Boom Preventer

[cwyckham]

Originally Posted by Capt. Don

Here is the best installation instructions for boom brakes and preventers I have seen.

"Preventers and brakes are notorious for breaking booms so should not be installed like most people do.
If its a preventer you use to prevent an accidental boom gybe on a monohull , you will want to fuse it with a small line that will pop if the boom was to drag in the water otherwise you will have catastrophic failure from the water load , on multis it's different, if you drag your boom, you are in bigger problems.
Either system you may use will need to attach to the boom somehow, and most attach it directly causing all the failures you hear about because of the point loading.
Here is what I have been doing for years and have never had any failures because of pure mechanics .
I use a very stout high tech line, either spectra/dyneema, vectran or one of the other flavors and run it along the underside of the boom from front to back. This line needs to be pre stretched to take out any sag and it is lashed tight one end to the other. Anything like a brake or preventer line is attached to this line and can be free running back and forth along the length as it will find its true center . The brake/preventer now puts the boom in pure compression load through the tensioned line, and unless the load generated is incredibly high the boom will never break.
I use a preventer line when sailing downwind (Leopard 47) to vang sheet the main to get proper leech twist. I also never sail any deeper than 150 degrees apparent as this is useless on any boat especially on a multi where you will be going slower than the wind. It is better to gybe downwind and keep the speeds up a little, it is also safer in the fact you won't round down causing an accidental gybe.
Hope this helps you understand the principle." - Troy Bethel

I have now started production (lost wax casting) of 1/2 thick very strong inexpensive 316L SS figure of 8 boom brakes and taking order reservations. Please see my site at: Boom Brakes
I am looking for help with sales so if you think you have the connections to help please contact me through the web site. Thanks

I couldn't disagree with Troy's analysis more. His system will put huge loads on the spectra line and the boom will have exactly the same bending load as if he'd just attached the preventer to the end of the boom, but now he has also introduced a massive compression load on the boom. This will greatly increase the chances of boom buckling.

Don't do this.

Also, his statements about wind angle are only correct in light wind. Once you hit hull speed on a mono, pointing deeper does get you there faster as your speed is the same but your route more direct

[Andrew Troup]

Originally Posted by cwyckham

I couldn't disagree with Troy's analysis more. His system will put huge loads on the spectra line and the boom will have exactly the same bending load as if he'd just attached the preventer to the end of the boom, but now he has also introduced a massive compression load on the boom. This will greatly increase the chances of boom buckling.
...

Sadly, I agree with cwyckham.

There's even a secondary effect, potentially: this system could in extreme circumstances apply enough thrust to the mast along the axis of the boom, added to the compression already in the mast from the rigging and the sail loads, to take the mast 'out of column' and cause it to fold up.

And the less stretchy the spectra line (ie the larger the diameter) the worse the compression, for no benefit in terms of bending load.

[cwyckham]

Originally Posted by Andrew Troup

Sadly, I agree with cwyckham.

There's even a secondary effect, potentially: this system could in extreme circumstances apply enough thrust to the mast along the axis of the boom, added to the compression already in the mast from the rigging and the sail loads, to take the mast 'out of column' and cause it to fold up.

And the less stretchy the spectra line (ie the larger the diameter) the worse the compression, for no benefit in terms of bending load.

Don't be sad about agreeing with me!

I thought about the gooseneck compression, but the line will be putting equal compressive force in each direction on the boom. ...

Wait a minute. It just occurred to me that there is also a lateral force on the gooseneck equal in magnitude and direction to the end boom force. So your right about putting extra unneeded forces on the mast. Just in a different direction and lower forces.

[Andrew Troup]

Originally Posted by donradcliffe

.... The main should always be reefed down so that a crash gybe will not likely break anything--if it will, you should have it down completely....

That's OK in theory, Don: it's sound advice, especially for conservative cruising practice.

I have to say, though, that I personally find the "should" label rather unhelpful, when things unforeseeably and rapidly get difficult, especially short-handed, because there's often a bunch of competing things which "should" have been done already.

