sleeving bent boom, fasteners, rod v wire etc

[Brian Duff]

John,
We have been extremely busy with work , unusual for January. Expect my dissertation on rivets over the weekend.

Love to hear more about the fasteners in the Selden Spar., TomP....

[Brion Toss]

Hello,
I have long been a fan of machine screws, properly done, but have developed grudging admiration for outfits like Selden who have done good work with rivets. Yes, you can't remove rivets for inspection and replacement, or not without drilling them out, yes, they are weaker than machine screws in both tension and shear (important exception noted below), yes they are a way for manufacturers to make things faster, cheaper, and requiring less skill, and yes they are easy to do wrong, but they have their place.
For instance, our favorite furler, made by Schaefer, has its foils held together with rivets. They are quick to install, plenty strong for the limited shear loads that each rivet sees, and quite smooth. Selden's spars have fittings engineered for rivets; notice that there are more of them on a given fitting than you would need with screws. I wonder what size hole remained when that Selden owner drilled the rivets out. Odds are that they were too darn close to the O.D. of the screws, but it wasn't practical to go to 5/16". This could have been a good application for Rivnuts.
One big problem is that screws need depth to develop full strength. That's why nuts are as thick as they are. And spars of course tend to be thin-walled. With masts, you can get a good bite for screws, but booms and spreaders are too thin for any but light loads. And for some items, like vang fittings on booms, no fastener will be up to the job, which is why the best vang fittings for booms are two-part, with one of them being a backing plate that is slid into the boom. The other part is fastened up to it, through the boom. With screws.
So as has been pointed out above, one must choose the right tool for the job. Screws are still my default fastener, just because they are easily removed/replaced. But there are many places where they just don't belong.
Getting back to the original question, if there are rivets there now, either stay with them or consider rivnuts.
As for the rod, while it is true that it usually fails at the ends (did you rehead aloft, too?), it can also part in the clear. Original quality, especially corrosion-resistance, can vary widely, even from Navtec. This appears to have to do with tolerances on when to replace the extrusion dies.
Tune has a lot to do with rod longevity, too, as does relative load (racers will tend to undersize rod, raising the relative load).
It sounds like your rod has been well-cared-for, and in a Northern climate, yes? So given a good tune and good scantlings, and limited use, it might have a lot of miles in it yet.
Fair leads,
Brion Toss

[Brian Duff]

Brion took my fun away, as I agree with him completely on this subject.

Thanks Brion !

I will only add that the thickness of a nut has a lot to do with trying to keep the outer diameter of the nut from getting to large. With more material around the fastener you can have a thinner surface and achieve the same holding power. What happens is that as a screw is loaded in tension, the slope of the threads trys to force the nut (or mast wall, or whatever) outward from the screw, so a nut (or mast wall) needs to be large diameter or thick, to get its holding power.

[John DeLong]

Thanks all for your advice and suggestions.

I sent a description and pictures to NAVTEC and talked through the history of the boat. Their advice was to rehead all the rod within the next year and replace the ss screws in the turnbuckles. We've disassembled and polished and inspected all the tangs, thru bolts, mast head fitting etc. We found some corrosion but it's pretty amazing how little for a boat that is almost 20 yrs old. We have switched all the fittings from rivets to fine thread screws. The large rivets (6mm) had to be tapped to a 5/16" ms. We're painting the aluminum under the fittings with zinc chromate paint and bedding with marine silicone. The ms are sealed with teflon sealant.

The boom is being reinforced with two 36" internal sleeves secured with epoxy and 1/4" ms - probably overkill but it provides sufficient reinforcement to install a vang.

Now we're interested to find open body turnbuckle screws (7/16" 20tpi, 5/8" 18tpi and 1/2" 24tpi) at a reasonable price - any suggestions?

Thanks again for your assistance.

Best regards, John DeLong