Crack in aluminum mast : options? - SailNet Community

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  #1  
Old 11-20-2011
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Crack in aluminum mast : options?

Hello,

I have a 33' masthead sloop. As i was doing a routine inspection of the rig, i found a crack just above the second spreaders. A previous owner had added a sheave for a staysil ( i am assume) as there is a padeye on the fordeck and running backstays. The crack begins at both of the lowest port and starboard screw holes of the stainless steel sheave housing, and spreads aft on either side for about 2 inches. The cause of the crack i assume is from possible corrosion ( there is minimal) coupled with the loss of structural integrity with the housing of the sheave, and the fact that the mast pumps when sailing to weather putting stress on this weak point.

Please see pictures.
The mast is keel stepped and 41 feet off the deck. I have rod rigging.
a cross section of the mast is ~6" x 4"

My plan is to drop the rig, cut the mast at the crack, square it off and insert an aluminum sleeve and rivet back together, in doing so i would fill the housing of the sheave and lose only a 1/4 inch of mast height.

It seems that the fabrication of the sleeve will be a challenge.
I would like to keep as much of the mast as possible in order to keep the rod rigging.
The new sleeve would fill the housing for the sheave, i.e the housing would remain, but the sleeve would reinforce it internally.

Questions:

Should i use a smaller mast or have the sleeve fabricated?
How snug does it need to be?
Tips?

Any insight would be greatly appreciated!
Attached Thumbnails
Crack in aluminum mast : options?-mast_crack.jpg   Crack in aluminum mast : options?-mast_crack1.jpg   Crack in aluminum mast : options?-mast_crack2.jpg  
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Old 11-20-2011
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That is a very critical structural problem you have there. I would replace the mast. This is a pay now or pay more later situation. You are going to spend a lot of $ on the repair, which may not work for long, put that towards the new stick.
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Old 11-20-2011
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I've never had a broken mast sleeved but I've heard it can work if done strong enough.
You are correct to be concerned about that crack.
I'm sure you will get more free advice here.
Good luck.
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Old 11-20-2011
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in my experiance with mast repair it can be sleeved but it is better to use an external patch. first they weld up the crack and then weld on an external sleeve on the front 2/3 of the mast diameter about 1 foot on each side. the cover sleeve is shaped like an elongated diamond. making a internal sleeve is very expensive and the rivets tend to cause more cracks. the line is probably a spinnaker pole topping lift. the pad eye is for the pole down haul. the facts that it is 33' with with 41' mast, rod rigging and running backstays says racer/ cruser what type of boat?
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Last edited by overbored; 11-20-2011 at 09:37 PM.
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Old 11-20-2011
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Why did it crack? Because of the slot cut in it? Seems an unusual place to cut into a mast? At any rate the mast is not structurally up to the loads place on it, and would have to be strengthened to accommodate the load.
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Old 11-21-2011
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Quote:
Originally Posted by samgary View Post
Hello,

I have a 33' masthead sloop. .... A previous owner had added a sheave for a staysil ( i am assume) as there is a padeye on the fordeck and running backstays. The crack begins at both of the lowest port and starboard screw holes of the stainless steel sheave housing, and spreads aft on either side for about 2 inches. The cause of the crack i assume is from possible corrosion ( there is minimal) coupled with the loss of structural integrity with the housing of the sheave, and the fact that the mast pumps when sailing to weather putting stress on this weak point.
Firstly, if the mast is Pumping when you are sailing to weather, you should have been using the running backs to prevent the mast bowing forward at mid span. I suspect your rig does not include forward or aft lowers (although perhaps a baby-stay forward). The runners are intended to replace the aft lowers and should be used in all but light air, particularly so if you've got a good load on the main sheet. The mast should never be bowed so much that the forward face is allowed to go into tension which is the only way such a crack could have developed. Runners are a pain in the neck but if you replace the original wire with 1/4" spectra, they are less so (been there, done that!).

From the regular shape of the cutout, I suspect that the fitting was made by the builder as few riggers or back-yard mechanics make nicely finished slots or cutouts. As previously mentioned the fitting is most likely for a sheave-box for a pole lift. If the fasteners were stainless, the pressure of corrosion around the fastener coupled with tension on the front of the mast due to bowing (i.e. pumping) from want of runners could have started the crack and, had you not noticed it, would eventually have allowed the spar to break over aft at some point.

Based upon your pictures, the crack isn't very serious or difficult to repair. One would begin by drilling a small hole at the aft ends of the cracks on either side of the mast and then machining a grove along the length of the cracks that would subsequently be filled with a full penetration weld and the weldment then ground flat. As previously suggested, a cover plate, itself slotted to accept the sheave-box, could then be laid over the front of the mast-covering the cracks on either side and welded in place with fillet welds all around. One could shape the cover plate as a "fish" (or the diamond shape previously mentioned) but that would add complication and really wouldn't be necessary. The fasteners for the sheave-box should be isolated from the spar with Tef-Gel or the like.

The foregoing repair is not that difficult and could be done relatively quickly by a competent spar-maker/welder for a few hundred dollars (excluding pulling and re-stepping the spar of course).

In future, use the runners. They were not included in your rig for no reason.

FWIW...
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Old 11-21-2011
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What HyLyte said. Drill a small (i.e. 1/8") hole at the end of each crack to stop it from propagating. Then find a good aluminum welder, sometimes listed in the yellow pages as "portable welding" meaning they can come to you instead of you taking the mast into a shop. A good welder can fill and grind the cracks so they are 100% again. But since 100% apparently isn't enough, then you have them add a shaped collar (plate) around that portion of the mast to reinforce it.

The hard part is finding a good craftsman who will come out and do it. Cutting and sleeving won't gain you anything over a proper weld and collar and if you can't find a really good local welder...the other option really is new/used replacement mast.
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Old 11-22-2011
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Hello and thank you to everyone for the insight.

I apologize for not making it clear, I always use the running backs because of how much the mast pumps.
The sheave-box is about 12 inches above the running back tangs.

As svHyLyte noted, i do not have forward or aft lowers, and i do have a baby stay. The line in the picture is only a place holder for the wire to rope halyard which was being replaced.



My one concern with welding the mast would be the loss strength in the process. By no means am i an expert but I have read about issues with the aluminum warping and weakening as a result of welding.

Please correct me if i am wrong for believing this.

What about drilling holes at the aft most points of the cracks to stop the propagation of the cracks then riveting a plate on as opposed to welding?

Thanks again for all the feed back!
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Old 11-22-2011
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"Please correct me if i am wrong for believing this."
AFAIK you're correct to worry. Which is why you would need a competent welder, not just an alleged welder, some bozo with a welding rig. And the reason you should consider having a collar (plate) lapped up over it, for a belt-and-suspenders approach.
Rivets can be applied properly or improperly--just like welds. Supposedly it was rivet failures that unbuttoned on the Titanic and let the ocean in.
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Old 11-22-2011
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I am a certified welder here in WA and I agree with svHyLyte completely. Have a pro TIG weld it as he described. Heat control is huge with any kind of welding, especially aluminum. Drilling the holes and full-pen welding the cracks is key. The plate should be added as well IMO. It sounds like this area of the stick gets worked pretty hard.
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