As for particular challenges: I don't not mention these because they're likely, but because they're unlikely. The fact they nevertheless happened to one person, indicates just how unpredictable the sailing environment CAN be.

Equally challenging things which have not happened to me will certainly happen to others:

I can recall on a good sailing day, close reaching into a bit more than 15 knots on port tack under full sail, and getting overtaken by a "downburst" gust of 45 knots from the stbd quarter, not visible on the water, at a notorious three-way junction surrounded by high hills, in the Marlborough Sounds. That's about as extreme as a situation can be, and the most savage crash gybe I've personally experienced.

Luckily it was a 20' boat with a very strong rig and minimal rotational inertia, so the rig spun the boat so fast the land went slightly blurry for a moment, effectively rolling with the punch.

On a bigger boat, I'm confident it would have taken out the rig, in the absence of a boombrake, and certainly would have killed outright anyone standing up. I remind you we were close reaching: one does not normally expect a gybe.

Setting that scenario aside, because it's a localised freak of nature, I have been caught with too much sail up on a couple of occasions where it was no longer possible to tack, and there was not searoom to take in a reef or drop the main on the tack we were on, even hove to. If you have to drop a large mainsail while running off, you need a lot of sea room.

On a big boat you might be doing well over ten knots, and if the sail is plastered against the stays, it can take a lot of time and effort to lower it. Ten miles of sea-room for every hour it takes could easily be more than you have to spend.

Those who have not sailed with a well engineered modern preventer or boombrake are unlikely to appreciate just how big a sail can be safely gybed in very strong winds, even single-handed. Long distance racing with big rigs in the Southern Ocean would not be feasible otherwise.

The loadings which build up when gybing with old-school systems, where the mainsheet provides the main control, are misleadingly high, and the dangers considerable. There is no way most people can bridge the gap to what a decent system can permit, purely by exercising their imagination.

Crash gybes are not compatible with some rigs (incl those with running backs), and outright dangerous with end-boom sheeting in particular.

The slack falls of the sheet can loop around winches or pedestals or crew.

If this happens with a proper preventer or brake, the boom can be stopped at any point in the swing to clear the problem.

[Andrew Troup]

Originally Posted by cwyckham

Don't be sad about agreeing with me!

I thought about the gooseneck compression, but the line will be putting equal compressive force in each direction on the boom. ...

Wait a minute. It just occurred to me that there is also a lateral force on the gooseneck equal in magnitude and direction to the end boom force. So your right about putting extra unneeded forces on the mast. Just in a different direction and lower forces.

You're right. I had misunderstood the proposal: I assumed the forrard end of the spectra line to be attached to the mast at the bottom of the gooseneck. It wasn't specified, but I think your assumption makes more sense.

Apologies for the confusion.

[Jim Cate]

FWIW:

After a friends new Van deStadt Samoa 47 broke their boom with a mid boom preventer, the spar maker (AllYacht spars of Brisbane) replaced the boom (a Selden single line reefing job), and fit just such a preventer rig. A big Dyneema line from end cap to gooseneck (not the mast) with a block running free along it for attaching the preventer tackle. Apparently it worked as a preventer and as a vang, but I never saw how you could use it to ease the boom through a gybe like we do with ours which is attached to a fixed point on the boom. Seems to me that as the boom crosses toward the midline the block would run up to the gooseneck and loose all control.

Sadly, the owner of that yacht had a sudden onset of vertigo which has prevented them from further voyaging, and we have lost touch with them, so I can't readily provide an update or further details.

Cheers,

Jim

[cwyckham]

Originally Posted by Jim Cate

FWIW:

After a friends new Van deStadt Samoa 47 broke their boom with a mid boom preventer, the spar maker (AllYacht spars of Brisbane) replaced the boom (a Selden single line reefing job), and fit just such a preventer rig. A big Dyneema line from end cap to gooseneck (not the mast) with a block running free along it for attaching the preventer tackle. Apparently it worked as a preventer and as a vang, but I never saw how you could use it to ease the boom through a gybe like we do with ours which is attached to a fixed point on the boom. Seems to me that as the boom crosses toward the midline the block would run up to the gooseneck and loose all control.

