broken boom- mid-boom sheeting problem?

Discussion in 'Boat Design' started by somedaypam, Oct 17, 2007.

  1. somedaypam
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    somedaypam New Member

    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
     
  2. alan white
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    alan white Senior Member

    The boom broke, I'm sure, because you went to loose-footed attachment.
    Mid-boom sheeting shouldn't be used with the sail attached only at the clew.
    You must see why this is so. The force of the sail coming up against the sudden limit imposed by the sheet turned the part of the boom aft of the last block aft into a six foot or thereabouts lever, the fulcrum being that breakage point. Corrosion may have contributed, but it would likely never have come close to happening if you hadn't gone loose-footed.


    Alan
     
  3. fastwave
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    fastwave Senior Member

    Loose footed main

    The boom did not break because it had a loose footed main. Any boom can carry a loose footed main. In the upwind condition the outhaul is so tight that there is no side load applied by the foot of the main to the boom. Thus it makes no difference.

    Either it was the corrosion, wrong design or you oversheeted it against the toppping lift.
     
  4. messabout
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    messabout Senior Member

    I respectfully disagree with Fastwave. It does indeed make a difference whether the sail is loose footed or attached to the boom. The boom is essentially a beam. In the case of an attached sail the load is distributed, tho not distributed evenly. In a loose footed sail the load is concentrated at the tack and the clew end. A mid boom sheeting position is the worst case scenario for the boom when considered in beam loading mode. ( think of a see-saw at the play ground or consider that you had a stick that you wished to break into two pieces. You'd intuitively place the middle of the stick over a rigid object and push down on both ends.) The worst loads and the ones most likely to cause failure may be while reaching or running. The sheet is placing a severe load on the boom at the most disadvantaged position. Consider that there is also a severe load placed on the boom when using the vang. The vang causes the boom to bend in the vertical plane while the sheet tends to bend it in the horizontal plane. Double jeopordy as it were. When going to windward, or even when running free, another set of loads are in play. There is a compression load (endwise load) on the boom to add to the problems of the vertical and horizontal deflection components. The drive of the sail is one of the contributors as the boom is doing its' best to hold the clew in the position that you have chosen for it. Meanwhile. the vang is putting even more compression load on the forward part of the boom. This is a stress analysts bad dream. The fixed loads are bad enough for the spar but during gusts there will be dynamic loads as well.

    All that said, the problem is more than likely a failure due to corrosion and/or age hardening and attending increase of brittleness of the alloy. Fractures are commonly found at locations where there were holes or deep gouges in the parent metal. Holes or gouges form a stress riser that promotes failure at that location.
     
  5. fastwave
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    fastwave Senior Member

    I respectfully insit. But I am not going to start a game.
     
  6. Cliff Pope
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    Cliff Pope Junior Member

    I agree with Messabout. The classic arrangement for a loose-footed main is to attach the sheet at a single point at the end of the boom. If the sail is attached all the way along the boom then the sheet commonly attaches mid-way, but better is to spread the attachment points along the outer half of the boom.

    Having a single mid-point attachment with a loose-footed sail is like placing the boom across two supports and jumping on it.
     
  7. fastwave
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    fastwave Senior Member

    Loose footed main

    Since this is beginning to get some interest I will go in a bit more depth.

    An attached sail does not distribute the load across the boom. Any load transfer along the bottom is minimal. Most of the load is at the tack and clew. To better understand this picture the foot of the main. If the outhaul is loose then the shelf of the main goes sideways out from the boom, which implies some sideways load but no vertical. This load is negtligible compared to the leech load. Also compression wise a loose outhaul is kind. Now tension the outhaul. The shelf of the sail is practically loose (which is a loose footed main setup). The compression load now is at the worst case and this is what is used for an upwind scantling case. Please find me any scantling rule that states anything to do with a loose or attached foot of the main.

    Fastwave
     
  8. Paul B

    Paul B Previous Member

    Fastwave, you are wasting your breath.

    There are some people who "know" that having a captured foot is magical. It "stifffens" the boom and also allows the sail to be "flattened by bending the boom".

    These things have been discussed on this site many times, and even if you provide a force diagram you will not convince those who "know better".
     
  9. tom28571
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    tom28571 Senior Member

    Sorry fastwave, but I go with the loyal opposition.There is much vertical and side loading on the clew attachment at the boom end, especially with a loose foot sail. You can't get rid of these forces just by tightening the outhaul. Most of the force on the sail is perpendicular to the boom and this MAY be why his boom broke. Even with an attached foot, the bending load on the boom is far greater for midboom sheeting than at the boom end.

