sleeving bent boom, fasteners, rod v wire etc

[John DeLong]

we have a five degree sharp bend in a 16' boom where two sheave boxes are located. I plan to cut and sleeve the boom. Can someone recommend an adhesive to use in this application. My first thought was 5200 but I read an old post where Brian vehemently discourages use of 5200 (why?).

We're also breaking all the fittings on the mast and boom and considering replacing all rivets with fine thread screws - any problems with this plan?

Finally, are there any opinions on whether to replace the 20 year old rod rig for wire before heading through the canal. The rig has been well cared for and carefully examined with substantial reheading and replacement of the connectors from the rod to the turnbuckle screws. Has anyone ever heard of rod (on a cruising boat) failing anywhere but at the fasteners? NAVTEC seems to think that with this kind of maintenance/inspection/service program that rod can be used for a long time.

Your thoughts and comments are much appreciated.

Regards, John

[Brian Duff]

John,
When glueing aluminum for structural repairs, use epoxy glue. There are specific glues available for each type of aluminum. Google ' Aluminum Epoxy'

Depending on the thickness of the mast and the diameter of the fasteners , fine thread fasteners can make sense. Rivets certainly do not, except for very thin masts used near-shore(dingys and small sport boats). ( Oh, rivets do make sense for high profit margin, low cost production spar building using semi skilled labor) << Read as heavily opinionated<<<

If the rig has been well tuned, the terminals replaced and the rod reheaded - all of this by an expirienced shop, and the installation has fair leads throughout - then the rigging is probably fine. If any of those conditions are in doubt - replace it. If the lead are not fair - redesign. The design of the rig has everything to do with weather you can switch to wire or must stay with rod- or go to fibers.

With more info we can help you decide.

[John DeLong]

Brian, thanks for your reply.

Our mast was constructed by MetalMast in Connecticut ans was OE on our Shannon 43 ketch. I don't know if it was designed for either rod or wire but the rod is original. A number of these rigs have been converted to wire but I don't know what modifications may have been required.

I'm interested in your comment on rivets v screws. The mast has a number of fittings that were riveted including the boom fitting on the main mast, the staysail sheave fitting and the spreader bases. I've heard that rivets are stronger in tension than screws but that screws are stronger in shear. The advantages of being able to regularly service the fitting (and it's sealant) is the main reason I'm interested to change over to screws (although the additional strength is a plus). But then some people argue that threads will corrode more quickly than rivets. Our experience with this spar suggests that rivets are not a problem as we pulled fittings off that are almost twenty years old with more more than 60k miles in mostly tropical waters with very little corrosion. But maybe we were just lucky!

Do you have a rigging consultancy or are you a dealer. We'd be happy to get a quote for rod and wire from competent riggers in the NE.

Best regards, John DeLong

[TomP]

In selecting the proper fastener one must look at all values that the fastener will be subject to: shear, tensile, torsion...there are cases made for either threaded fastener or rivet and even a newer approach: the rivnut which is essentially a rivet with a threaded insert to allow the use of machine screws. One must carefully calculate the loads that the fastener will be subjected to, allowing for a margin of safety and error. When those parameters have been discovered then it is time to determine the appropriate fastener.
When the fastener is selected, the installation of that fastener becomes paramount. The best rivet is useless if the installation was not performed correctly, which also holds true to an improperly tapped machine screw.

I have seen many cases of booms and masts where the owners hated the rivets, drilled them out, replaced them with machine screws, only to call me in to ask why their outhaul was pulled out of the boom, or the tang on the boom for the vang ripped the screws straight out. Or, the best yet, the owner drilled out his gooseneck on his Selden Spar...replaced them by tapping in 1/4-20 MS...first jybe and the gooseneck parted from the mast, the threads pulled smooth.

Use caution, and practice solid engineering principals.

[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