Boom Preventer

[Bash]

Originally Posted by daddle

Say one has a preventer rigged when the breeze is up and then they accidentally gybe? What does the boat do?

I am unlikely to ever know first hand as I would only rig a preventer is light air and rolly seas. Usually I simple steer up a bit to stay far from the slow and dangerous gybe sector. I've pondered the consequences but don't know. Anyone here had this accident?

In what position does the boat end up if the rig remains intact? Beam-to and luffing?

I had that happen many years ago on a previous boat. I was using some pretty heavy snatch blocks at the toe rail, one of which blew off its snap shackle. The force was so violent that the block went through the dodger eisenglass. Someone could have been seriously injured had the dodger not taken one for the team. We were 1,200 nm from home, which is a long time to be staring at a duct-taped hole in your dodger.

Ever since then, I've only used preventers in light air with big swells. (Actually, most of the time I use a preventer these days I'm at anchor trying to keep the boom shadow off the solar panels in late afternoon. Works great.)

[Andrew Troup]

Originally Posted by Bash

Great idea! I'm wondering how much the midsection reinforcement would cost. Would you put it right where the vang attaches, or further aft?

The midlength of the reinforcement should be at the vang attachment point.

This is because the 'bending moment diagram', looking down from above, will be a triangle, whose base is the boom and whose apex is offset, perpendicularly from the vang attachment point, by a distance corresponding to the amount of side load *

The significance of this diagram is that it's a graph of how the loading affects the boom, in terms of making it bend. The peak of the graph coincides with the point of application of the force from the vang preventer.

So an ideal reinforcement would taper from a maximum at the vang tang, to nothing at the clew and gooseneck.

Of course in practice, you'd simply stop short well before either of these, but that principle could be used to govern how you taper away the profile (or if you get really fancy, the thickness) in the zone you did decide to reinforce.

It should not stop abruptly, because that could conceivably make things worse, rather than better, by creating a 'bottleneck' where the stresses carried by the reinforcement have to make a sharp turn to rejoin their colleagues running along the boom.

Sharp turns amplify the local effect of stress on the metal.


*(in reality it's not a side load but a download, because the boom will swivel so the sail is pulling "Up" at the clew and the vang preventer is pulling "Down", in the frame of reference of the boom.

This is important because the reinforcement should primarily aim to add material near the top and bottom of the section, rather than at the sides. Imagine you're installing 'vang sheeting', with a semi-circular track and a vertical vang and a very powerful traveller, and dispensing with the mainsheet altogether - in terms of the boom implications, it's just the same.

In practice, it could be uniform thickness, particularly if it's not a racing boom, but you could beneficially cut apertures along the midheight if they didn't have corners)

[hpeer]

With the Duthman it is continuously attached. The degree of "preventing" is adjusted by the tension on the line. Lots of tension makes it a preventer. Less tension makes it a break. Not a lot of difference between the two.

Poorly rigged it has slack, which gives it momentum, which makes it darn near useless.

Pros and cons.

[estarzinger]

You have a question about almost any mistake that can be made and you have come to the right shop window, because I (at least) have almost certainly made it.

We use preventers almost all the time when offshore. . . . mostly because there is a lot of lighter wind and they are nice to keep the boom stable . . . but, unlike some, we do also like them in heavier wind.

So, we have certainly accidentally gybed in strong winds (40kts) with them set. What happens will in theory depend on the boat, but for almost all our heavy slug like cruising boats it will be no drama. As Andrew said, if someone is helming or close enough to grab the helm relatively quickly, you just steer back onto course and the main pops back. If you wait longer you loose boat speed and can't steer back thru and the easiest thing is to then ease the preventer and ease the boom across and build boat speed back up and then do a proper gybe back to course.

As Paul has mentioned, if you have a spinnaker up, it adds another level of complication, but most of us cruising will not in strong winds.

Originally Posted by daddle

Say one has a preventer rigged when the breeze is up and then they accidentally gybe? What does the boat do?

I am unlikely to ever know first hand as I would only rig a preventer is light air and rolly seas. Usually I simple steer up a bit to stay far from the slow and dangerous gybe sector. I've pondered the consequences but don't know. Anyone here had this accident?

In what position does the boat end up if the rig remains intact? Beam-to and luffing?

Andrew, Yes my boom section is massive, and I have a really long sleeve both inside and outside centered over the vang lug. I am a bit surprised you think mid-boom is better. Does not mid-boom have a 3m long (with Hawk's dimensions) lever arm acting on it (from the clew load) while end boom has zero lever arm?

[Andrew Troup]

Originally Posted by estarzinger

...

Andrew, Yes my boom section is massive, and I have a really long sleeve both inside and outside centered over the vang lug. I am a bit surprised you think mid-boom is better. Does not mid-boom have a 3m long (with Hawk's dimensions) lever arm acting on it (from the clew load) while end boom has zero lever arm?

Evans

What you say is true from the narrow perspective of what's best for the boom, considered as a "beam in bending" (forgive the engineer-speak).

My concerns, in the general case, are the wider implications for the gooseneck and mast, and for aspects of operation.

