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Old 06-04-2012, 20:41   #1
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Snapped Boom

12-15 knots of wind. We were under engine power rounding Redonda, sail was up but not under load as we were heading directly into the wind at the time for people to take pictures. Then I heard something from one of the crew that I could not process "your boom is broken!"

Main sheet was tight, traveller locked. There was no tacking, no swing.. just poof and it snapped as you see here. The rope on the back was slack before the break, the boom snapped up like that in one go.

Rigging was recently inspected a few months ago and stays were replaced and mast was taken up and down. Traveller car was also replaced and new.

Has any one seen anything like this?
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Old 06-04-2012, 20:47   #2
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Re: Snapped Boom

Do I see a topping lift at the end of the boom in the first pic? Main sheet acting against topping lift?
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Old 06-04-2012, 20:53   #3
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Re: Snapped Boom

Not an old boat either. could be the topping lift against the mainsheet but I would think you had a solid vang.
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Old 06-04-2012, 21:00   #4
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Re: Snapped Boom

Topping lift was not tight, it was loose. In fact I had new crew and I made a big deal of making sure it was loose because otherwise it pushes on the sail and deforms the shape when under load.

And even if it was tight, I would think there was no way that rope could exert enough pressure to do this.
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Old 06-04-2012, 21:02   #5
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Re: Snapped Boom

Quote:
Originally Posted by Charlie View Post
Not an old boat either. could be the topping lift against the mainsheet but I would think you had a solid vang.
Yes boom has a vang.
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Old 06-04-2012, 21:03   #6
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Re: Snapped Boom

Quote:
Originally Posted by Charlie View Post
Not an old boat either.
Boat is a 2001 model, Im fourth owner and have had it since late 2009.
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Old 06-04-2012, 21:13   #7
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Re: Snapped Boom

Examine the crack: you probably can see, it started at the bottom of the boom, you should see lines, small fractures, aged, probably oxidized. Then the rest of the fracture should be clear, shiny metal.

It broke right behind the main sheet. There had to be a force pointing up, at the end of the boom. Probably just the clew of the mainsail was enough force, over time.

I don't see any reefing lines going thru the boom? This shouldn't be too hard to repair, I probably would stiffen this section a bit, to several feet behind the mainsheet blocks.

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Old 06-04-2012, 21:19   #8
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Re: Snapped Boom

And that's why I run my main sheet blocks at the end of the boom. I'm willing to bet the crack started right where the aft block support is riveted to the boom.
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Old 06-04-2012, 21:20   #9
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Re: Snapped Boom

I looked at the crack but will look closer with the clues you mention. Its fairly straight.

Reefing lines were not in. The sail was recently repaired and we had just put it back up. We did not have a chance to put the reefing lines in yet, and the conditions today did not call for above 14 knots of wind. If winds had gotten too much we had planned to simply take down the main sail and use fore sail and/or engines.

What really amazed all of us was that it snapped while it was not under load.... but it was also lucky as dealing with it under load would have been more difficult.
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Old 06-04-2012, 21:31   #10
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Re: Snapped Boom

After a closer inspection it looks as though those block supports are welded on, rather then riveted. Is that the case?

If so, that's where the crack started. On aluminum boats, cracks always start right next to the welds. One should never weld across the stress area of an alum. structure. Always weld inline with the stress.
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Old 06-04-2012, 21:46   #11
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Re: Snapped Boom

this is a picture of a broken piece of aluminum. The black portion is where it started. It slowly developed, see the lines? Then, at some point, the material is weakened enough, to break off.

What I am trying to say: The damage was done earlier. Maybe the weld, maybe a crash-jibe in the past. Then the crack kept advancing. Today it broke

http://www.brightspoke.com/img/fatigued_crank.jpg
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Old 06-04-2012, 23:06   #12
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Re: Snapped Boom

Aluminum welds reduce the strength of the material by something like 30% Unlike steel whose strength is unaffected or maybe even strengthened by a proper weld. Looks like the break was caused by the welded bale. A bale with drilled and tapped fasteners probably would have been fine.
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Old 06-04-2012, 23:08   #13
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Re: Snapped Boom

That sucks man. =( Hope they didn't weld too much other stuff on your boat.
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Old 06-04-2012, 23:18   #14
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Re: Snapped Boom

Yes, I've seen that lots of times on aluminum racecar parts that were welded right on the stress points. That only works if you weld on a round or diamond shaped plate under the fitting that stretches away from the stress point about the same distance as 1/2 of the tube's diameter or more. Better to bolt or rivit there.
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Old 06-04-2012, 23:56   #15
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Re: Snapped Boom

Hi,

We had exactly the same thing happen to us 2 years ago although our conditions were a little different. We were sailing down wind in strong breezes and lumpy seas at 3am in the morning on a moonless night.

It certainly took us by surprise! We had certainly sailed in worse conditions and didn't expect it.

Surprisingly after retrieving the boom we sailed with the main loose footed to the main sheet ( like a head sail) and were able to average better than 6 knts





This shot was taken the following day after we managed to get into Escape River to lick our wounds.

Our investigations confirmed what others have suggested. The boom snapped right behind the vang attachment that was welded to the boom.

A close inspection showed that there may have been a small crack starting some time before the boom snapped.

The rigger who built the rig suggested that the cause in our case was possibly the mast pumping in the middle as a result of the stay sail collapsing putting upward strain on the boom at the vang attachment.

We did a very temporary repair in an isolated town at the top of Cape York then replaced it in Darwin thanks to insurance!!! We made sure this time that the vang attachment was riveted rather than welded.

This a link to our blog when it happened. Unfortunately we were in a very remote part of the Australian Coast so we didn't blog a lot of detail.

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