Overthinking?

[Scooter]

I need to add a cleat to the aft end of my Potter 19 aluminum boom. A 3" nylon looks appropriate but then I started looking at fasteners. My first thought was self tapping aluminum screws; it looks like that's what was used on the mast and that sounds correct. Then I asked google and found some other options. Stainless steel self tapping screws with tef-gel, drill and thread and use aluminum or stainless machine screws, pop rivets, rivnuts. There are lots of options and lots more opinions.

Anyone here like to chime in?

[Spot]

If you can reach the inside of the boom at the cleat location, SS machine screws with lock nuts and anti-seize goop would be the bee's knees. If you can source the right machine screws with an Allen/hex socket versus slot/blade or Phillips drive, you'll use less sailor talk trying to remove them if needed.

Rivets and riv-nuts only if you are tooled up and good at it.

Aluminum fasteners: nice for preventing some galling but more fickle to install and uninstall without breaking.

[Charles Brennan]

Scooter, In 2016, I had occasion to rework my mast as a result of a mishap.   
The Sea Grape tree at John U Lloyd State Park had grown in the 25 years I had been launching there and I forgot about the overhang. 
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So on the NEW mast I replaced it with, I used bolts . . . 
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 . . . pop-rivets  . . . . .
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and sheet metal screws on the aluminum mast for the various parts of the project.
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The first question is:
Pop-Rivets?   
For a 3/16" aluminum pop-rivet, the shear strength is around 310 lb/ft of force and the tensile strength is around 500 lb/ft.
If you're attaching something fairly light, that is never going to have any significant forces applied to it (i.e. NOT a cleat) it is probably OK to use them as fasteners.

The second question is:
Sheet metal screws vs. threaded machine screws? 
On my Dwyer mast, I quickly discovered the mast wall was thinner than the average machine nut was in thickness, which means my average tensile pull-out strength would be reduced by a considerable amount; meaning it would be easier for fastened items to pull out of the tapped holes.
So I discarded the idea of tapping the aluminum and using machine screws and instead, opted for sheet metal screws.
I'm guessing the boom on your Potter is probably even thinner, so I don't think tapping holes and using machine screws is a viable option.

The third question is:
Aluminum vs stainless steel?   
The two considerations there, are shear and tensile strength.
In aluminum, using a #10 sheet metal screw as an example (in 6061 alloy) shear is about 880 lb/ft and tensile is 1775 lb/ft.
In 316 stainless steel, a #10 sheet metal screw has a shear of 2700 lb/ft and a tensile is 2540 lb/ft.
Notice in the aluminum screw, the shear is half the tensile strength, compared to the stainless steel screw which are roughly equivalent.
This is due to the softer aluminum being easier to shear, compared to stainless.

My experience with aluminum screws has been that I usually have two diametrically opposed problems:
A) I don't drill a large enough pilot hole, and the screw shears off as I attempt to install it. 
B) I DO drill the correct size pilot hole, and then the screw strips out when I attempt to tighten it down. 

So I (personally) prefer to use stainless steel sheet metal screws, when I can't use nuts and bolts.

The fourth question is:
What if I ever want to change my mind, or replace a damaged or defective item?   
You can spend a half day with some PB-Blaster penetrating spray and a butane torch . . . .
 . . . . .  OR you can invest in some Tef-Gel.
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And just unscrew whatever you screwed in, the last time.
The stuff is hideously expensive, OTOH, the small tube I bought in 1985 is still half full.
You don't need much, just where the fasteners go through, so buy the smallest package you can find.

Hope this helps,
Charles Brennan

[Scooter]

Yes, helps a lot. Also ouch. I don't think I'll be able to get nuts in place so stainless screws seems to be the best option. I do have some tef-gel here somewhere but mines a paste I think. I'll be using it. Thanks!

[Wyb2]

You may have already completed this project, but if not, I would strongly recommend reviewing what it might take to get a nut in place inside the boom.  I'm not above taping a nut to the end of a wrench and taping the wrench to a the end of a stick to get nuts in place inside a boom or mast.

With any option that taps threads into the thin aluminum wall, just make sure it never loosens up.  Any play will cause the stronger threads of the hardware to quickly chew at the hole.  You may be able to snug it down, but the damaged threads will likely loosen up again, and after a couple rounds of this the hole will strip out completely.

[Brian N.]

My Precision P165 came without the boom vang attached (Dwyer brand mast and boom). A call to Precision indicated it was an easy job, and standard pop rivets were sufficient. A friend had done similar work on his mast and used aluminum pop rivets over 20 years ago, so I figured that was the way to go. So far the rivets have held since 2008, and appear to be very solid. Be careful to prevent the drill bit from "walking".

[Riley Smith]

I hate the thought of putting threads in aluminum. Aluminum is the butter of metals and it SUCKS in that application. I've seen STEEL threads strip from some applied force and then I expect aluminum to withstand that? JMHO. Don't pay attention to my wool gatherings.
 Maybe that's why my mast is wood? The only experience I have with such problems was the cast aluminum mast step fitting on the old Victoria. It was riveted to the base and loose. I asked about rivets here and there and didn't get much information on it before deciding to find the absolute best rivets I could find and using OEM installation. Done after drilling the old ones out. All I can say about the effectiveness of it was we went on one of the most thrilling sails I've ever had and the mast stayed on And I assume it stayed on in a pond where the Victoria ended it's life as an advertisement, mast up and out in the weather. Don't poo-poo rivets. They're a great attachment means although I WOULD check strength ratings and do SOME research before.

[Scooter]

So I ended up going with stainless screws and tef-gel. I figure if it doesn't hold I can ream out the holes and switch to bolts. Most of the other fittings are set up the same so I'm not too worried.

One more question though, is a bowline sufficient for the clew or should I use a shackle or something else? Here's how it turned out, any suggestions are of course welcomed.

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[Wayne Howard]

I believe a bowline will be sufficient. That's what I use on the jib of my 37 foot Tartan.