Preventer Rigging -- Lessons from the Platino Disaster

[cyan]

Originally Posted by accomplice

Back of the envelope static analysis for a backwinded main of 100m^2 in 35kts.

The force on the sail would be as follows:

100 m^2 [sail size] * (1.229 kg/m^3) [density of air] * ( (35 knots) * (0.5 m/sec/knot))^2 * 0.102kg-f/Newton= 3.8kg-f.

Assuming that CE for the (reefed to 100 m^2) sail is 2 meters aft on a 7meter boom, and that the preventer line is around a mast of 1/10meter radius, that would multiply the force by 20x.

Thus the preventer line in that scenario would experience approximately 75kg static load. (Of course, dynamic loads could be much higher)

Even a static analysis shows they're going to need a stronger line, much stronger attachment points and clutch. They're off by 1-2 orders of magnitude.

I think it's MUCH worse than that. Your math appears to be off by 3 orders of magnitude, on the gentle side!

This particular "preventer" design seems like an oil filter wrench with an extender arm. Not sure if the line or gooseneck would fail before the mast was twisted thin, collapsing like a plastic straw. Thankfully (for the potential victims), their website no longer appears live.

[RaymondR]

Suitable for pram dingys only.

[Tillsbury]

Back to the topic a little, I was in quite a rolly anchorage this lunchtime, and I thought I'd have a go at "controlling" the boom. I found it remarkably easy to throw the jib sheet up and over the boom when it was to one side, then loop it around the stern cleat and back to the winch. The sheet was secured by virtue of the genoa being furled, so it was leading from the usual sheet lead point. Being inside the topping lift it cannot drop off the boom. It was very easy to control -- I'd fancy my chances at being able to use it as a makeshift mainsheet in an emergency for a reefed main. It certainly stopped the boom moving in the swell.

An experiment well worth trying, IMHO.

[Dockhead]

Originally Posted by Tillsbury

Back to the topic a little, I was in quite a rolly anchorage this lunchtime, and I thought I'd have a go at "controlling" the boom. I found it remarkably easy to throw the jib sheet up and over the boom when it was to one side, then loop it around the stern cleat and back to the winch. The sheet was secured by virtue of the genoa being furled, so it was leading from the usual sheet lead point. Being inside the topping lift it cannot drop off the boom. It was very easy to control -- I'd fancy my chances at being able to use it as a makeshift mainsheet in an emergency for a reefed main. It certainly stopped the boom moving in the swell.

An experiment well worth trying, IMHO.

That's what I thought, but our booms don't weigh 700kg, do they?


I guess not having been there, we'll never know whether it could have been possible.

[Tillsbury]

Originally Posted by Dockhead

That's what I thought, but our booms don't weigh 700kg, do they?


I guess not having been there, we'll never know whether it could have been possible.

No, certainly not, although if mine did it wouldn't have changed much. I'm not asking whether it was possible in the case of Platino (obviously it wasn't easy at least), but making sure that I know how best to do it on my boat should the need arise.

[Dockhead]

Originally Posted by Tillsbury

No, certainly not, although if mine did it wouldn't have changed much. I'm not asking whether it was possible in the case of Platino (obviously it wasn't easy at least), but making sure that I know how best to do it on my boat should the need arise.

And that is commendable Your experiment is great practice and I might do the same on my boat in a rolly anchorage sometime.



I have also been rethinking this entire issue after reading this hair-raising horror story.


I am going to have a good hard look at my preventer system, and no 700 kg booms for me, ever!

[fivecapes]

Originally Posted by Tillsbury

If they sell any of these, expect them to come up in maritime reports in the future.

Unlikely, only thing for sale is their domain www.sailsafeinc.com.

[accomplice]

Originally Posted by Montanan

Ah, but then what's an added zero or two.

Whoa, that much more than nothing, Who Knew!

Thanks much for the analysis. Great envelope, far better than the napkin the system was apparently drawn up on.

I shouldn't be one to talk... I missed a few zeros myself. That's 75,000 kg. (75k kg)

[cyan]

Originally Posted by accomplice

I shouldn't be one to talk... I missed a few zeros myself. That's 75,000 kg. (75k kg)

Yeah, that's another example of where the "smell test" (or "sanity test") is good- much like the picture of that very thin, weak padeye from Platino. You don't need to be an engineer to run that test.
Your equation seemed to show that a 35 knot wind would create only a 3.8kg force on a large sail. If a marine equipment installer showed you such numbers, you would probably shake your head and find a better one.

[jmh2002]

Another thread was started on the subject of DIY Boom Breaks and I'll quote my post which has a few interesting links from the thread:

http://www.cruisersforum.com/forums/...ml#post2956418

Originally Posted by GILow

Have you looked at the Wichard boom brake for inspiration?

Originally Posted by jmh2002

https://marine.wichard.com/fiche-A%7...000000-ME.html

That looks similar in concept to the DreamGreen model that @groundtackle posted.



