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Old 17-10-2007, 21:48   #1
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Mid-Boom Sheeting And A Broken Boom

Does anyone know if there is a history of (or higher than "normal" incidence of) booms breaking when rigged with mid-boom sheeting? My boom broke recently and I'm needing to deal with the insurance company and want to educate myself on the dynamics of a break of this type. On my Beneteau Oceanis 390 the main sheet bridle is located in the middle of the boom. There are three blocks- 1 is connected to a bale that's attached to the boom (this is where the boom break occurred) and the other blocks are on bales that slide along a track (groove in extrusion) on the underside of the boom. I was tacking in 15kt winds and going from a beam to a beam, didn't pulled my sheet in and when the boom came over it just kept going and split in two pieces just at the fixed bale. Not sure if its significant or not but I'd just changed to a new loose footed mailsail. I also had the vang on but not too tight and I'd tightened the toppinglift about 3 inches earlier in the day in order to take up excess stretch from the new line. The insurance company wants to say that its a result of "corrosion" in the aluminum under the fixed bale but there's certainly more to this story. I've been sailing for 28 years and never seen anything like this. Any info, insights or similar stories would be helpful. Thanks
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Old 17-10-2007, 22:23   #2
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Aloha Pam,
Was your previous main also loos footed? If you do a search on this forum you'll find lots of discussion on this very subject.
Kind Regards,
JohnL
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Old 17-10-2007, 22:54   #3
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Personally, I have not been fond of mid boom sheeting just for this reason (broken boom). And with a loose-footed sail I can see how it would be worse.

The fore and aft stresses with a loose-footed sail are between the tack & clew. If you put a line in the middle of the boom with out the help of the sail foot, that boom is going to bow backwards. With the foot of the sail pushing in the mid point it would help relieve some of the shock. And under a skock load, all it takes is a small spot to start a fracture. With enough tacks, there you go, snap!

Even with a sail I don't really like the looks of a boom bending from one side to the other. I can imagine what an accidental jibe would do with mid-sheeting.

Another reason I put on a boom brake, so I don't get that sudden shock of the boom tacking hard over. The brake keeps a continous pull on the boom as it goes over w/o the ending result, bang!
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Old 18-10-2007, 01:08   #4
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Ditto what Delmarrey said.

Switching to loose a loose footed sail put all the stress at each end of the boom and the support in the middle.

The corrosion idea can easily be put to bed by taking pictures of the boom and the crack surface. Corrosion could be a contributing factor but my guess is that you changed the entire dynamics of the load bearing system by changing the sail configuration.
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Old 18-10-2007, 04:46   #5
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The mid boom location is more highly stressed (loaded) because it lacks the mechanical advantage of end boom sheeting. Move after and the load on the sheet and attachment points / hardware etc will be less.

The loose footed man exacerbates the problem by not distributing mainsail loading aling the bottom of the boom and transferring all the loading which would have been distributed along the boom to the end where the clew is. Now you have a concentrated load at "mid span" the main sheet and a concentrated cantelever at the end boom. So you have to examine the section of the boom because, by changing the type of loads, from simply supported (end boom) to cantilever (mid boom) and see if the section is robust enough for the latter. Whether it is or not, the simply supported beam (end boom) of the same section will perform better, less bending.

Sound like you loading was getting close to or exceeding the design limits of the section... and for 15 knots that seems awfully under designed.

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Old 18-10-2007, 08:36   #6
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I really doubt that having a loose footed main has anything to do with the forces mid boom. Unless your main is cut absolutely flat, especially in the bottom, there just isn't much force at the foot, mid boom. We had a bolt rope footed main converted to a loose footed and we can be hard on the wind I can easily pull the foot in the middle down to the boom. There just isn't much force there...
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Old 18-10-2007, 09:05   #7
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Quote:
Originally Posted by rtbates View Post
I really doubt that having a loose footed main has anything to do with the forces mid boom. Unless your main is cut absolutely flat, especially in the bottom, there just isn't much force at the foot, mid boom. We had a bolt rope footed main converted to a loose footed and we can be hard on the wind I can easily pull the foot in the middle down to the boom. There just isn't much force there...
Read carefully. It was the "sheeting to the boom" that is in question. And we're not talking about dinghy's here. It's a 39 footer with a 12' - 16' boom.
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Old 18-10-2007, 11:25   #8
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Strange how this thread has kind of done a 180 degree turn from the last time we talked about the subject. Del, I made the same points that you did and got a bit stomped on.
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JohnL
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Old 18-10-2007, 11:32   #9
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You might find an earlier* discussion, “Loose footed main sails”, interesting & informative:
Goto: Loose footed main sails
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Old 18-10-2007, 11:41   #10
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Oops sorry for that. That was from Gord May and didn't have much to do with mid boom sheeting.
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Old 18-10-2007, 11:45   #11
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Cruisers & Sailing Forums > Engineering & Systems > Deck hardware: Rigging, Sails & Hoisting Loose Footed Main OK?

This is the one I had in mind. The search engine isn't working very well is it?

