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Old 20-12-2010, 08:27   #1
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Tapping for Mast and Boom Screws

I want to install some rope clutches on the mast and eye straps for reefing line attachments on the boom.

The loads on these fittings could be pretty substantial but I'm assuming that since they are sheer loads drilling and tapping for screw attachment would be strong enough even if the spar walls are relatively thin ?

I can't think of a good alternative to tapping and screwing but a sanity check seems in order so I thought I'd ask before creating a new problem.

(Yes, I'll be using anti-seize to prevent SS to AL problems)

Thanks,



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Old 20-12-2010, 08:47   #2
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You can use monel rivets if the loads aren't expected to be huge and are in shear. The mast manufacturers use them a lot in places I wouldn't. Personally, I prefer tapping and using coarse thread fasteners for the ease of removing/repairing things. Of course, use TefGel on the threads to prevent seizing.
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Old 20-12-2010, 09:04   #3
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A yard I worked at used rivets for everything... on my own boats I prefer s/s self tappers.... set the gear so's it works the loads 'in line' and you should be good to go
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Old 20-12-2010, 09:39   #4
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Originally Posted by SvenG View Post

The loads on these fittings could be pretty substantial but I'm assuming that since they are sheer loads drilling and tapping for screw attachment would be strong enough even if the spar walls are relatively thin ?


-Sven
My only concern would be how many threads would be on the thin spar walls. I've tapped my mast before mounting winches and even moved the gooseneck higher on the mast, but I found that the walls of the mast was pretty substantial and allowed a number of turns on the mounting machine screws.
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Old 20-12-2010, 09:45   #5
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are rivnuts used on boats ?

"Rivnut® – The Original blind installed internally threaded rivet, was invented (patent #2,149,199) in 1936 for the purpose of attaching a rubber aircraft wing de-icer extrusion to the leading edge of the wing. The Rivnut® riveted the aluminum wing skin to the inner aluminum spar and then accepted a mating screw to attach the rubber de-icer.
The Rivnut® fastener was given a military specification of MS27130 and an aerospace standard of NAS1329 and NAS1330. Use of the Rivnut® expanded within the military and aerospace markets, and soon became popular in general industry because of its many design and assembly advantages."



Rivnut Fasteners & Bollhoff Installation Tools, Puller EZ & Custom Rivnuts - Blind Fasteners & Inserts - Bollhoff
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Old 20-12-2010, 09:59   #6
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I typically us more threads per inch if the wall is thin as in some booms, ie, #10-32 rather than #10-24 and align the loads as suggested by boatman.
Steve.
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Old 20-12-2010, 10:01   #7
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Having just drilled and tapped over a hundred holes in my mast(for steps) recently I can offer my opinion.

Use regular thread screws as opposed to self tapping and go with a finer thread. I used 10/32 which is, I know, overkill for steps. The fine threads will spread the load over a much larger(longer?) area and will stay in the hole better even if loosened. Self tapping are tapered and when they get loose have a much higher tendancy to quickly work their way out. Be careful not to over tighten ANY screw you use in aluminium or you'll bugger the threads. Make sure you use plenty of lube when tapping and plenty of Tefgel or some other anti corrosive.

All of this is from info given to me by several people who build things for a living not just theoretical engineering types(not that those types don't have some good info to occaisionally)................m
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Old 20-12-2010, 10:16   #8
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I prefer not to use self tappers because the sharp points can play havoc with internal lines, at least the ends of rivets and machine screws are fairly smooth, of course you should always try to keep the length of any fasteners to the minimum.
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Old 20-12-2010, 10:59   #9
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I sell threading tools for a living. The self taping screws are never as good as using a quality machine screw in a tapped hole. If you have not tapped before, get a chunk of aluminum the same thickness as your spar and practice a little first. I like to use a tap with a longer lead chamfer on it as it can help get the tap going in straight as there is more point on it. It will also last longer. What happens is that each thread on the end is cutting less material per revolution. You do have to turn it more times but you should end up with a better thread. When tapping aluminum by hand on my mast, I have not needed to use any lubricants. Any kind of oil would help if you are in a pinch and it is too hard to turn. Just be careful when you first start to back out. If chips are loaded up on the tool it could break if you force it.

