broken boom mid-boom sheeting problem? - SailNet Community

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  #1  
Old 10-18-2007
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broken boom mid-boom sheeting problem?

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 10-18-2007
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Pam,

There was a bit of discussion on the merits of mid-boom sheeting not long ago:
Mid-to-End Boom Mainsheet

Certainly changing to a loose-footed main would increase the force on the end of the boom (it's no longer even distributed along the length of the boom) and could break it. Having extra tension on your topping lift and vang wouldn''t help either.

--Cameron
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Old 10-18-2007
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I have a gut feeling that the problem is due to the loose footed mainsail. There is a difference in the loading on the boom when the mainsail is not attached; when the sail has a bolt rope on the boom the load is distributed linearly and the loads are higher at the luff than at the clew. When you put a loose footed main on, the load at the foot is now concentrated at the end of the boom and puts a much larger bending moment on the boom. This torque gets translated into a concentrated stress at the mainsheet bail. The failure point is going to be where the load is high and the strength is weakest; which would be at the thru-bolted bail.

I would contact Beneteau and ask if there has been any record of boom failures on the 390 and describe to them the problem and ask if they could tell you what the cause was (you might ask without telling them the main was loose-footed at first).

JMHO/HTH...
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Old 10-18-2007
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Even before I read the insurance company conclusion, I suspected corrosion at the bale. I think you need to take the end of the boom to be analyzed at a metal lab to prove differently if visual inspection does not show evidence of galvanic corrosion from dissimilar metals.
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Quote:
Originally Posted by somedaypam View Post
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.
Tacking from one beam reach to the other, all in one go, with the boom out that far, in 15 kts, letting the boom go uncontrolled... isn't that something one wants to avoid, if possible? Sounds almost as forceful as an unintended gybe.

Jim
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Old 10-18-2007
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If you were actually tacking (ie going thru head-to-wind) then it's really odd that there was enough shock loading on the boom to do this damage (barring a weakening of the boom).

However if you were gybing from beam to beam then in fact the loading could be significant if not controlled.

I wonder if in fact the loose footed sail is a large factor - I'd like to know if anyone ever actually analyzed the boom loading of loose footed vs bolt rope mains.... The major loads will be on the clew and tack in any case, I suspect. Esp with the outhaul eased I wouldn't expect the bolt rope to really spread and absorb a lot of load. But who knows......

Also odd that 2 of your blocks were installed in the provided slot, while the other was through drilled... doesn't sound like a factory/rigger install.
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Quote:
Originally Posted by Faster View Post
If you were actually tacking (ie going thru head-to-wind) then it's really odd that there was enough shock loading on the boom to do this damage (barring a weakening of the boom).

However if you were gybing from beam to beam then in fact the loading could be significant if not controlled.

I wonder if in fact the loose footed sail is a large factor - I'd like to know if anyone ever actually analyzed the boom loading of loose footed vs bolt rope mains.... The major loads will be on the clew and tack in any case, I suspect. Esp with the outhaul eased I wouldn't expect the bolt rope to really spread and absorb a lot of load. But who knows......
...And I've only just had the bolt-rope cut out of our mailsail to make it loose-footed!! With mid-boom sheeting, I, for one would REALLY like to know before I go out again.

Which begs the question: I must have seen hundreds of different arrangements over the years, but does anyone know the "optimal" ("best") way of attaching main sheet blocks to the boom in a mid-boom sheeting set-up??

--Cameron
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Quote:
Originally Posted by Hartley18 View Post
...And I've only just had the bolt-rope cut out of our mailsail to make it loose-footed!! With mid-boom sheeting, I, for one would REALLY like to know before I go out again.

Which begs the question: I must have seen hundreds of different arrangements over the years, but does anyone know the "optimal" ("best") way of attaching main sheet blocks to the boom in a mid-boom sheeting set-up??

--Cameron
Most sailmakers these days are making loose footed mains as a matter of course.... I think a bolt rope main is almost a special order item nowadays. I'd think if that alone was a big issue it would have come to the fore by now, and there would be recommendations re reinforcing the boom, or at least recommeded practice for block attachment with loose footed mains.

Drilling holes into spars is always an iffy thing... but many boats have such installations and do not experience boom failure, and I'd venture to say that everyone has had an unintended gybe at least a time or two!

From a practical standpoint I'd think that the booms with extruded tracks along the bottom, and the proper padeye fittings to fit those tracks would be the best method (no drilling involved exc perhaps for a set screw to lock the position)

Looking at the Carbon booms on some race boats, they simply put a sling around the boom and attach to that - no drilling either (but requires a loose footed main!).

Mid boom sheeting gives up mechanical advantage - ergo the sail forces have increased mechanical advantage vs the sheet mounting point(s) - I think that any mid boom sheeting arrangement is more prone to boom failure than an end-boom attachment, and the more forward the mainsheet is the higher the potential for this problem.
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Quote:
Originally Posted by Faster View Post
Looking at the Carbon booms on some race boats, they simply put a sling around the boom and attach to that - no drilling either (but requires a loose footed main!).
I don't want to hijack this thread - but you wouldn't, perchance, have any photos, diagrams or anything of this set-up that you could post?? A picture tells a thousand words..
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Something like this....
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broken boom mid-boom sheeting problem?-boom-strap.jpg  
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