Preventer Rigging -- Lessons from the Platino Disaster

[Dockhead]

Originally Posted by fxykty

Wouldn’t you just put the preventer in the same position as the main sheet? The boom is built to take main sheet loads in various directions at that point.. . .

In my opinion this is exactly right.

[Tillsbury]

Originally Posted by Dockhead

In my opinion this is exactly right.

It's only matching two of the forces, though. The third one (a loose-footed main) also needs considering. If your boom is sheeted at the end then a preventer at the end is absolutely correct, as all three forces on the boom are acting at the same point. With mid-boom sheeting it isn't quite as easy as you can't move the clew of the sail. Mind you, if you have one or two reefs in you will be a little further back. But then most mid-boom systems have three or even more attachment points to the boom, so where on there do you fit it? Somewhere between the sheet attachment points and the clew of the sail?

Many booms have a preventer attachment marked at the end of the boom, and it makes sense to put it there if the manufacturer has planned it.

[ErnestV]

Originally Posted by fxykty

Wouldn’t you just put the preventer in the same position as the main sheet? The boom is built to take main sheet loads in various directions at that point.

So if end of boom sheeting, then preventer at end of boom. If mid boom sheeting then preventer mid boom. Is this too simple and missing something?

Is possible load on preventer higher than the load of the mainsheet? If so, perhaps we shouldn’t use preventers at all other than as flopper stoppers? Unless of course your boom and mast is designed and built for those additional loads.

Read dockheads math above. The farther out the preventer is the less the forces get multiplied with when the main gets backwinded. Its all about the load that goes up logarithmically when moving the preventer away from the boom end.

[a64pilot]

Several issues.
First don’t most of our main sheets attach to the boom in more than one location? That is done I’m sure to distribute the load, so logically you maybe would want to attach a preventer the same way?
Wouldn’t be hard, a block on a line would do it. Attach the line ends to the sheet attach points on the boom and attach the preventer line on a snatch block and connect it whenever you wanted to rig the preventer? snatch block makes it easily removable, just leave the short line attached to the boom, the snatch block will move as needed to ensure equal distribution of loads to the boom.

I think Transmitter Dan’s comment about best to have a 90 degree angle attachment from the boom is absolutely correct, and also not possible.
Maybe on a Catamaran, but a mono that attachment would be outside of the hull, but bringing it to the bow does decrease the angle, but also decreases the downward angle.
Ideal would be 90 degrees from the boom and level with the boom, not achievable of course.

A boom brake won’t cause a bent boom if you dip the boom into the water, because a brake will release, if it doesn’t release, it’s a preventer, and some brakes can be tightened so much that it takes excessive force to release.

[a64pilot]

Originally Posted by ErnestV

Read dockheads math above. The farther out the preventer is the less the forces get multiplied with when the main gets backwinded. Its all about the load that goes up logarithmically when moving the preventer away from the boom end.

Yes, but. There is always a but.
If the line is attached to the center of the boom, then there should be no forces applied to the gooseneck as each side will balance the other out. Not really true as I doubt the center of pressure of the sail will be the center of the boom, I bet it moves around quite a bit, but you get the point.
If attached to the very end of the boom, The forces the preventer is under is minimized, but there may be significant forces applied to the gooseneck.
Emphasis on may be, they may be way below what the goose neck is designed for too.
I think there is logic to attaching a preventer to the sheet attach points especially if there are two, somebody ought to have designed the system to take a significant load at those points, most probably someone a lot smarter and more experienced than I am.

[wingssail]

Originally Posted by a64pilot

Several issues.
First don’t most of our main sheets attach to the boom in more than one location?

No, my main sheet attaches to the end of the boom, the same place where I attach the preventer.

My preventer holds the boom forward and down. I place the preventer depending on the point of sail. On a reach it is aft of the shrouds holding the boom down at a good angle. On a run, it is forward of the shrouds, holding the boom also at the best angle. The load on the gooseneck is insignificant.

No-one needs to design a new system, you just put a preventer where it needs to be and Bob's your uncle, it's done.

