Jibe preventer rigging

[goboatingnow]

Originally Posted by Dockhead

no, from mid-boom


Indeed. And another factor against midships preventers is the angle in the VERTICAL plane, which is much greater when you run the preventer to midships. This is beyond my engineering knowledge, but wouldn't you MULTIPLY the force again by this factor?


So you've got the following situation:


Force on a backwinded sail WITHOUT snatch loads -- say 1400kgf for a 50m2 sail in a 40 knot gust (using cD of 1).


You rig to mid-boom -- 2800kgf (if in a vang position 1/3 of the way from the gooseneck, as recommended in that egregious Cruising Club article, then 4200kgf).



10 degrees angle in the horizontal plane (you're doing better than Platino because your cleats are further forward -- 5x, so 14 000kgf


45 degree angle in the vertical plane -- 1.4x, so 19 560kgf.


Breaking load of 3/8 StaSet? 2000kgf. Of 1/2"? 4454kgf.



A block and tackle will be stronger than a single line, but it's obvious if you do the math that this is BAD ENGINEERING. Again, "never" is not too strong a word for this.






And what's the point here? That we shouldn't try to make going to sea safer? Then why rig a preventer at all? It's all down to the autopilot?

I think the maths getting thrown around here are a bit off

There are far more complex interactions going on here including the effect of stretch and as the boom moves the horizon and vertical angles change. ( and improve )

Hence it’s not quite as simple as being portrayed nor is mid cleat mounting as bad as portrayed.

Also there are the strength of the end boom fittings etc.

Furthermore running bow based preventers is often quite difficult , especially properly running back the line to the cockpit in my case this needs guides and deck organisers and significantly to the clutter on the side decks and additional trip hazards.

Nothing is that simple as being presented

[Dockhead]

Originally Posted by goboatingnow

I think the maths getting thrown around here are a bit off

There are far more complex interactions going on here including the effect of stretch and as the boom moves the horizon and vertical angles change. ( and improve )

I'm not an engineer so will be very happy to be corrected with better calculations.

Originally Posted by goboatingnow

Hence it’s not quite as simple as being portrayed nor is mid cleat mounting as bad as portrayed.

I would be very happy to see objetive reasons as to why mid cleat mounting is not as bad as this. I'm not seeing it with my weak engineering imagination. Stretch just adds snatch loads so makes it even worse for preventers rigged like that as far as I can see. If the boom is moving around, then all the more you want a good angle on the preventer, don't you? Or if not, why not?

My preventers are dyneema, chosen because it seems to me that stretch is really bad here.

Originally Posted by goboatingnow

. . . Furthermore running bow based preventers is often quite difficult , especially properly running back the line to the cockpit in my case this needs guides and deck organisers and significantly to the clutter on the side decks and additional trip hazards. . .

Those arguments don't go to strength. If it's not strong enough otherwise -- an objective, engineering question -- then one needs to deal with the convenience issue one way or another.

In my experience, on the boats I've sailed, it's not that difficult to run preventers to the bow. On my present boat in particular I have short bulwarks and two sets of midship cleats, so it's quite easy to run the preventer back from the bow cleat back through both midship cleats as leads, keeping the preventer quite close to the bulwarks and out from under foot. But if it were difficult to rig, that wouldn't magically make rigging it differently strong enough.

The deck does get fairly complicated when running a spinnaker with our "cheater pole", which requires a jockey pole when squared forward for sailing higher, so a plethora of tack lines, downhauls, pole lifts, guys, etc. etc., but rigged right is rigged right. Sometimes spaghetti on deck is unavoidable.

[jannw]

Boom brakes correctly set up do double duty as a preventer, can be easily attached and removed with a snap shackle, and can be purchased economically from a climbing store as a "figure 8 descender" ... they also do a great job of controlling hair-raising gybes as an added extra.

[Happ]

Sail just had an article on this. Wichard makes a very nice piece. You just weave a line that is attached on both sides at the beam. Easy.

[argonauta1]

The attached JPG might help. The vessel is a Beneteau 440. The encircled bungee was very successful in mitigating shock loads to the rig in the event of inadvertent jibe.

[tweitz]

I am learning a lot from this thread, and also from the very good video from the Maryland School of Sailing at https://youtu.be/qAhUWCa-KoQ . By the way, I also saw another video on line that casually said that all one needs to do is turn the boat as the jibe begins, so that it does not strike the far end, and the rigging on a modern boat should be able to handle the jibe. That advice seems to me to be incredibly dangerous.

A couple of questions for the experts on here: It seems to me that it makes an awful lot of difference in the force if the preventer is tight or loose. The big danger of an accidental Jibe is that once the boom builds a little momentum, the force quickly becomes enormous and potentially lethal. No matter how one rigs the preventer, unless it is tight the force might be enough to break things. If it is tight, then it is unlikely to pick up the necessary speed to do the damage.

Also, the discussions on angles seem to me to be very dependent on how far the boom is out. It seems to me that the angles would change considerably. On a boat like mine, with spreaders angled back, the boom can never be out at ninety degrees. So presumably the calculations of the angle would be quite different.

Thanks for the discussion.

[sailorladd]

What I did when sailing my 32 foot Hartley across the Atlantic and around the Pacific to prevent unplanned jibes, was merely to attach a line from the clew side of the boom, lead it outboard of the side stays and to the bollard/samson post on the bow...

After numerous dangerous jibes in my early days, this jibe preventer I used (from the wonderful book of Eric Hiscock, "cruising under sail") I never once had another jibe, and all it cost was the price of some polyester line...

