Screws or rivets hardware to boom?

[amoret]

Hi there, I have to install hardware to boom for #2 reefing line, but can't decide between stainless machine screws or monel rivets. I'm worried about the boom not being thick enough to take a decent tap for the screws, and will those monel rivets really hold in a big gust? How thick does the boom tubing need to be for tapping? Should mention its a Ross 780 (25ft ) 17sqm mainsail area.
Many thanks, all suggestions much appreciated.

[billknny]

A properly sized monel rivet will do you just fine. They are quite strong in shear loading.

Monel or stainless, do use tef-gel or similar to prevent galvanic corrosion.

If I was going to put a screw in a tapped hole, I'd look to engage 2 threads at a minimum. Three would be better. For a 10-24 screw I'd look for a thickness of at least 0.09"

[amoret]

Thanks, most helpful. Do you have suggestion for cheek block size or model? Harken or Ronstan are available here in NZ. Thanks again

[thinwater]

I did a bunch of testing for Practical Sailor that should be out at some point, testing shear and pull out strength of machine screw and rivets in a range of thicknesses. Without being a spoiler, somewhere around 0.1 inches thick there is a cross over. Thinner spars are better off with rivets, and thicker with machine screws (always coarse threads in soft metals).

Yes, you can remove machine screws, but they can seize and rivets are easy to drill out. Both require some skill. More important is avoiding pull-out forces, since they tend to focus the strain on one fastener, and sharp bit pointing into the mast. Make everything in shear and use enough of them.

Bad design is responsible for most failures.

[john61ct]

Look at stainless rivnuts / nutserts.

Specifically PlusNuts.

[GordMay]

Originally Posted by amoret

... How thick does the boom tubing need to be for tapping? ...

Often, the minimum recommended length of tapped thread engagements are:
Hardened Steel – 1 times diameter of the screw
Soft Steels or Cast Iron – 1 and 1/2 times diameter of screw
Aluminum – 2 times the diameter of the screw.

However ➥ https://www.engineersedge.com/thread...engagement.htm

[mikedefieslife]

My main is 14sqm so a little smaller than yours, though it's on a 35' cat. My boom hardware is all riveted and is strong enough.

It is however a bit annoying as I have to drill them out now to install a new track for the furler. I'll probably replace with rivet nuts, which will then allow me to remove hardware in the future.

[P3sailor]

As was stated previously, Riv-nuts or Nut-serts may work for you, relatively easy to install, only problem may be the size of hole required to receive them properly. Not a big fan of standard pop-rivets but a good aviation grade (NAS type) rivet should work just fine.

[john61ct]

The smaller thread size PlusNuts require a smaller hole, but still have amazing load-bearing and pullout strength specs.
The original rivnuts were invented for securing heavy equipment to thin body panels on military aircraft.

[billknny]

Originally Posted by GordMay

Often, the minimum recommended length of tapped thread engagements are:
Hardened Steel – 1 times diameter of the screw
Soft Steels or Cast Iron – 1 and 1/2 times diameter of screw
Aluminum – 2 times the diameter of the screw.

However ➥ https://www.engineersedge.com/thread...engagement.htm

Gord,

That's a nice theory, but not useful in boat practice. Think about what you just said: A 1/4-20 screw into an aluminum mast (which is VERY standard practice) would require a mast wall 0.5" thick... Not likely on anything less than a 100 footer...

Almost all mast fittings are primarily loaded in shear, where the formula you just quoted does not apply.

[stormalong]

Whatever fastener you decide to use be sure to isolate the stainless from the aluminum with some thin stiff plastic sheet.

To prevent electrolysis use Duralac, Tefgel or some other inhibitor. I prefer Duralac for high vibration or cyclical loading because it hardens up where Tefgel is a grease and screws can loosen.

On the subject of fastener choice you have already received good advice. I currently have both screws and monel rivets in my spars and both work well.

[thinwater]

For a resent article on fasteners, I tested some rivets from a 40-years old Hobie Cat mast where none of this was done. They tested at 85% strength and there was moderate corrosion. Look at cats that have been on the beach for 40 years, and think you will find mostly good rivets. The hounds don't pull out, and these boats have been abused.

Later, I was working on the bowsprit of a 25-year old F-24. I was able to drill the rivets out in seconds and drop in new rivets. The machine screws were hopeless; I had to grind them flush and re-tap next to them. Just sayin'.

I've also fought with corroded fasteners that were quite young. I like to use Tefgel or similar. That's good for at least 10 years. But if I have to deal with an old, corroded fastener, I always hope it is a rivet.

[a64pilot]

I have said before, rivnuts are evil, the invention of the Devil.
The reason is once the screw seizes in the nut, and it will in a salt water environment, then the nut just spins when you try to remove the screw, and then you have a difficult to drill out rivet, and your left with an over sized hole in the mast.
Monel is far superior to SS in this instance, in my opinion

[amoret]

Thanks to all for informative input. What a great forum this is! Looks like it's monel rivets with duralac for this project. Tools and materials readily sourced.
Just another query: what is the range of thickness in hardware that can be riveted? Oh and is monel much harder to set than aluminum rivets? (May need bigger tool)
Thanks again

[a64pilot]

Rivets come with different grip lengths for different thicknesses of metal. So you ideally need to know material thickness to get the correct length rivet.
Max thickness? Depends on what of course your riveting and what tools you have. Of course they used to build bridges and ships from rivets

Monel is I believe a type of steel, and yes they are harder to pull, however I believe a good hand set will pull them, I used to pull Cherry Max all the time with my Craftsman set, but of course not very many at all, more than one of two and you would want the pneumatic gun.
https://en.m.wikipedia.org/wiki/Monel