However you've been doing it with conspicuous success, in your specific case, for long enough that I'm sure you can safely treat my concerns as mildly interesting, at best.

So anyway, here they are:

(reminder to others reading this: these comments are to do with TOERAIL preventers, not bow preventers.
In a few cases, you might be able to extend or adapt the observations to apply to aspects of a bow preventer

(further note: they're based on beam at the chainplates falling within the 'normal' monohull range relative to boom length... and to me Hawk's powerful form does fall within that, under current norms.

My concerns become more strident as beam reduces):

When connecting the preventer to the toerail rather than the bow, I personally think mid boom is better, provided the boom can handle it.

My reasons:

1) The tension in the mid boom preventer, and hence the bending moment on the boom, reduces as the boom is eased across with the main backwinded *. If the preventer runs to the boom end, the tension increases in the same situation.

While it is unlikely to bend the boom, because it's applied ever more in line with it, this increasingly steep tension rise could create a "toggle press" intensification effect, applying thrust loads into the mast via the gooseneck, for which the geometry alone does not define an upper limit. (Engineer-speak for potentially crippling). Operation, however, can judiciously apply limits, and in Evans' and Beth's case, clearly that has worked well.

2) If end-boom preventers are both set up to stabilise the boom in extreme weather when the main is down, or when reefing (performing the function of a gallows) the acute angles resulting from the boom being middled make this a suboptimal strategy (I'm picking my words carefully because it's nevertheless a great strategy, and 'circumstances alter cases').
The preventers have to resist the inertia of a (hopefully heavy) boom, when the boat may be taking serious sideways hits. In extreme cases, a wavecrest might land in the foot of the sail while it's being reefed. I would personally prefer the boom to attempt to take this on the chin, rather than kicking the stress into the mast, and with the deeply reefed main implied by such conditions, the mid boom preventer is actually better placed, even for boom bending considered in isolation.

In more normal conditions, there is still a case to be made that the higher tensions mean such preventers will stretch more (even with low-stretch line, which I consider desirable) than a mid-boom setup, allowing increased boom motion in a big swell – with the possibility of extra chafe to the blocks and the line. A mid boom preventer will need a multi-part tackle to achieve the same line loads as a single-part end-boom preventer, and this further reduces stretch (although admittedly it increases the localities for chafe)

3) Mid boom preventers remain permanently rigged. End-boom preventers need re-leading every time they transect the lifelines, as Evans mentioned. This means the connection must be easily disconnectable, and even at grand-prix levels of hardware, failures are inseperably linked with complexity of form and function.
This also implies leaving the cockpit, and could expose the boat or crew to risk during the time they're disconnected. For solo sailors, especially when in confined waters with frequent course changes, this is particularly undesirable.

4) Permanently rigged preventers are useful in light variable winds, even hard on the wind. I often remove the mainsheet altogether under such circumstances, use the topping lift to control leech tension, and use the windward mid-boom preventer to slew the boom.
The preventer hauls the boom delicately sideways, whereas a mainsheet, with traveller up to windward, hauls downwards, fighting with the topping lift: "Lots of sound for very little action".

There is also gratifyingly little line to handle, given the rapid and radical changes in boom angle implied by shifting wind direction. It's like being back in a dinghy....

5) For booms which are at head height:

If it's sloppy, the leeward preventer can also be hardened in situation 4) without unduly loading things up. This can be a worthwhile precaution.

The hardest I've ever been hit by a boom was when it was sheeted amidships, in light winds in sheltered waters, and my head was close by it on the windward side.
I was hanging onto the mainsheet falls as I looked up to windward, so I'd be warned of a shift which hadn't made it down to the surface yet. We hit a wake I hadn't noticed. The sail slatted once, the boom gave a twist (about the long axis) and caught me on a tender cranial contour. Boy ! did that hurt!

An end boom preventer does little to prevent the boom twisting in this way, because the force is largely horizontal, rather than downwards.

I think I've got others, but I've run out of time.
Thanks for reading !
YMWV ...


* many sailors whose rigs rely on runner support never initiate a gybe without first middling the boom, in which case this might seem academic .... but even they might need to recover from an inadvertent gybe.
There may come a time when the opportunity to do so by turning the bow down has passed by. Generally they will be able to come up on the new tack, but in rare circumstances this will not be an option

[Andrew Troup]

I thought of a simpler explanation just as I was walking out the door.

Picture a HIAB crane (other brands are available!)

The main lifting ram could equally well push at the outboard end of the main boom, where the hook is, but it doesn't.

The boom is engineered to take the push further in.

As, from the sound of it and from what I remember, the boom on Hawk is....

[rtbates]

Years past I've messed around with all kinds of preventers. I found that running it as far forward, from the boom end (end boom sheeting) as possible worked the best. Then I got really smart, or so it felt, when I started dropping the main all together and using twin headsails when running down wind. It was one of the best things I've ever done. I still really dislike sailing dead downwind, but sometimes it's just necessary and having no main, and no boom (it's sheeted in on center) to trip in the waves was a god send.

[estarzinger]

Randy +1 on running with twin headsails. It's marvelous.

