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Go Back   SailNet Community > On Board > Gear & Maintenance > broken boom mid-boom sheeting problem?
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Thread: broken boom mid-boom sheeting problem? Reply to Thread
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Topic Review (Newest First)
10-23-2007 06:22 PM
KeelHaulin Yep; there may be multiple factors at work. If there was no visible corrosion I'd say possibly a combination of metal fatigue and the change in loading due to the loose foot. Still hard to say why it happened without seeing some detailed pictures and another post from Pam about the incident.

We "chicken gybe" my boat regularly in 25 to 30 kt winds and I would not say that the loading on the boom when doing this is signifigantly higher than if we were sailing upwind and tacking. When you turn to windward the sails develop a bit more power until they go luff but for the most part it is less stressful on the rig than attempting to do a direct jybe and having the boom crash if not sheeted or released quickly enough.
10-23-2007 05:56 PM
sailingdog Hmm...Yes, I believe that there is a significantly higher percentage of booms breaking due to mid-boom sheeting. However, there are many other factors that could have affected the boom and caused it to break as well.

Galvanic corrosion is a strong possibility, especially if the bail and bolts are stainless steel and no galvanic isolation washers or compound was used during their installation.

The change from a bolt-rope main to a loose-footed main could have certainly contriibuted. A bolt rope main spreads a lot of the load across the boom more evenly, rather than just point loading the clew and tack.

Fatigue of the boom is also another possible contributor. Aluminum doesn't have to actually bend to fatigue, and flexing around the point where the mid-boom sheeting attaches seems more than likely.

Finally, if you were gybing, instead of tacking, the loads could be high enough to snap a boom, even if it weren't previously compromised. Tacking, I think it is far less likely to have been the cause, since the boom will generally move across the boat with significantly less force, since the wind is essentially pushing the boom into luffing, rather than loading it and then switching the loading forces almost instantly.
10-23-2007 10:58 AM
Faster A couple of further thought on this....

When a bolt rope main is tightly outhauled, most of the material (the "shelf") in the foot area is slack and folded under the taut part of the foot between tack and clew.. as such I don't believe there's a great deal of support for the boom there. I think you could treat that situation as essentially "loose footed".

A reefed main is close to the same thing... all the loads are between tack and clew, the reef cringles really are not supporting sail loading even if they are tied in (nor are they meant to). Whether the forces on a reefed main are reduced would depend on just how windy it is, of course, and how appropriately reefed the sail is.

btw the OP has yet to clarify (unless I missed it) whether they were tacking or gybing when this happened.. I suspect they were gybing reach to reach .. so obviously that would increase the potential shock loads involved, as you say, Keelhaulin, loosefooted or not.

I suspect that mishandling, mishap or structural issues would have more to do with this failure than the sail type. I do think mid boom sheeting in general is going to be more prone to this problem.
10-23-2007 02:22 AM
KeelHaulin
Quote:
Originally Posted by Faster View Post
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......
I have not done any sort of "analysis" on this; but one thing that tells me that the loads are higher on a loose foot is that when you adjust an outhaul on the main with a bolt rope it is easier to tighten than when reefing the same sail. I know that a reefing line goes through the eye which increases friction (and the force on an outhaul is reduced because the outhaul car is helping); but you really can't reef a sail without easing the sheet until the sail is luffing and even then it is not nearly as easy to tighten than when the sail has a bolt rope and outhaul. With most of the lift force generated 2/3 of the way forward on the sail I would suspect there to be significant vertical pull on the bolt rope across the length of the foot. When there is no "foot" the outhaul would have to take all of this load in addition to the outward pull it normally sees.

It's possible that the stock boom on the Beneteau 390 was not designed for a loose foot main. Many of the 'newer' boats were FEA designed and considerations for how the loads were placed on the boom may have been made specifically for a bolt-rope main; that was the "stock" sail in the early 90's (loose footed mainsails are relatively new in design). If you look at the new Catalinas they are using a "roll-bar" so that the mainsheet is end-boom controlled, with mast-furling mainsail that is loose-footed. It's possible that this is done to reduce the bending load on the boom; that roll-bar looks like a pricey item for MFR costs. I could be wrong it might only be there to help sell user-friendly boats...

In a normal low windspeed tack I would not expect to see a failure like this; it's hard to say if the loose foot caused it or if it was just a contributing factor. If it was an uncontrolled gybe then certainly the shock loading could have caused the failure (loose-footed or not).
10-19-2007 09:27 AM
SEMIJim
Quote:
Originally Posted by Tartan34C View Post
... or adding material to the bottom surface. Inserts are made for some sections just for this reason.
I should probably look into this. The PO moved the sheeting from end-boom to 2/3 boom (approx.) and the high-performance main is loose-footed, whereas the original sail was not.

Jim
10-18-2007 11:03 PM
GeminiDaze Most, if not all, in mast furling systems use a loose footed mains. However, you will find that the Main Sheet traveler is located somewhere between approx 3/4 to full aft on the boom.

The beam reach gybe is never good for any boom if not sheeted in and then released. Could be a combination issue.
10-18-2007 10:49 PM
Faster Something like this....
10-18-2007 10:00 PM
Classic30
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..
10-18-2007 09:09 PM
Faster
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.
10-18-2007 07:25 PM
Classic30
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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