Mainsheet/Boom fitting

[Jason K]

The fitting on the end of my boom to which the mainsheet blocks attach to is more loose than I think it should be (and has been since I've had the boat). While pondering another boat quandary, I was looking for an easy distraction and found it in the fitting. I could just imagine it giving way while under sail and decided I'd have a closer look.

The end cap on the boom, through which the bolt that secures the main sheet is bolted, was held with four screws. After drilling them out to remove the fitting, I saw that the bolt, as it passes threw the end cap, has a gas coupling as a large spacer, then a nut separated from two other nuts with a lock washer.

This struck me as odd, but it has been there a long time and appears functional. So, two questions:

1) How much tolerance should there be on the fitting that attaches the boom and mainsheet?

2) For those with a similar setup, how is the bolt secured on the inside of the end cap?

Thanks.

[Tim]

It sounds like someone once "repaired" your fitting with whatever bolt they had on hand (that is, one that was way too long); hence the additional long spacers and what not. I expect the third nut and washer are part of the spacing arrangement, and also allow some "adjustment" of the tension while the two locked-together nuts prevent any of it from spinning loose.

The bolt should be just loose enough to allow the mainsheet (and topping lift) bails to turn freely. Ideally, the tension should be enough that the bails don't just flop around loosely, yet turn easily when even a slight load is applied. This will require a locknut on the inside, or two nuts locked against each other, to prevent the whole arrangement from falling apart, similar to what you have described.

Whatever works here works, but it's far from an ideal setup to begin with. I don't recall how my old one was secured, or how tightly, but I never liked the arrangement and don't miss it.

Britton experienced the failure of this part during the past summer, and I imagine will have something to say about that aspect of the question. Obviously, this is not something one would care to have happen.

[Eric]

Someone, somewhere warned me about replacing the aluminum bolt through Sophia's aluminum end-boom casting with stainless as the dissimilar metals [mumbles something about a catfight...], which will result in my head being knocked clean off my shoulders or some other equally demoralizing tableau.

Of course now I only perform my annual inspections of this area of the boat using binoculars from the safety of the dock...

-- Eric

[Tim]

Actually, the fittings on nearly all aluminum spars are attached with stainless steel fasteners, and many of the fittings themselves (tangs, spreader bases, etc) are also stainless. Maybe an aluminum bolt is a good idea in a particular situation, but aluminum fasteners are generally soft and weak.

While there is inevitably some galvanic corrosion between the fasteners and the spar, it's usually limited to bubble the paint, or freeze the fastener in place, not enough to ruin the spar. (Of course, there are exceptions..as with everyting.) Modest means of isolation, such as plastic gaskets or use of sealant and/or thread seal, takes care of this effectively. Obviously, even minor corrosion that is neglected can eventually turn into something serious.

The biggest concern with dissimilar metallic corrosion occurs underwater, or areas frequently bathed in salt water. Masts do get salt spray, particularly near the base, so one should have some concern and not ignore the possibility completely. But the worry is generally way overblown. Some of this sort of action is unavoidable if your boat is in salt water.

[bcooke]

As Tim mentioned, I had that bolt on the end fitting let go on me while reaching out in a decent breeze. It went off with a bang and I had to turn around and go back to the harbor to change my pants. I walked around and found a general mechanics shop where I could buy some replacement hardware so I reinstalled another bolt with the nut on the outside so I could keep an eye on it. I noticed the nut was backing off slowly when under tension (easier to see than explain why). My final fix was using a spare clevis pin I had onboard (for the standing rigging) with a washer and cotter pin. It worked great for another month and a half. My real fix will be a new boom. A little overkill perhaps but I have been on the forum too long and the "buy lots of shiny new stuff" philosophy is wearing off on me. It is all you guy's fault.

What the boat wants, the boat gets. Live it. Love it. :-)

So, two questions:

1) How much tolerance should there be on the fitting that attaches the boom and mainsheet?

2) For those with a similar setup, how is the bolt secured on the inside of the end cap?

My original bolt was always a little loose and a cause of concern but I had so many concerns with my rigging that it all blended in. Because of how the end fitting is made I think some slop with the bolt is inevitable. The aluminum cap just doesn't seem strong enough to resist the elogation tendency of the bolt.

I don't know what the original hardware was like inside the boom because when it let go it blew the parts out with about as much force and probably equally lethality as a bullet.

I think Tim is right about the aluminum bolts. I would be afraid they will fatigue and fail quicker under the stress than any corrosion problems you might have.

-Britton