Boom Preventer

[cwyckham]

Andrew's comment about progressive fuses triggered a thought.

Climbers use something called a "screamer". Here's an example: Petzl Nitro Shock Absorbing Sling - Mountain Equipment Co-op. Free Shipping Available

They are cheap, readily available, and work well. They will absorb impact by ripping threads, but will not completely let the load go (once the thread is done ripping, you still have a high strength attachment). The idea is only to smooth out a shock load. It won't help with a steady load.

The thread starts ripping at 2.5 kN in the example I've attached. It isn't clear to me what the typical breaking load of a boom would be. Does this seem like it would be useful, or is the load too high before the stitching lets go?

[cwyckham]

Originally Posted by Capt. Don

A boom with a line stretched between ends is no different than a mast with a sail stretched between both ends.

It is completely different. The sail is supported along the length of the mast. The load case is almost completely shear, not compression.

[cwyckham]

Originally Posted by Capt. Don

Folks, appreciate the feedback. Let me state that Troy was a professional racer. If you look at most of the Open 50 and Open 60 class boats they are rigged with a line underneath the boom. No it is not attached to the mast. I have posted some images here: Boom Brake Installation images

Thanks for the images. I see that the position of the preventer attachment is fixed somehow, so that answers Jim's question about it sliding back.

You can also see the very large sections on these booms. It is possible that they were designed for the very large compressive forces provided by this set-up. It is also possible that the riggers who did this didn't understand the forces they were creating and were inadvertently putting the booms at risk. Don't presume that just because somebody's a professional they understand vector mechanics.

I wouldn't say that because some racers do it, or all racers do it, that it is a good idea to do on a cruising boat. A very high compression load greatly increases the chance of buckling the mast due to any side load (from, say, the vang or mid-boom sheet).

The concept isn't easy to visualize, so here's a diagram. To put a very modest 250 lbs of force on the boom end, you create a 2900 lb of compressive force!

What is the reason for doing this on the Open 60 boats? What is the advantage of having a moving preventer?

[bornyesterday]

I'd have thought most readers here would be fully familiar with the concept through having swigged halyards?

[cwyckham]

Originally Posted by bornyesterday

I'd have thought most readers here would be fully familiar with the concept through having swigged halyards?

You would think so. but then people anchor and do a stern tie or stern anchor bar taught and wonder why they drag when even a small broadside breeze comes up. I don't think people really understand what they're doing and why it works when they "swig" a halyard (I've always called that "sweating" a halyard).

[Andrew Troup]

My understanding of the setups in the photos is that the sliding of the thimble setup along the underboom line is what provides the braking friction: IOW that the underboom line is the BRAKE line, so the load on that line is limited to the retardation force.

Whereas there is no such ceiling if a bridle, full length of the boom, was used for a preventer.

[cwyckham]

Originally Posted by Andrew Troup

My understanding of the setups in the photos is that the sliding of the thimble setup along the underboom line is what provides the braking friction: IOW that the underboom line is the BRAKE line, so the load on that line is limited to the retardation force.

Whereas there is no such ceiling if a bridle, full length of the boom, was used for a preventer.

Now that makes a lot of sense. So it is a boom brake, not a preventer. The forces will be limited and the boat won't get pinned. I'd love to see it in action.

[Jim Cate]

Originally Posted by cwyckham

Now that makes a lot of sense. So it is a boom brake, not a preventer. The forces will be limited and the boat won't get pinned. I'd love to see it in action.

Looking at the photo of Alan Nebauer (an old friend of ours) it appears that one, the line along the boom has considerable slack in it, not tight up against the boom, and two, that the attachment point is hitched in place, not free to run. We will likely be catching up with Alan soon, and I'll try to remember to ask him how it worked.

Now, as to it being a brake, relying upon the friction of sliding along that Dyneema line... surely not! Ain't much friction there! ONe of the photos shows a line leading fore and aft from the slider, and perhaps that is used to adjust and then fix the position of the slider. Or perhaps they are attached to some friction device like that of a typical cruising boom brake... who knows?

Finally, it is true that these systems add compression loads on the boom. But I note that most cruising booms are pretty stiff, and tolerate considerable compression loads from conventional hardware (outhauls, regular vangs, etc) and I don't recall ever seeing one fail from buckling, even with the considerable loads of mid boom sheet attachments. So, I have to wonder if your worries are valid, especially now that we see heavily loaded extreme race boats utilizing that system.

