DaySailer.org Forum

Hello,

I just discovered that the aluminum mast on my “new to me” boat this summer is shearing at the bottom tabernacle plate connection/bolt. Couple questions:

1: could the cause be the new standing rigging installed/too much tension in the rigging? I didn’t think I was overdoing it, but I suppose it’s possible. Maybe it’s a problem that came to a head with the proper tension (or a bit too much). Just looking for the root here.

2: would an appropriate repair be to simply remove the plate, cut the mast 1.5” higher than the shear and reattach the plate? Any tips for a square cut? I’m thinking slow and steady with a hack saw. Any other tips on re attaching the plate are also welcome…

https://share.icloud.com/photos/0ae7quqc5OJ7O3O3ANxByp6Jg

The local rigging shop recommends simply adding another bolt nearby, so as not to have to adjust stay lengths, etc. Not sure that I am totally on board with that solution (it was just over the phone, so he didn't actually see the damage), but figured I'd throw that out as follow up information. I may do that at least in the interim and see if things hold together...

At the mast step, you can also sleeve the mast without affecting its bend. Your damage looks like that the mast end has started to split or widen so it doesn't sit securely on the tabernacle fitting. My thought would be you could sleeve it with a strip of aluminum, either riveted or epoxied (or both) around the bottom. That would also give you clean material for bolts. The aluminum you are likely to find, unless you can get a bit of a mast section, is likely not able to withstand the elements as well as the mast, so I would give it a coat of epoxy and paint.

From the picture you shared, that would seem to me the most expeditious "simple" repair possible.

My reasoning:

The mast is loaded in compression. If the bottom opening weakens, it can widen and then the mast slips down over the tabernacle fitting. A "strap" will prevent that. The bolt is used when raising the mast, and to prevent the mast from jumping off the fitting, due to, say, wave action. Neither of those should rival the force with which the mast pushes down when the rig is tensioned. The bolt normally also should not be what takes those downward loads. A strap, together with the existing mast, should be strong enough to support the bolt for its intended purpose.

Rivets are quite strong enough for this purpose, and doubling up with epoxy should make this bomb proof. I wouldn't do more than 4 rivets, two on each side, to avoid the effect of creating a "perforation" in the mast by too many closely spaced holes. Another reason to double with epoxy.

Any thoughts?

PS: mast sections can be ordered from DR Marine, but you may find one locally, if you have rigging shops in your area. Just a foot, and then saw out a strip. It doesn't even have to be a DS mast. (This would be different if you tried to sleeve a repair further up the mast, that gets more complicated).

Your reply makes a lot of sense I think! I do believe that the compression load could transfer in part to the bolts or rivets even with the strap, but the addition of epoxy would help spread that load out as well. Will aluminum of approximately that thickness bend reasonably well?

After inspecting it a bit more it became apparent that when the mast was cut to install the tabernacle, it was left a little proud right below/at this failure- so it was taking much more of the compression load, it even mushroomed out a little. So I also filed that nub down a bit, relieving compression from that spot on the mast. It’s pretty near the internal layer of tubing behind the sail track, so I think should be OK until I have time to apply the strapping. I’m also wondering if a weld could solve the issue. Not a welder myself but I know a few. I also know that welding aluminum is not easy and there is some risk involved in that approach.

Any further thoughts welcome, and thanks for the reply!

My recommendation was based on what should be quick and easy for somebody who's not a professional and durable enough for the purpose. A professional may have other tools available, but they don't need advice from the forum. If you go that route, pay them for their expertise and done.

As you noticed, the loads are supposed to be taken by the mast profile resting on the "lip" of the tabernacle fitting. You even saw the result of where that load turned into a point load because of bad prepping. The purpose of a strap is just to counteract any tendency for the mast foot to open. Riveting a bit of aluminum is quick and easy, and all it has to do is "not stretch" to force the loads to be transferred correctly to the mast profile. The bolt should never hold the mast up, it's meant for the opposite: to prevent the mast from slipping off the tabernacle while being raised or due to slamming action in waves.

From your picture, a strip that's 1" wide would be enough to hold the opening together. If you can source 2" it'll also strengthen the bolt hole, but that doesn't seem necessary in your case. Don't rivet it into the fitting, just into the mast profile.

You could of course also make a collar from fiberglass / epoxy laminate. If done carefully enough and with enough layers, it should be equally effective. But probably not as quick as bending a bit of aluminum and riveting it in place. (Plus a bit of epoxy glue if you like belts and suspenders.)