Sadly, the owner of that yacht had a sudden onset of vertigo which has prevented them from further voyaging, and we have lost touch with them, so I can't readily provide an update or further details.

Cheers,

Jim

That's pretty surprising. On the other hand, I've had to draw the vector diagram of a transverse load on a tight line many, many times, even for aircraft structural engineers. It isn't an obvious concept.

The tighter the line, the worse the problem. Of course, a loose line will take somebody's head off which wouldn't help much. It could be done safely with a fairly stretchy line, I suppose, but I fail to see the advantage. The best thing would be to just attach the preventer at the same place, roughly, the sheet's attached.

I hadn't even considered your important point about the block sliding to the mast when trying to let the backed main down under control!

[carstenb]

I have end boom sheeting (for now), and plan to rig a dutchman boom brake. Off hand, the place that comes to mind is where the rod-kicker is attached. This is about half way along the boom, which I realize may result in very high loads midway along the boom.

I won't be sailing in much more that 35-40 knots (probably not even that, although you can never be sure), this summer. It will be int the baltic, so no chance of catching the boom in the water (waves simply aren't big enough).

I plan on mounting a different boom and rigging a german style sheeting next year.

Any input on if my scheme for this year will work? Or should I rig a preventer from the end of the boom?

carsten

[Andrew Troup]

"Letting the backed main down under control" is the Achilles' heel of any end-boom preventer, unless on a multihull (or mono with deck spreaders with foreguys), it seems to me. It's like a toggle, or overcentre linkage, as used to devastating effect on a toggle press.

Obviously it's even worse for the

<< big Dyneema line from end cap to gooseneck (not the mast) with a block running free along it for attaching the preventer tackle.>>

New topic: Which part of the gooseneck, Jim? hopefully the part attached to the boom?

[Jim Cate]

Originally Posted by Andrew Troup

New topic: Which part of the gooseneck, Jim? hopefully the part attached to the boom?

Yes, Andrew, that's what I meant to convey. IIRC, that single line reefing gooseneck had a suitable attachment point not far from the end of the extrusion. I wish I had taken some pix, but that was a while ago and the magic of digital photography had not quite penetrated my foggy brain!

Cheers,

Jim

[Andrew Troup]

Originally Posted by carstenb

I have end boom sheeting (for now), and plan to rig a dutchman boom brake. Off hand, the place that comes to mind is where the rod-kicker is attached. This is about half way along the boom, which I realize may result in very high loads midway along the boom.

I won't be sailing in much more that 35-40 knots (probably not even that, although you can never be sure), this summer. It will be int the baltic, so no chance of catching the boom in the water (waves simply aren't big enough).

I plan on mounting a different boom and rigging a german style sheeting next year.

Any input on if my scheme for this year will work? Or should I rig a preventer from the end of the boom?

carsten

Carsten

I strongly recommend you find a way to fit the boombrake (or a 2:1 purchase, which in some ways is ever better) at the kicker location.

I think you are probably at a crossover, where if your boat and mainsail were smaller, you would be fine, and if they were bigger, you would definitely need to reinforce the boom at that point.

Two possibilities (if I'm right!)

1) Get a welding shop to build two tangs, out of say 5mm stainless plate, one for each side of the boom. The baseplates should be curved to conform to the boom profile, and maybe 18cm long (fore and aft), and the tang lug should lie in the plane the leeward preventer will occupy when the boom is well outboard.

The fastenings should either pass right through the boom, with spacers to prevent crushing, or there should be backing plates inside the boom, on either side, which can have tapped holes for the fastener. These backing plates should be longer than the baseplates.

2) Fit a progressive fuse at the sidedeck, so that it fails in several stages, each absorbing increasing amount of energy, and by the time the boom is far enough inboard to be nearly over the gunwhale at the cockpit, something at least as strong as the preventer/boombrake line is taking all the load, and will not fail.

If you're interested, I'm happy to discuss in more detail.

If you're changing the boom next season, option 2) probably has a lot more appeal.