[cyan]

I think Dockhead is correct, as his loose-footed main is a key point here His example is very similar geometry to our boat. (but his is much bigger)
My math shows his end-of-boom preventer sustains 1.74 times the tangetial force applied by the sail's clew versus the mid-boom preventer at 2.69 times this force. (never mind the mid-boom-bending torque)
It might be fun to also calculate a64pilot's concern too: the gooseneck loads.

Anyway, here is the formula I used, treating this as a simple torque problem with a couple of law of cosines and my obnoxious substitution-without-simplification because Excel doesn't care...
Force multiplier:
=L/(Db*SIN(ACOS(((SQRT(Dc^2+Db^2-2*Dc*Db*COS(RADIANS(beta))))^2 + Db^2 - Dc^2)/(2*(SQRT(Dc^2+Db^2-2*Dc*Db*COS(RADIANS(beta))))*Db))))
where
L = Boom length
Db = Distance from mast to boom preventer attach point
Dc = Distance from mast to cleat
beta = boom angle

[sailingharry]

Originally Posted by transmitterdan

The boom can bend if the preventer is at the end. In fact, for my boat I feel confident the bending moment is not minimized by preventing at the boom end. By moving the fixing point away from the end the bending moment is reduced. The force on the attachment point goes up but the boom bending moment goes down. May not be intuitive but it’s true nevertheless.

I'd like to see the math on that. Ideally, with a free body diagram. If there are point loads at each end, I can't see how you can have even 1 ft-lb of bending moment. But Statics was 40 years ago and my mind ain't what it used to be.


Can you describe how you get any bending moment on a boom with the preventer one end and a gooseneck at the other end?



Harry

[sailingharry]

Originally Posted by Dockhead

I get what you're saying, but it certainly cannot apply to loose footed mains like mine.

I wouldn't see it any different on an old-school main. There never was much load transfer to the boom along the foot. Most were built with a shelf foot to allow sail shape -- and the sail was basically loaded like a loose-foot main even then.

[sailingharry]

Originally Posted by a64pilot

Yes, but. There is always a but.
If the line is attached to the center of the boom, then there should be no forces applied to the gooseneck as each side will balance the other out. Not really true as I doubt the center of pressure of the sail will be the center of the boom, I bet it moves around quite a bit, but you get the point.
If attached to the very end of the boom, The forces the preventer is under is minimized, but there may be significant forces applied to the gooseneck.
Emphasis on

Sorry, no. Assume a simple diagram, not real life but the results are the same. Assume the sail is applying 1000 pounds to the clew, backwinded. Assume the preventer is exactly mid point, tied to an imaginary point dead ahead (90 degrees) to eliminate compression forces. Assume a 20 foot boom. Results:
* Clew force 1000 pounds aft
* Preventer force 2000 pounds forward (simple lever arm, made easy by being the mid-point)
* Therefore, sum-of-forces-equal-zero, the gooseneck is pushing aft with 1000 pounds.

* Bending moment will be 1000 pounds (clew force) times 10 feet (moment arm) or 10,000 ft-pounds.



For boom end:
* Clew force 1000 pounds aft
* Preventer force 1000 pounds forward
* Clew and preventer cancel each other, no force at gooseneck.
* Bending moment along entire boom is 0 pounds times 20 feet, or zero.

[transmitterdan]

Originally Posted by sailingharry

I'd like to see the math on that. Ideally, with a free body diagram. If there are point loads at each end, I can't see how you can have even 1 ft-lb of bending moment. But Statics was 40 years ago and my mind ain't what it used to be.


Can you describe how you get any bending moment on a boom with the preventer one end and a gooseneck at the other end?



Harry

My main is not loose footed. It has numerous strong attachments to the boom using slides smaller but similar to the vertical slides. So a back winded main puts force all along the boom. The main sheet also has multiple attachments to the boom which respect the side forces the boom experiences. So for my boat (cutter with mast further aft than a sloop) a preventer at the boom end going all the way to the bow isn’t optimal. A free body diagram is also complex. Want some fun? Send me a free body diagram of an inflated car tire on a rim.