Cheers,'
Jim

[Andrew Troup]

Jim

Any preventer to the end of the boom applies a pure compression load when the mainsail goes aback.

There is no bending resulting from the preventer tension in such cases, because the clew sideforce is exactly balanced by the preventer sideforce, and they're both attached at the same location.

This wacky idea just manages to apply more compression, by amplifying the tension (just as "swigging" a halyard does), theoretically up to the breaking load of the Spectra line,

if the Spectra line is used to attach a preventer, or if a fixed preventer tether slides along it.

[transmitterdan]

Something I have not understood is why you see preventers attached to the boat well forward of the mast. This doesn't make sense to me because it creates a force trying to bend the mast forward at the gooseneck. My preventer is attached to a points located on either side deck at a point where the preventer line is perpendicular to the boom when the boom is centered. The attachment to the boom is near the center (maybe just aft of center) of the boom. To me this makes sense but I see books and magazines with the preventer line rigged way forward on the toe rail. Which way is "correct"?

[Andrew Troup]

Originally Posted by transmitterdan

Something I have not understood is why you see preventers attached to the boat well forward of the mast. This doesn't make sense to me because it creates a force trying to bend the mast forward at the gooseneck. My preventer is attached to a points located on either side deck at a point where the preventer line is perpendicular to the boom when the boom is centered. The attachment to the boom is near the center (maybe just aft of center) of the boom. To me this makes sense but I see books and magazines with the preventer line rigged way forward on the toe rail. Which way is "correct"?

Both are correct, in the right context, but (in my opinion) a side-deck attachment is appropriate for a preventer whose other end is mid-boom, while a foredeck attachment works well with an end-boom preventer, both for reasons of simple geometry: to keep the broadest possible angle of preventer to boom, avoiding (or postponing, in the end boom case) the dreaded "over-centre toggle" effect.

There are two main benefits I can think of to an end-boom preventer: if you dip the end of the boom, the preventer which is attached at about the "centre of effort" of the dipped area will obviously impose no bending loads on the boom. However if in that situation, the other end was at the chainplates, rather than on the foredeck, the angle would be narrower, converting the dipping load into more compression, trying to drive the boom through the mast.

Some boats will never dip the boom, so this is not always a consideration.
Maybe the boom is short and the boat is beamy .... and there are other situations: if the spreaders are well-swept, for instance...
And for slow boats, it's less of a consideration.


Second benefit: the end-boom preventer does not impose bending loads on the boom when the sail goes aback, as explained in my last post

Contrary to popular opinion, I don't think it's a showstopper to combine end-boom sheeting with midboom preventer, or vice versa, but it DOES require a degree of crew awareness and discipline which is hard to guarantee, even for a single hander.

This is because you can bend the boom simply by forgetting to release the preventer before you sheet in the main. This mainly applies on boats big enough for a powerful mainsheet winch*, particularly if they sail at night. (Some boats never do!)

(*or a power winch - in which case, I'd say it was Bad Idea, no question)

A reason not to fit a mid boom preventer on an end-sheeted boom is that the boom may not have the bending strength to cope, because it doesn't need it
(this is often the case with racing booms, and helps explain why ex-racers tend to be implacably opposed to mid-boom preventers).

But if it does, there's no merit in that line of argument.

Progressive fuses which absorb the hit but do not fail completely (mentioned further up the thread) are the other weapon, if you have to persuade an ex-racer to discard the end-boom layout, but unless they're open minded, I've not have much luck with that suggestion.

Linesmen and windowcleaners stake their lives on them, as do people on "via ferrata" climbing routes in Europe.
I find that reasonably persuasive, as to reliability.

- - - -

The overwhelming advantage of mid-boom, sidedeck preventer/vangs is considerable: they're even better than a boombrake when it comes to gybing a big sail area in lots of breeze.

The main advantage is this: if something goes amiss (like the mainsheet falls looping around an obstruction), or if you have to move something, like running backstays, for instance, you can arrest the process partway through.

You can either gybe back while there's time, or clear the problem and finish the gybe.

[thinwater]

This is really simple on my cat.

With the chute up or broad reaching it is generally not needed. If I do use one--either to stabilize the boom or to move the boom shadow--I simply route the spin sheet through a turning block and clip to the boom end. Boom end sheeting, loose footed main, and no vang (wide traveler) so no interesting structural questions. A wide traveler also helps.

Drag a boom in the water? Impossible.