Andrew, so to summarize you see a trade-off between loading/breaking the gooseneck (with an end boom system) and loading/breaking the boom (with a mid-boom system) and prefer to break the boom . . . right? Let's just say that I am not too highly worried about breaking either boom or gooseneck. So, I guess I am interested in other trade-offs?

Bash mentioned breaking the toerail block - I presume mid-boom put rather higher loads on that block than end boom? That block and the clutch are probably the weakest pieces of the system. The block breaking could hurt

You say "Mid boom preventers remain permanently rigged. End-boom preventers need re-leading every time they transect the lifeline", but if you look at the sketch I posted above I think I would also have to detach a 'mid-boom' preventer. I have not measured it but visually it looks like it would need to be at the 1/3 point to always stay inside the life lines - any concerns about that?

[s/v Jedi]

Why do you guys want to break something when it's not needed? A weak point can be engineered to be soft like a slipping rope clutch. Now you have an engineered weak point without damage when it is triggered.

[estarzinger]

Jedi . . . Agreed . . . Like you I have a clutch in the system, and Andrew has suggested fuses.

I will note that the toe rail blocks I use for the preventer are quite old and quite low strength snatch blocks (probably no more than 1500lb swl) and have never broken, despite having taken windy accidental gybes. So it seems empirically that the loads (on an end of boom system) can't be all that high. I initially started using these 'junk' blocks just to temporarily figure out where the best placement was, and then planned to install much better more premenant ones, but somehow never got around to swapping the only ones out. I have loads of better spare blocks on board and should really do it sometime.

[Paul Elliott]

I broke a preventer block during a big crash. The block was grossly undersized though, and I knew it, Maximum Working Load of only of 330 lbs. Rather expensive for a fuse. I had intended to only use it during light air, but you know how these things happen...

[Matt Johnson]

[QUOTE="estarzinger;1242676" but visually it looks like it would need to be at the 1/3 point to always stay inside the life lines - any concerns about that?[/QUOTE]



The Walder Boom Brake suggest setting it at the 1/3 point on the boom. Not sure if this is due to loads or convenience of lifelines and leaving permanently rigged.

[charliehows]

Originally Posted by funjohnson

The Walder Boom Brake suggest setting it at the 1/3 point on the boom. Not sure if this is due to loads or convenience of lifelines and leaving permanently rigged.

the boom brake does a different thing - it acts before loads have built up to the point where somethings gotta give - thats what i love about them - you look up and you're gybing but its not even an issue cos she just rolls over and settles on the other tack - hmm that doesnt sound too good for my sailing technique, does it...

[Andrew Troup]

Evans

I normally fit the tang as far aft as it can be while staying within the lifelines. This should put it near the midpoint of your reinforced section, I imagine.

And generally somewhere about 1/3 L (preferably not less)

As for preferring not to damage the gooseneck: That's not my concern; I'd certainly rather damage a gooseneck than a boom.

The loads I was drawing attention to were loads THROUGH rather than ON the gooseneck. In other words, trying to stove-in the mast.

It would be hard to apply enough axial thrust, when the boom is near to midships, to bust a decent gooseneck without running the risk of crippling the bottom panel of the mast in the process, and that's something not to be trifled with, I'd have thought.



One bad scenario would be to do a leeward broach/massive round-down (autopilot failure, or a wickedly bad unexpected cross-sea) followed by a knockdown and end up lying momentarily on your (previously 'windward') side with the prevented main sticking up skywards.

In that condition a rogue crest dumping against the 'aback' side of the main could easily bust the boom with a mid-boom preventer, but almost as easily break the mast with an end-boom ditto. If the main was reefed to the sailnumber, a third possibility would be to break the boom the opposite way for the opposite reason, because the end-boom preventer is now well aft of the reefed-down clew.

Someone might well ask where I'm sourcing this succession of rogue crests: all I can say is that they're proportionately more frequent in bad blows, and that they not infrequently travel in packs.

(Shades of the apocryphal midshipman, sitting for his lieutenant's commission, who was presented with a succession of improbable screaming squalls, each exposing the ship to an escalating succession of implausible dangers while incrementally damaging the rig and impairing his ability to manoeuvre, to which his answers inevitably involved, among other things, fetching another spare spar.
The senior post captain finally blurts out, in exasperation "Where the deuce are you getting all these damned spars?" "Same place you're getting all these white squalls, sir")

Because the tension rises so steeply as an end-boom preventer comes amidships, I'm not entirely sure that fuses or slip clutches would be feasible, due to 'false positives'. I'm on shaky ground there, no experience and limited analysis.

[Andrew Troup]

Nick

One thing about progressive fuses is that their breaking loads can be more accurately calibrated, and be less prone to drift with time, than a slipping rope clutch, unless you know something I don't.

But the main thing I like is the progressive way the breaking loads can be escalated as the boom comes inboard.

A clutch would need some sort of 'deadstop' as well, to prevent the boom definitively before it came inboard far enough to cause injury.

And it's hard for me to imagine a simple slipping clutch being able to slow the boom down in preparation for that sudden stop.