VS



Also linked when I searched was this test which has some interesting points: https://www.yachtingworld.com/yachts...eventers-65517

There was also a different concept in that test that I hadn't seen before, the SailFuse, which is a weak link type system:

https://www.yachtingworld.com/review...ailfuse-tested

[accomplice]

Originally Posted by jmh2002

Another thread was started on the subject of DIY Boom Breaks and I'll quote my post which has a few interesting links from the thread:

http://www.cruisersforum.com/forums/...ml#post2956418

Those might be ok for smaller boats but are unsuitable for 35ton displacement/ 100-150m^2 mainsail. The Gyb'easy is only rated for 40 m^2. In the yachtworld article they compare to other boombrakes and review the Walder. Walder does have larger models, the largest of which is only spec'ed for displacement to 30 (tons?).

[jmh2002]

Yes it for conceptual ideas only (I wrote 'interesting ideas') in relation to this thread. But possibly bigger versions could be manufactured, even on a DIY basis.

[accomplice]

Originally Posted by jmh2002

Yes it for conceptual ideas only (I wrote 'interesting ideas') in relation to this thread. But possibly bigger versions could be manufactured, even on a DIY basis.

If placed 1m from the gooseneck, static working load of about 7,600kg in 35kt -- more with gusts or higher wind. What would appropriate safety factor be? Perhaps 5:1? At that, it would need to be designed for 38,000kg minimum breaking load. Line of this strength is potentially feasible, but large (1" Amsteel). The attachment points to the boom and deck are what trouble me. Lewmar 65 winches (our primaries are 66's) are rated for only 1700kg WLL. In other words, the deck and boom attachment points would need to be 5 times stronger than the primary winches. I'm not sure if that is realistic. This is comparable to the yield strength on a 1 1/8" 316 bolt (which is just a little smaller than the bolt securing our gooseneck).

I think what this points to is that mid-to-end boom attachment for preventer is necessary on boats of >30t displacement . By comparison, the working load if the preventer is run at 90 degree angle and attached to the end of a 7meter boom is around 1,100kg -- much more manageable. I don't know how one could rig a boom-brake, though, that attached end-boom and ran at near 90 degrees when running.

[Dockhead]

Originally Posted by accomplice

If placed 1m from the gooseneck, static working load of about 7,600kg in 35kt -- more with gusts or higher wind. What would appropriate safety factor be? Perhaps 5:1? At that, it would need to be designed for 38,000kg minimum breaking load. Line of this strength is potentially feasible, but large (1" Amsteel). The attachment points to the boom and deck are what trouble me. Lewmar 65 winches (our primaries are 66's) are rated for only 1700kg WLL. In other words, the deck and boom attachment points would need to be 5 times stronger than the primary winches. I'm not sure if that is realistic. This is comparable to the yield strength on a 1 1/8" 316 bolt (which is just a little smaller than the bolt securing our gooseneck).

I think what this points to is that mid-to-end boom attachment for preventer is necessary on boats of >30t displacement . By comparison, the working load if the preventer is run at 90 degree angle and attached to the end of a 7meter boom is around 1,100kg -- much more manageable. I don't know how one could rig a boom-brake, though, that attached end-boom and ran at near 90 degrees when running.

I think you're right. I think we need that lever arm like we need a hole in the head.



Would be expensive and possibly overcomplicated, but you could use an industrial brake -- an elevator brake:


https://www.mayr.com/synchronisation...24_10_2008.pdf


Boom end to appropriately strong turning block in the bow, then back to wherever you could mount this. I'm not sure what kind of winch you would need between the brake and the preventer line, but I'm sure an appropriate device exists. It would need to be something which captures the line probably with several turns, and turns the shaft whenever the line goes in or out. The brake is electrically operated so you could turn it off when you're adjusting the preventer line, then clamp it down.


Can't imagine that would quite be worth it, but interesting thought-experiment.

[accomplice]

Originally Posted by Dockhead

I think you're right. I think we need that lever arm like we need a hole in the head.



Would be expensive and possibly overcomplicated, but you could use an industrial brake -- an elevator brake:

https://www.mayr.com/synchronisation...24_10_2008.pdf

How long would a non-marine brake work. It would need to be mounted somewhere protected, but then I'm not sure how to isolate it from the brake/preventer line that gets repeated doused in seawater.

Boom end to appropriately strong turning block in the bow, then back to wherever you could mount this. I'm not sure what kind of winch you would need between the brake and the preventer line, but I'm sure an appropriate device exists. It would need to be something which captures the line probably with several turns, and turns the shaft whenever the line goes in or out. The brake is electrically operated so you could turn it off when you're adjusting the preventer line, then clamp it down.


Can't imagine that would quite be worth it, but interesting thought-experiment.

Although they cannot be used on larger boats, one of the advantages of the boom brakes that mount forward on the boom is that they utilize a continuous line. There is no need to take up slack, or release. I wonder if that principle could be employed from boom-end without leaving a loop of line to catch items on deck or someone's neck.

If you're going to require manual operation to engage/release the brake, as you suggest, you might as well lead the lines aft and put them through a clutch near the cockpit -- and call it a preventer.

Taking that a step further, perhaps one could employ a continuous preventer line -- boom end -> bow -> fairlead towards cockpit -> clutch -> fairlead to bow -> boom end? The clutch would need to lock the line from moving either way, of course.

One concern of running a brake or preventer line this way is that as the boom starts to swing over, the preventer line has a clear lead to the bow. As the boom comes in, the line rests against the shrouds. If there is any brake/friction on that line, this would impart lateral forces on the shrouds -- not a good thing.