You also want to be sure to use the proper size drill for that thread. Anything bigger will add some slop in the connection. You want a firm fit. You can google for a tap/drill chart to be sure what size drill to use.

If you can't find what you need at your local hardware store, check with your local industrial distributors. Larger ones will usually carry some inventory. You can also buy through a Fastenal store. On line you could go to McMaster Carr, Fastenal or MSC.

Good luck.

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Old 20-12-2010, 21:43   #10
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On the rivet vs screw issue: rivets are fine for things that never need removing. So far, I've NEVER found anything on my boat that never needs removing! Undoing screws is one hell of a lot easier, faster and more repeatable than drilling out Monel rivets.

Cheers,

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Old 20-12-2010, 22:04   #11
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You will want aluminum backing plates for both items. The boom straps can use rivets. The rope clutches should be mounted with s/s bolts and nuts. It can be a bit of a trick installing the nuts. Patience and cleverness pay off. You can use match-drilled and tapped s/s backing plates too, if the nuts just cannot be installed. Threaded fittings tapped into aluminum are problematic.
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Old 20-12-2010, 22:07   #12
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Personally, I think your decision should be determined by the wall thickness (relative to the diameter of the fastening). If the wall thickness is significantly less than the diameter of your fastening, I'd go with monel rivets. If the wall thickness is equal to or greater than 1 diameter of your fastening, I'd go with threaded fastenings.

Applying the above logic logic, I used monel rivets to attach blocks to the boom, for reefing lines. If using threaded fasteners in the mast, I tend to use helicoil inserts if there is any significant load involved, shear or otherwise.
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Old 20-12-2010, 22:08   #13
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Quote:
Originally Posted by michaeldsusa View Post
are rivnuts used on boats ?

"Rivnut® – The Original blind installed internally threaded rivet, was invented (patent #2,149,199) in 1936 for the purpose of attaching a rubber aircraft wing de-icer extrusion to the leading edge of the wing. The Rivnut® riveted the aluminum wing skin to the inner aluminum spar and then accepted a mating screw to attach the rubber de-icer.
The Rivnut® fastener was given a military specification of MS27130 and an aerospace standard of NAS1329 and NAS1330. Use of the Rivnut® expanded within the military and aerospace markets, and soon became popular in general industry because of its many design and assembly advantages."



Rivnut Fasteners & Bollhoff Installation Tools, Puller EZ & Custom Rivnuts - Blind Fasteners & Inserts - Bollhoff

Good advise I will follow myself-----Thank you
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Old 20-12-2010, 22:16   #14
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Quote:
Originally Posted by clockwork orange View Post
I prefer not to use self tappers because the sharp points can play havoc with internal lines, at least the ends of rivets and machine screws are fairly smooth, of course you should always try to keep the length of any fasteners to the minimum.
Steve.
The trick is start the tap to 1/2, remove and then snip that end of....
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Old 21-12-2010, 10:49   #15
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Quote:
Originally Posted by michaeldsusa View Post
are rivnuts used on boats ?

"Rivnut® – The Original blind installed internally threaded rivet, was invented (patent #2,149,199) in 1936 for the purpose of attaching a rubber aircraft wing de-icer extrusion to the leading edge of the wing. The Rivnut® riveted the aluminum wing skin to the inner aluminum spar and then accepted a mating screw to attach the rubber de-icer.
The Rivnut® fastener was given a military specification of MS27130 and an aerospace standard of NAS1329 and NAS1330. Use of the Rivnut® expanded within the military and aerospace markets, and soon became popular in general industry because of its many design and assembly advantages."



Rivnut Fasteners & Bollhoff Installation Tools, Puller EZ & Custom Rivnuts - Blind Fasteners & Inserts - Bollhoff
So that's what they are called ! Thanks.

They won't be flush so fittings might not sit flush unless you countersink the bottom of the fitting itself but great idea.



-Sven
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