Because I use an asymmetrical chute, I don't run deep enough to need a preventer. I center the boom and sheet in before jibing. In fact, I do that for all jibes.

I use dynamic line for the traveler. No sharp bang at the end of a bad jib, just a few inches of stretch. Less than one inch of stretch under working loads.

Only when wing-and-wing do I ever sail by the lee. In that case, the spin sheet (not in use) is routed through a turning block and to the boom end and hauled tight. On a cat (lottsa beam) that block can be well aft of the mast, giving better control of the boom angle. I also keep a short utility sheet (20 feet with a clip on the end) that can be used for this and many jury rigs, including MOB, over rides, towing something, or lashing something. Very handy.

[cwyckham]

Originally Posted by Andrew Troup

Jim

Any preventer to the end of the boom applies a pure compression load when the mainsail goes aback.

There is no bending resulting from the preventer tension in such cases, because the clew sideforce is exactly balanced by the preventer sideforce, and they're both attached at the same location.

This wacky idea just manages to apply more compression, by amplifying the tension (just as "swigging" a halyard does), theoretically up to the breaking load of the Spectra line,

if the Spectra line is used to attach a preventer, or if a fixed preventer tether slides along it.

That would only be true of a loose footed sail at full hoist.

[Andrew Troup]

Originally Posted by cwyckham

That would only be true of a loose footed sail at full hoist.

No question about the full hoist. I could argue that in practice the boom is unlikely to break as a result of a deep-reefed sail going aback, but we're talking about vectors, and you're dead right.

But on the other point .... I'm pretty confident the 'foot shelf' of the average mainsail does not impose an appreciable sideload on the boom.

As a thought experiment, imagine (while sailing in a decent breeze) running a box-cutting knife all the way along the sailcloth just above the boom. I think the change in the shape and stress distribution in the sail would be minimal, even with the main over-sheeted.

And this, I think, is why loosefooted mains look much like the other sort.

Most of the load in the lower region of the sail is concentrated in a path from the clew curving up towards the head. If you look at the 'stress map' below there are no stresses normal to the boom elsewhere, but there are bucketloads converging on the clew from above.

Whereas doing the knife thing parallel to the mast, up the luff, would give a rather different result, I reckon, at decent angles of attack.

Especially in the lower quarter
(ie where slugs or boltropes usually fail)
Which is consistent with the stress map.

Looseluffed mains are not popular.

[cwyckham]

Originally Posted by Jim Cate

Looking at the photo of Alan Nebauer (an old friend of ours) it appears that one, the line along the boom has considerable slack in it, not tight up against the boom, and two, that the attachment point is hitched in place, not free to run. We will likely be catching up with Alan soon, and I'll try to remember to ask him how it worked.

Now, as to it being a brake, relying upon the friction of sliding along that Dyneema line... surely not! Ain't much friction there! ONe of the photos shows a line leading fore and aft from the slider, and perhaps that is used to adjust and then fix the position of the slider. Or perhaps they are attached to some friction device like that of a typical cruising boom brake... who knows?

Finally, it is true that these systems add compression loads on the boom. But I note that most cruising booms are pretty stiff, and tolerate considerable compression loads from conventional hardware (outhauls, regular vangs, etc) and I don't recall ever seeing one fail from buckling, even with the considerable loads of mid boom sheet attachments. So, I have to wonder if your worries are valid, especially now that we see heavily loaded extreme race boats utilizing that system.

Cheers,'
Jim

Hi Jim,

I hear you. In the case of Alan's preventer set-up, I'm sure you're right. I'm not sure if I understand the advantage, exactly, but it's the slack that makes it ok from a buckling perspective. Remember that the original case was using "pre-stressed" Spectra done up as tight as possible. That will give you very small angles and very large compressions, especially since the peak loads may be very high.

If you have some slack or use a rope that has some give in it, then the geometry changes dramatically. Also, by using normal rope you greatly reduce the shock loads because of the stretch, so you get a double benefit. I showed a case with a 250 pound end load giving a 2900# compression load. A shock load could have been much larger than 250#.

In Alan's case, if we assume that under full load during a back-wind type event, the slack rope has more like a 30 degree angle, then the compression along the boom from a 250# force would only be about 450#. Much more manageable!

My take-away from all this is that I think that a preventer with some give in it that is attached at the same place as the main sheet (very important point from Andrew about user error on that one) seems to be the winner. I don't understand why you'd want to make a bridle, but if you do it isn't a big deal as long as it makes a decent angle with the boom when under load.