw in the main sheet before it was ripped out. Apparently, there were enormous issues in controlling the steering of the boat as well, and that must have prevented lessening the loads on the mainsail I would imagine.