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Old 07-04-2012, 01:34   #16
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Re: Snapped Boom

The root cause of this is mid-boom sheeting. This arrangement creates tremendous stress on the boom since the main force of the sail is imparted to the clew. The sheet attachment then becomes a fulcrum. So the whole driving force of the sail is taken in the boom as bending moment. It's a very common failure on mid-boom sheeted boats.

It's as if you laid your boom down on the edge of a dock, and had two guys stand on one end, and one other guy jump up and down on the end hanging over the edge. A good, strong boom might hold up for a while, but for how long? It only takes one crash gybe to get the cracks started, after which it is only a matter of time. . .
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Old 07-04-2012, 05:42   #17
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Re: Snapped Boom

Way back when, mid boom sheeting actually put less bending stress on booms but with modern sail cuts, it is putting more. The reason that it has changed is that the load distribution on the boom has changed. When the foot of sails carried a relatively even load all along them, placing the sheet so that approximately 1/3 of the load was outboard and 2/3 inboard resulted in the least bending stress in the boom. If you imagine end boom sheeting and a lot of load pulling up on the middle of the boom, you can see that the bending stress in this arrangement is higher. However, what has changed is that less and less of the load is distributed along the boom and most is now at the clew. In the case of a loose footed sail, all of the load is at the clew. In this application, if you attach the sheet at the same point that the clew is attached, then you end up with a boom that is only in compression which is much better than one in compression and bending. So yes, mid-boom sheeting likely contributed to this but the load distribution that the sail placed on the boom must be known to say for sure.

The comments about the welding and a crack developing over time are spot on as well. It looks to me like there was a stress concentration in an area of high bending stress which was made even worse by some weakened aluminum.
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Old 07-04-2012, 07:26   #18
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Re: Snapped Boom

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Originally Posted by delmarrey View Post
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.
Yes its welded. Its a Sparcraft, apparently still quite a large rigging manufacturer.

A long shot - but is there any liability in such a case to Sparcraft?

And Im expecting not, but several have suggested that insurance covers this kind of incident?
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Old 07-04-2012, 07:30   #19
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Re: Snapped Boom

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Originally Posted by LeaseOnLife View Post
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
Thanks. This pic helps a lot to show me what to look for. Im going to go back to the boat today to look closer and get close up pics.
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Old 07-04-2012, 07:31   #20
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Re: Snapped Boom

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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.
We were thinking something similar. Thanks for the info. It snapped exactly at the weld.
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Old 07-04-2012, 07:33   #21
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Re: Snapped Boom

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Originally Posted by Target9000 View Post
That sucks man. =( Hope they didn't weld too much other stuff on your boat.
Mast and boom were from Sparcraft, separate from the boat. Boat is fiberglass and from Fortuna.

But now Im worried about the mast..... Mast was recently inspected by FKG though because of a lightening strike. Mast does have welds on it though.....
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Old 07-04-2012, 07:40   #22
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Re: Snapped Boom

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Hi,



Thats kind of what I expected a boom to look like that snapped. The extreme 90 degree break of ours still has us shaking our heads a bit. I know stress fractures and catastrophic failures can do weird things... but....
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Old 07-04-2012, 07:45   #23
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Re: Snapped Boom

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Originally Posted by Dockhead View Post
The root cause of this is mid-boom sheeting. This arrangement creates tremendous stress on the boom since the main force of the sail is imparted to the clew. The sheet attachment then becomes a fulcrum. So the whole driving force of the sail is taken in the boom as bending moment. It's a very common failure on mid-boom sheeted boats.
Ive done some reading about this before I posted and I see others have said this too. Ours is a catamaran though and I dont see any other way to do it with the traveller as otherwise it would have to go on top of the bimini. This of course would not work nor would it take the stress.

How common is such a failure? Searching turned up some results. It seems that boom failures while not unheard of, are not very common either.

I didnt even consider that repair was an option, but it seems that this can be repaired by inserting an insert and epoxy of all things. In fact just about a week ago I was inquiring about a boom furler... what a coincidence. Unfortunately the cost of a boom furler is pretty expensive, so repair might be my only option at this point.

That being said - anything that can be done for the future when ever I do replace the boom to minimize this possibility again since I have to have a mid boom mount? Or is it pretty rare and I just had some bad luck with a bad weld that degraded over time?
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Old 07-04-2012, 07:52   #24
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Re: Snapped Boom

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Originally Posted by PyotrBee View Post
I just had some bad luck with a bad weld that degraded over time?
Probably, just wondering how much a new boom is without the fittings and then have a rigger change it over. Be curious to see the price.

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Old 07-04-2012, 08:05   #25
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Re: Snapped Boom

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Probably, just wondering how much a new boom is without the fittings and then have a rigger change it over. Be curious to see the price.

Pete
Thats on my list to check too... but I just refitted this boat after a double lightening strike and insurance from hell... just put about $25,000 of my own money into it....

At least for the next few months I'll be sailing foresail only.... Fortunately this boat sails pretty darn well that way.

Now this
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Old 07-04-2012, 08:11   #26
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Re: Snapped Boom

If our insurance will buy you another - go that route - otherwise find a good tig welder, add an insert and weld it back together - you will likely never have another such problem.The break was caused by a stress crack at the end of the weld by wrapping the weld at the end of the pad eye - they should have just run the weld by the end and left the end open. Common mistake in welding.
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Old 07-04-2012, 08:21   #27
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Re: Snapped Boom

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If our insurance will buy you another - go that route - otherwise find a good tig welder, add an insert and weld it back together - you will likely never have another such problem.The break was caused by a stress crack at the end of the weld by wrapping the weld at the end of the pad eye - they should have just run the weld by the end and left the end open. Common mistake in welding.
Just called the insurance. Not sure if they will cover or not but they want to come take pics today.
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Old 07-04-2012, 09:00   #28
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Re: Snapped Boom

I would be calling Sparcraft and putting a bug in their ear! Send them pictures! They have been around for years and should know better then to weld a stress fitting right in the middle of a boom. Maybe they're trying to make it perdy.

If it were mine I would have riveted bails on the out side and hung the blocks from those.>>> Boom Bails
Or reinforced from the inside and then added with rivets, like my old Sparcraft that lasted 25 years, and probaly still going on someone else's boat (gave it away). The only welds were right on the ends with only longitudinal welds.

Most booms have a slot in the bottom for sliding in swinging bails that can be positioned in place with large rivets. Maybe they are cheap-ing out now.
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Old 07-04-2012, 10:26   #29
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Re: Snapped Boom

I like the way Selden attaches the mainsheet bails to their booms: the bail padeye slides in along a channel extruded under the boom. No welds or rivets to break and no threads to strip. Very good engineering.
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Old 07-04-2012, 16:46   #30
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Was the vang loose or tight and holding the boom down? If the vang was holding the boom down then the leech of the mail will pot a lot of load on the boom. I have seen external tracks peeled right off of the boom this way. Throw in some stress cracks from welding and there you have it.
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