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Old 04-08-2017, 17:08   #1
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Damage to mast at lower shrouds.

Attached pictures are of the attachment point for my lower shrouds. I've already spoken with a rigger, just getting as much information/opinions as I can. I'm told that a doubler plate can be made (by the rigger) and that installation is easy enough with glue that I can do it. Proper prep is necessary etc.

I race offshore, and will be making ocean crossings, so a repair that is fine for day sailing and weekend outings might not be good enough.

Thoughts? Is this really as simple of a repair as I have been led to believe, and will it hold up to offshore racing?
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Old 04-08-2017, 17:14   #2
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Re: Damage to mast at lower shrouds.

I would want the edges cleaned up so the hole would need to be made much bigger. I would also want mechanical fasteners in addition to glue.
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Old 04-08-2017, 23:33   #3
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Re: Damage to mast at lower shrouds.

The Rigger is correct a well designed doubler plate (to ensure the load is distributed evenly) over the mast will rectify your problems,

Modern Epoxies are great but like above input I would add some well placed Monel rivets installed with Duralac - The rivets spacing needs to be well laid out so you dont induce a sheer line around the mast,

Cheers Steve
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Old 04-08-2017, 23:58   #4
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Re: Damage to mast at lower shrouds.

The above posted ideas sound about right, though I'd want to know why the mast wall is tearing/tore. Since if the metal in the mast tube is below spec, then another approach needs to be taken. The below spec thing meaning that perhaps the tear was caused due to the metal being too weak or soft. It's worth checking.

BTW, if it helps, here's the blueprint for the doubler & spreader attachment on a Cal 40' http://www.cal40.com/files/TEMP001.pdf
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Old 05-08-2017, 11:06   #5
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Re: Damage to mast at lower shrouds.

The cause was improper installation when the PO replaced the rigging. Threads from the bolt were against the hole, and the nut (nylock) backed out about 1/8" and the threads sawed the hole.

So I know why it happened and can prevent that from occurring. Just want to be sure the repair is done correctly.
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Old 05-08-2017, 11:09   #6
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Re: Damage to mast at lower shrouds.

When my mast was manufactured it had a double (or maybe it's even triple thick) plate welded to the mast to provide extra strength and rigidity where the through-bolt comes out. This design also aligns the plate between the bolt and the wire terminal better so the wire terminal does not contact the mast below the plate and create another chafing spot.

I would think a plate could be added and attached with a powerful adhesive of some kind which would work as well as welding on a plate. There should be very little movement in that area anyway if everything aligns well.
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Old 05-08-2017, 11:20   #7
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Re: Damage to mast at lower shrouds.

It looks to me that the inner bushing was not properly engaged fully in the bore (either too short or improper torque on the nut). Then the thru bolt shoulder contacted the edge of the larger bore and caused a stress concentration and thus began the tearing of the mast. Just a guess. Easy fix with a doubler plate, larger internal bushing, epoxy adhesive, and your choice of fasteners.
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Old 05-08-2017, 11:43   #8
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Re: Damage to mast at lower shrouds.

I'm suspecting that there is a compression post inside the mast, sleeving the bolt?

If so, for that to work properly it needs to be evenly and cleanly seating at both ends, i.e. flush to the mast. With the hole all chewed up that way...it will never properly take the compression load against new outer plates. So I'd suspect that you'd have to cut nice new round holes, back to clean metal, in the mast. Followed by making up a new compression post (sleeve) to meet the new doubling plates.

"Basic" simple engineering, but something to be done carefully by someone who knows what they are doing.

Might be worth asking the spar maker (any of them if that one is out of business) about the details and what they'd recommend. They might say to clean up the holes and fill them, welding and rebuilding the original metal before adding plates over it. Or that a lot depends on just how robustly the mast on that boat was spec'd to begin with. Bendy mast, more concern. Tree trunk...less so.
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Old 05-08-2017, 16:41   #9
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Re: Damage to mast at lower shrouds.

If you want this repair to last then you should address corrosion as a major issue. The stainless steel through bolt and rigging fittings against the aluminum mast material will result in pitting and weakening where they meet, especially since your rigging is a static electricity path from your mast to earth (water).
My suggestion is to drill through the mast 1/4 inch oversize for the through rod. Have SS shoulder washers made about 1 1/2 inch OD with the shoulder a press fit into the through holes and the shoulder height at least the mast thickness. Of course the center hole would accept the through rod. To minimize corrosion coat the shoulder, mating surface of the washer and through bolt with an anti-corrosion agent. I like Duralac. Run a bead of silicone sealant around the washer / mast joint. You can only seal the joint to a certain extent be cause water will find its way inside the mast.
There are other solutions but all require that you minimize the mating of unprotected incompatible materials and eliminate the presence of an electrolyte such as salt water.
Good Luck.
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Old 05-08-2017, 16:59   #10
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Re: Damage to mast at lower shrouds.

Sorry, l dont buy into the explaination of threads working on Alu. (BS)


To me, it seems than the fittings are pulling aside, jerking and bending.

Mechanically speaking, it looks like such a poor design, I would barely accept on a 470

Slacks are quite evident. The use of a nut that way is over simplistic...

I'd use a tapered plate matching a larger U recess.

Adhesives? OMG
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Old 05-08-2017, 17:10   #11
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Re: Damage to mast at lower shrouds.

It appears the hole is elongated...? The inside tube/bushing should sit in the mast hole, not inside the mast. It provides more bearing area as well as keeps the bolt tightening from crushing the mast tube. I would want a doubler with mechanical fastenings to the mast wall... and plenty of them as they need to withstand the shear strength of the bolt (1/2"?) . A new tube that is long enough to stay in the doubler also. Or weld the doublers on.
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Old 05-08-2017, 17:14   #12
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Re: Damage to mast at lower shrouds.

Quote:
Originally Posted by TheThunderbird View Post
Sorry, l dont buy into the explaination of threads working on Alu. (BS)


To me, it seems than the fittings are pulling aside, jerking and bending.

Mechanically speaking, it looks like such a poor design, I would barely accept on a 470

Slacks are quite evident. The use of a nut that way is over simplistic...

I'd use a tapered plate matching a larger U recess.

Adhesives? OMG
It's hard to not agree with pictures. It's obvious the mast is eaten away. Many many masts are done this way with a simple bolt.
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