JohnL
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Old 18-10-2007, 12:22   #12
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Originally Posted by SkiprJohn View Post
You might find an earlier* discussion, “Loose footed main sails”, interesting & informative:
Goto: Loose footed main sails
Yeah! I remember that thread long ago. I was in question about even getting a loose footed main. But now that I have a well made sail, I think I'd have a hard time switching back.
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Old 18-10-2007, 12:28   #13
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Originally Posted by delmarrey View Post
Read carefully. It was the "sheeting to the boom" that is in question. And we're not talking about dinghy's here. It's a 39 footer with a 12' - 16' boom.
Thanks for the suggestion? I did read it correctly thank you very much.
I understand that the original poster was interested in the potential problem caused by mid boom sheeting, which of course places much more bending stresses on the boom than end boom sheeting. The problem arises when boom sections that are quite capable of handling end boom sheeting loads are converted to mid boom. Whether the main is loose footed or not has nothing to do with the forces to counter mid boom bend. That WAS my only point. Some folks believe that the foot of the sail carries forces to counter any boom bend tendancy and that's in-correct. As far as it's not being a dingy but a 39 footer,,,well sure, that's why it has a boom with much larger sections than a dingy. It's all relative to the forces generated and the hardware that counters those forces...Bigger boats have bigger booms and bigger forces for sure.
Now back to the authors question...
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Old 18-10-2007, 23:07   #14
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Without modelling the loads here is what I suspect is happening.

Imagine a sail supported only at the head, clew and tack. Imagine this sail had a total of 3,000 pounds of force on it and the center of pressure was approximately in the geometric center of the sail.

Each corner would support 1,000 pounds and through the mast and rigging all 3,000 pounds is distributed into the boat as motive force.

Now if the sail is mounted from head to tack with a bolt rope approximately 1,500 lbs is distributed evenly up the mast. The clew has 1,000 lbs as before and the tack has the residual 500 lbs for a total of 3,000 lbs.

If the boom is 16 foot as Delmarrey says there is 16,000 foot pounds acting at the end of the boom. If the sheet is mid-point the lever arm is 8 feet from midpoint to clew and 8,000 foot pounds of bending moment are acting at the boom end.

If the sail is mounted as a bolt rope the 1,000 lbs is distributed along the boom with center of effort at the mid point. In addition, the 500 lb residual from the clew would be distributed. Call it 1,500 pounds distributed along the boom.

So what? Well 750 lbs would be distributed evenly forward of the sheet and 750 distributed evenly aft. The center of effort would be 12 feet from the tack or 4 feet from the sheet 4 X 750 = 3,000 foot pounds.

The bending loads are more than double in the loose footed example. Add to that the shock of letting the boom "slam" on a tack or gybe and I can see the failure happening.
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Old 20-10-2007, 02:50   #15
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Originally Posted by Ex-Calif View Post
If the sail is mounted as a bolt rope the 1,000 lbs is distributed along the boom with center of effort at the mid point.
I'm sorry, but I believe you're incorrect in stating that a mainsail with a shelf built into the foot will apply a significant load to the boom - the foot shelf is intended to provide an end-plate to the sail, and if the shelf is built correctly it is of lightweight material with enough length to allow the sail's draft to be altered via the outhaul. If the sail doesn't have a shelf and instead is directly attached to the boom (similarly to how the luff is attached to the mast) - then the sailmaker screwed up and the sail has a draft that cannot be altered in the lower third of the sail.

The main sail (loose footed or no) causes the boom to go into compression and the boom must be strong enough to avoid buckling. This is usually pretty easy - witness how must compression a thin-wall aluminum spinnaker pole tube can handle.

The boom's section modulus and moment of inertia must handle the upwards force applied by the mainsail at the clew and the gooseneck, and the opposing downwards force applied by the mainsheet at the mainsheet attachment point. When the boom is eased beyond the traveler, the opposing downwards force is predominantly supplied by the vang.

As for the original posted question, a boom can be engineered that is strong enough to withstand the loads generated by mid-boom sheeting if the loads are known. What is unknown except to the engineer is whether or not the boom was designed, built, and installed for mid-boom sheeting and that it would handle the loads generated by the mainsail, and what those loads might be when shock-loaded during a beam-to-beam tack in 15 knots of wind (though empirical evidence would suggest it was not!).

Beneteau is a reputable builder - I would suggest talking with Beneteau and determine whether or not the boom was intended to be used for mid-boom sheeting and verify that nothing was done to the boom that weakened it at the point of failure - such as drilling a hole thru the boom to install the mainsheet block bail. Is that bail the normal way Beneteau attaches the mainsheet hardware? If so then I would get that in writing from Beneteau and use it to counter the insurance company's argument. It may also turn out that the bail weakened the boom (metal was removed from the boom to drill holes for fasteners), damaged the boom's side wall due to corrosion (stainless and aluminum are not friendly), or point-loaded the boom in a way that the sliding bails on the underside of the boom do not. If the bail is fixed and cannot pivot to align itself with the mainsheet loads, then additional unpleasant forces are locally applied to the walls of the boom where it attached to the boom. Bails should be free to pivot into alignment with loads applied to them.

Clearly the forces generated by the deceleration of the sail 's loads were beyond what the boom could handle and the boom broke at it's weakest point. And yes, this happens more often with mid-boom sheeting than with end-boom sheeting. Though in my experience most booms seem to fold up at that vang or preventer during a heavy air gybe, not at the mainsheet point while tacking in moderate breeze...

- beetle
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