[carstenb]

Originally Posted by Andrew Troup

Carsten

I strongly recommend you find a way to fit the boombrake (or a 2:1 purchase, which in some ways is ever better) at the kicker location.

I think you are probably at a crossover, where if your boat and mainsail were smaller, you would be fine, and if they were bigger, you would definitely need to reinforce the boom at that point.

Two possibilities (if I'm right!)

1) Get a welding shop to build two tangs, out of say 5mm stainless plate, one for each side of the boom. The baseplates should be curved to conform to the boom profile, and maybe 18cm long (fore and aft), and the tang lug should lie in the plane the leeward preventer will occupy when the boom is well outboard.

The fastenings should either pass right through the boom, with spacers to prevent crushing, or there should be backing plates inside the boom, on either side, which can have tapped holes for the fastener. These backing plates should be longer than the baseplates.

2) Fit a progressive fuse at the sidedeck, so that it fails in several stages, each absorbing increasing amount of energy, and by the time the boom is far enough inboard to be nearly over the gunwhale at the cockpit, something at least as strong as the preventer/boombrake line is taking all the load, and will not fail.

If you're interested, I'm happy to discuss in more detail.

If you're changing the boom next season, option 2) probably has a lot more appeal.

Andrew, as noted, I'm fitting an adjustable brake, so the system will ahve as much "give" in it as I adjust the brake to. I've just gone out at looked at the boom (I'm a very fortunate sailor - my boat lies 8 meters form my ktichen doo LOL) The rod kicker is attached about 1/3 of the way out from the gooseneck. There is room on the attachment to fit the boom brake.

I'm expecting to adjust the brake so that in a gybe situatin the boom moves slowly enough for everyone to be aware and get out of the way, yet fast enough to allow the gybe to take place in a timely manner.

I had thought about rigging a fuse - this is probably a good idea.

carsten

[Cruisin Cat]

Originally Posted by Capt. Don

Here is the best installation instructions for boom brakes and preventers I have seen.

"Preventers and brakes are notorious for breaking booms so should not be installed like most people do.
If its a preventer you use to prevent an accidental boom gybe on a monohull , you will want to fuse it with a small line that will pop if the boom was to drag in the water otherwise you will have catastrophic failure from the water load , on multis it's different, if you drag your boom, you are in bigger problems.
Either system you may use will need to attach to the boom somehow, and most attach it directly causing all the failures you hear about because of the point loading.
Here is what I have been doing for years and have never had any failures because of pure mechanics .
I use a very stout high tech line, either spectra/dyneema, vectran or one of the other flavors and run it along the underside of the boom from front to back. This line needs to be pre stretched to take out any sag and it is lashed tight one end to the other. Anything like a brake or preventer line is attached to this line and can be free running back and forth along the length as it will find its true center . The brake/preventer now puts the boom in pure compression load through the tensioned line, and unless the load generated is incredibly high the boom will never break.
I use a preventer line when sailing downwind (Leopard 47) to vang sheet the main to get proper leech twist. I also never sail any deeper than 150 degrees apparent as this is useless on any boat especially on a multi where you will be going slower than the wind. It is better to gybe downwind and keep the speeds up a little, it is also safer in the fact you won't round down causing an accidental gybe.
Hope this helps you understand the principle." - Troy Bethel

I have now started production (lost wax casting) of 1/2 thick very strong inexpensive 316L SS figure of 8 boom brakes and taking order reservations. Please see my site at: Boom Brakes
I am looking for help with sales so if you think you have the connections to help please contact me through the web site. Thanks

In my humble opinion, this is a bit of smoke and mirrors. If you get any structural engineer to draw a vector diagram of the forces involved in this line under the boom arrangement, three things will become apparent...
1. There is a lateral force on the boom (not just compression) otherwise the boom would not be prevented from moving backwards
2. There is an additional large and totally unnecessary compression force introduced into the boom, and
3. Since the lateral force acting on the boom is transmitted to the boom through the point where the aft end of the under boom line is attached, the same lateral force (with much less compression) can be transmitted to the boom by simply attaching the preventer line at the same point

Barrie

[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

[Capt. Don]

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

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