I said that boats and rigs differ. Each boat thus has to be treated differently when thinking how to rig a preventer.

[Dockhead]

Originally Posted by transmitterdan

My main is not loose footed. It has numerous strong attachments to the boom using slides smaller but similar to the vertical slides. So a back winded main puts force all along the boom. The main sheet also has multiple attachments to the boom which respect the side forces the boom experiences. So for my boat (cutter with mast further aft than a sloop) a preventer at the boom end going all the way to the bow isn’t optimal. A free body diagram is also complex. Want some fun? Send me a free body diagram of an inflated car tire on a rim.

I said that boats and rigs differ. Each boat thus has to be treated differently when thinking how to rig a preventer.

I still don't see how the small improvement of angle between the preventer and boom could possibly make up for the added leverage, of a mid-boom preventer attachment.



It seems to me that a traditional cutter like yours, so relatively larger "J" and relatively shorter boom -- has on the contrary EVEN LESS to gain from a mid-boom preventer attachment, e.g.:






This angle is now almost full strength:






Am I missing something, or is it not indeed the case that a boom end preventer attachment will ALWAYS be stronger?


You say that for some rigs mid-boom might be stronger, but I'm struggling to see it. Maybe a boat with tiny J and giant boom, but not your boat, where the strength loss to preventer angle will be even less than my boat.

[HeinSdL]

With the boom hitting the water with a preventer fitted, the boom is very likely to break at the point where the kicking strap is attached to the boom. The kicking strap is likely to be quite tight (it has to be, after all, when running) and will see maximum load keeping that boom digging into the water. The load it sees is quite a bit higher (still!) than the preventer.

This from first hand experience racing around Britain and Ireland, many years ago, with a spi up in 25-30 knots of wind, and we ended up in a roll... we spent most of the coming night riveting the spar back together using a spare section of spi pole, in order to carry on to Lerwick.

[sailingharry]

Originally Posted by transmitterdan

My main is not loose footed. It has numerous strong attachments to the boom using slides smaller but similar to the vertical slides. So a back winded main puts force all along the boom. The main sheet also has multiple attachments to the boom which respect the side forces the boom experiences. So for my boat (cutter with mast further aft than a sloop) a preventer at the boom end going all the way to the bow isn’t optimal. A free body diagram is also complex. Want some fun? Send me a free body diagram of an inflated car tire on a rim.

I said that boats and rigs differ. Each boat thus has to be treated differently when thinking how to rig a preventer.

I think that the boom loading by the sail can be looked at as a distributed load along the boom by the slugs (perhaps a non-uniform load, heavier at the boom end), and a point load at the clew imparted by the huge loads running down the leach. But, the load along the foot is fairly trivial -- in a loose footed sail, it is easy to pull it toward the boom. The vast majority comes of the force even without a loose foot is at the clew. So, yes, there could be a minor bending moment. But I'd suggest it is minor.



The shape of your boat, cutter or otherwise, does NOTHING to support moving in from the boom end, or moving aft from the bow. The lowest force on the preventer is always at the boom end and the stem fitting.

[rgleason]

My transatlantic friends use a dual port and starboard preventers attached to the end of the boom and stored cleated near the gooseneck. When deployed the preventer is attached with a bowline on a line from the bow back to the cockpit.

What is the best way and place to attach to the boom?
On my 32' boat 12.6' boom, could I use two wichard eyes on either side through bolted? How far from the end of the boom? 1/3?

What is the most reliable way to attach the bow line to the boom preventer?

Since hardware can fail, is it easier/better to just run the bowline around one of the 1.25" vertical bow pulpit stanchion bases which is welded to the bowsprit main support?

Would a 3/4" rubber snubber used for moorings help reduce shock or would it get in the way?

Would it be possible to rig a good boom brake at the bow that would go to port/starboard preventers?

Reading these comments, I have begun to consider the loads on my traveler and one or two important shackles in the system, which have held for 38 years, but if broken would become a real hazard.