    I agree with you that the reduction in bending with mid boom sheeting is less than most might think with an attanched foot over a loose foot. But this might have been enough to cause the breakage. In that case, Beneteau probably cut the boom strength margin too close.

    I suspect that once interpretation problems are overcome, we all are in pretty close agreement.
     
  10. messabout
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    messabout Senior Member

    One of the reasons that we subscribe to these forums is to learn. That there is a difference of opinion is a plus feature. I applied the word "respectfully" in a previous posting. That is because I have learned, after some embarassment, to take account of other peoples' position even though I might have been pretty sure of my own.

    I think Fastwave is onto something when he refers to shelves. I had a sail for an A Cat one time that hade a really big shelf. Sure enough, the loose shelf did not place any load on the main body of the boom. For all practical purposes the sail was loose footed with an end plate. I have also had sails with a baggy foot that did not seem to pull on the interior of the boom. When I tied off some reef points there was a small but perceptable difference in the bend of the boom. The boom bent less even though the wind was somewhat stronger.

    So it would seem that the construction of the sail may have some bearing on our differences of opinion. Could it be that we are both right?

    I reckon that Somedaypam does not give a damn about the technicalities. A new boom is the solution. Insurance companies are not famous for being generous so breaking the piggy bank is the obvious remedy. Meanwhile, why not quiz some of the Beneteau people?
     
  11. alan white
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    alan white Senior Member

    My comment about the liklihood of the boom-end attachment coupled with mid-boom sheeting causing the breakage was based on the idea that the boom was well out and it gybed, and the breakage occurred because the sail area aft of the sheet bail was cantilevered.
    Likely the sheet came up hard before the boom hit a shroud (broad reach). The sheet with several ropes is as good as an iron cable in comparison with a shroud, which wouldn't have imposed as severe a shock.
    In any case, the damage occurred right after the sail attachment was changed to loose-footed. It is common sense to look at what was done just before the accident as being the probable culprit.
     
  12. Faster
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    Faster Junior Member

    I'm going to come down on Fastwave's side... there was a similar debate (begun by the same poster) on another forum.

    I, too, think a tightly outhauled main has the same boom loading whether loose footed or boltrope style. For that matter, anytime a main is reefed it is also into "loose foot" mode. Anyone who thinks the reef cringles support the boom (and use them that way) will soon be headed to the sailmaker for repairs to same.

    Mid boom sheeting arrangements, esp in uncontrolled gybes, simply have to be better candidates for failure than end boom sheeting, regardless of mainsail configuration.
     
  13. alan white
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    alan white Senior Member

    Boom end sheeting is the way to go if at all possible. All loads become compression loads if the sail is loose-footed. The boom can actually be lighter, and fewer blocks are needed (which reduces effort and cost).
    Any time a (especially) substantial portion of the boom is cantilevered past the bail, whether loose-footed or not, the boom takes on lateral loading to some degree. More so with 100% of the sail loading concentrated on the clew (loose-footed).
    If the boom has the bail close to the clew, it will be very hard to break the boom under any conditions short of wrapping it around a shroud. But if the sheet is set up to take the strain before the boom contacts the shroud, it would be more likely that hardware would fail first, which is unlikely under any normal conditions.
    There's no "side" to this discussion, as Messabout pointed out. Each boat and rig vary enough to allow everyone to be spot on with at least some instances.
    Maybe I was too quick to say I thought the loose-footed arrangement had caused the breakage, but anyone here headed offshore would likely upgrade the boom section in replacing it as cheap insurance. I know I would.
     
  14. ABoatGuy
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    ABoatGuy Member

    Interesting thread, but I don't think it is an either/or situation. My 2 bits:

    I think the distribution of the load in the boom is dependent on the aspect ratio of the main. With a very high aspect ratio main the distributed boom loading would be minimal and would be concentrated at the clew. With a very low aspect ratio the loading along the boom could be considerable (a J-boat for example).

    It also seems unlikely that with a loose footed sail that all the load in the boom is in compression. The lift being generated by the sail has a significant component perpendicular to the plane of the sail which creates the heeling moment. That load path has to go through both the mast and boom and thus side load at the clew.
     

  15. alan white
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    alan white Senior Member

    All the loading (100%) would have to be in compression if the clew and the sheet attachment were located in the same place at the end of the boom.
    Aspect ratio shouldn't make any difference at all. The load on a sheet attached at the clew would always be the same---- when running, wung out to perpendicular, high or low aspect, it would be exactly half of the force acting on the sail. At mid-boom, it would be the full force.
     
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