[argonauta1]

Further to my JPG I posited in my earlier reply, I thought it would be useful if I posted the "why" behind the bungee. About eight days out, enroute from the Galapagos to the Marquesas we had been running all night with 20 to 25 knots of wind about 10 degrees off the stern from starboard making a good 7 knots. I had the genoa poled out to starboard and full main out to port with a jibe preventer to the toe rail. Early morning the main sail dropped. Yup, the main halyard had parted at the mast sheave. Why? Well I lacked a full understanding of the stress relationship between the luff and leach of the mainsail. In short, I had the preventer too tight thus restricting the boom from rising and falling with the flexing of the mainsail. Hence, all the stress was transferred to the luff and the main halyard eventually parted at the sheave. I keep the lower part of the halyard as a souvenir and as a prop when I talk about rig stress. Later, we modified the preventer adding a very strong bungee between it and the boom. I also avoided metal to metal connections and used looped lifeline strips to secure the preventer to boom and toe rail. The arrangement took us the rest of the way of a circumnavigation and beyond. Again I picture it below:

[Marathon1150]

Originally Posted by Dockhead

I agree. "Just let it go"? That's a recipe for mayhem, including even losing the rig.


Furthermore, a preventer does more than just hold the boom against a fully backwinded situation. It keeps the boom from flopping around in light wind, which may allow it to be caught backwinded.

Preventers? Use them! On my boat, ALWAYS, when the boom is out beyond the rail.


Always rig preventer to the boom end and ALWAYS take it all the way to the bow.

The other reason that I always deploy a preventer as per the above, is to stop sail and boom from slatting in light wind. It drives me crazy.

We run a line from the end of the boom (lots of stories out there regarding booms breaking when the attachment is mid boom) to a an oversized cleat at the bow and back to a cleat at the stern. I agree that it is important to be able to quickly release a prevented boom after an accidental gybe. So far this has been an issue but we are very aware of the risk. Bill Siefert's book describes a system where part of each line (best to have one on both sides) lives on the boom and other part lives in a locker until needed. His approach involves installing some cleats on the boom. Someday the cleat installation project will make it to the top of the "to do" list.

[requiem]

Originally Posted by Dockhead

I would be very happy to see objetive reasons as to why mid cleat mounting is not as bad as this. I'm not seeing it with my weak engineering imagination.

Agree. I see the point that the geometry would might with the midcleat attachment and even worsen with the bow attachment as the boom moves, but the point is to prevent the boom from moving to begin with, so that shouldn't come into play.

Originally Posted by Dockhead

My preventers are dyneema, chosen because it seems to me that stretch is really bad here.

I recall such a discussion in older threads, and I think good reasoning was provided, but I could use a recap here as my instinct is that shock loads on static line would be far worse. (This comes from the climbing world, e.g. reference funkness devices, how to break dyneema slings, etc.)

[Bill_Giles]

There is thing called a figure of 8 descender used in climbing. They are cheap. Shackle the small end of one of these to your boom. Secure a long rope to a point on the bow take it though the large ring on the descender take a few turns until you are satisfied with the friction. Take the end of the rope through a block on the bow and back to a cleat in the cockpit. With the boom well out tighten the rope. The friction with control any inadvertent jibe. Loosen to carry out a controlled jibe.

[Dockhead]

Originally Posted by tweitz

. . . A couple of questions for the experts on here: It seems to me that it makes an awful lot of difference in the force if the preventer is tight or loose. The big danger of an accidental Jibe is that once the boom builds a little momentum, the force quickly becomes enormous and potentially lethal. No matter how one rigs the preventer, unless it is tight the force might be enough to break things. If it is tight, then it is unlikely to pick up the necessary speed to do the damage.

Your analysis is exactly correct. You are talking about snatch loads. You don't want them in this system.

Originally Posted by tweitz

Also, the discussions on angles seem to me to be very dependent on how far the boom is out. It seems to me that the angles would change considerably. On a boat like mine, with spreaders angled back, the boom can never be out at ninety degrees. So presumably the calculations of the angle would be quite different.

Mine are also swept back, and the angles are worse as a result.


The calculations don't change. The measurements do however.

[Dockhead]

Originally Posted by requiem

. . . I recall such a discussion in older threads, and I think good reasoning was provided, but I could use a recap here as my instinct is that shock loads on static line would be far worse. (This comes from the climbing world, e.g. reference funkness devices, how to break dyneema slings, etc.)

Certainly, if you get a shock load, low stretch cordage like dyneema is worse. But the point is to prevent the shock loads in the first place. Dyneema kept good and tight should be better, in my opinion.

[limte]

For many years we have used a four part tackle and nylon kernmantle rope for our preventer system. We attach a double block to the middle of the boom using a sling around the boom of one half inch nylon double braid. A similar sling attaches a double block to a hawse hole amidships. We keep this preventer tight when it is needed, and a gybe, either intentional, or very rarely unintentional, results in virtually no movement of the boom. The preventers are cleated off, and released slowly after the sail backwinds. The dynamic kernmantle line allows a very little bit of stretch, which absorbs some of the backwinding force.
I recall some years ago reading that Hal Roth used a similar midship system, and noted that if the system is kept taught it is an appropriate way to rig preventers. Another point to note is that it may be appropriate to reef sails when on a downwind run, even when it may not appear necessary. On our boat, we frequently reef on downwind runs, to the same extent as if we were beating. The speed is not reduced by much, the wind vane works better, and if a squall comes up, we are already prepared.

Regards,
Bob S

[Joli]

I read that report, mostly they had never sailed the boat in a breeze. Stuff wasn't sorted.