Screw or rivet?

[Graham_Wright]

Having enjoyed(?) several hours swinging around the mast sitting in an excrutiatingly painful bosun's chair (doesn't anyone make a comfortable one?), I have given up trying to shake out my teeth with a lazy tongs rivetter and reverted to drilling and tapping. With the hope that Duralac will act as a thread locker as well as a corrosion inhibitor, is it reasonable to assume that such an approach will be just as secure as rivetting?

Any views please?

 

[sailorman]

What are you fixing /forums/images/graemlins/confused.gif

 

[Skyva_2]

Duralac is not an adhesive; a rivet is to be preferred for most mast fittings. How critical is the attachment?

 

[vyv_cox]

Most masts in my experience have a fairly small metal thickness. Tapping a hole in such thin metal, in aluminium, is asking too much for anything other than a very small bolt, maybe 4 mm maximum. I agree though, lazy tongs riveters are very difficult to use up a mast. I leave mine at home for boat jobs but keep one of the sort with two long arms on board.

 

[Graham_Wright]

[ QUOTE ]
What are you fixing /forums/images/graemlins/confused.gif

[/ QUOTE ]

Everything! Radar scanner support (12 M5 A4), radar reflector (8 M6 A4), mast steps (4 M6 A4), nav lights (4 M6 A4).

There is a safety issue with the steps but I console myself with the thought that the screws are in shear (and bigger than the rivets) and they are only used while wearing a safety harness.

Agreed Duralac is not an adhesive but when you try to scrape it off your screwdrivers, it's not disimilar.

Side benefit of screws;- they can be removed (Duralac permitting!).

Disadvantage of rivets;- the steel core doesn't always drop off and can rust.

 

[rogerthebodger]

Having rebuilt my mast last year for my new boat I riveted all fittings I did not wont to be able to remove sealing sikaflex. All items I would wish to remove at a later date I riveted a stainless and tapped into that using stainless screws. Some existing items stainless threaded screws into the aluminium mast could not be removed due to corrosion .

IMHO screws into tapped holes are size for size stronger that rivets but could come loose except to the corrosion that takes place if Duralac is not used. If strength is a factor the weekest point is the tread in the aluminium mast, in these cased i.e. winch fittings I riveted a stainless plate on to the mast using monel rivets and screwing the winches on to the stainless plate into tapped holes.

If your concern is the stainless screws in tapped holes in the aluminium mast coming loose why not leave out the Duralac and the corrosion will lock the screws into the mast. If strength is the factor fit a stainless plate or larger stainless thread inserts into the mast and fix the screws into the plate of thread inserts.

 

[npf1]

or you could rent a cordless electric riveter.

 

[chuzzlewit]

Embrace the mast from the far side and pull the lazy tongs towards you.

 

[VicS]

[ QUOTE ]
Embrace the mast from the far side and pull the lazy tongs towards you

[/ QUOTE ] Then when the rivet pops the tool skids off the top, as they do, and whacks you in the face. Nice one /forums/images/graemlins/mad.gif

 

[vyv_cox]

Not just whacks you in the face, but the sharp end of the snapped off rivet pokes you in the eye! I scored a lot of paint off my garage door like this just a couple of days ago.

 

[Tranona]

With that amount of serious work to do, surely the only sensible thing is to take the mast out and do it all in comfort AND safety!

 

[Bilgediver]

is it reasonable to assume that such an approach will be just as secure as rivetting?
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I would have a rethink of anything you have done using screws.

If attaching to mast material only then there insufficient thickness to fully utilise the power of the threaded connection. Not enough threads in engagement. Here a rivet is definitely safer.

Where attaching through doublers then it is possible that the screw connection is OK.

 

[smonard]

Have you thought of rivnuts?. You can buy a cheap setting tool on ebay for about 20 quid but the expensive ones are probably better. You can get low profile rivnuts but you still need to have an indentation on the back of the thing you are securing to take the head. I wouldn't trust screws.

 

[Graham_Wright]

Yes;- did think of them. How much projection is there for, say, M6 and does it really matter?
When I enquired, my supplier couldn't source ali ones.

The other consideration is that, by definition, the holes in the mast need to be bigger thus weakening more than a screw or a rivet. With 12 required for a radar scanner, that could be a concern.

 

[charles_reed]

there's really no comparison, especially on a thin-wall section like a mast. I trust the self-tappers aren't holding something critical. Even alloy rivets are better than self-tappers!!

 

[Graham_Wright]

What self-tappers? Read the post!

 

[2Tizwoz]

Monel rivets are recommended.

Placing the larger sizes is difficult with lazy tongs.

These make the job much simpler.

Available on ebay

 

[RAI]

I've used tapped holes and bolts plus Duralac instead of rivets. The oldest installation is the vang mount and it's held well after 7 years. Maybe my old Selden mast is a bit thicker than the more modern bendy ones. But a thin wall is going present a pull-out problem for both pop rivet and screw thread when the force is perpendicular. I would worry about setting the rivet properly when working up the mast.

I theorize that, in shear, the rivet's weak point is its own wall thickness, whereas the bolt is much stronger. If the Duralac does not prevent corrosion, both methods are compromised.

 

[2Tizwoz]

"Points to remember when attaching fittings
"To minimise corrosion, always insulate between spars and fittings with vaseline, zinc chromate primer, Evomastic or similar material. (This also applies to nuts and bolts). At Selden Masts we always use acid-proof stainless steel nuts and bolts, and cadmium-plated monel metal or stainless steel rivets. So far as we know, no aluminium alloys suitable for this purpose are available. Self-tapping screws should be used as little as possible as they promote corrosion, and there is also a risk of halyards inside the mast catching on the points of the screws.

Special care must be taken to provide insulation beneath larger stainless steel fittings. In general, brass and other copper alloys should not be used in conjunction with light metal components."

 

[Graham_Wright]

I did try to find a rivetter with a more progressive action than the lazy tongs variety without success. The lazy tongs came with collets that were barely effective with monel and the bang and subsequent violent shaking of the mast started all the local dogs barking. I imagined a screwed rivetter could be available (like a nut puller) but failed to find.

At the first refit, if any of the screws have loosened, I will replace them with monel rivets but with the mast down. My mast thickness is around 3mm which is not much different from the equivalent nut. However, I accept that the aluminium thread is weaker than SS.

I shall ask one of my staff to mock up a lever simulating the attachment and see what the failure forces are like. Lot to be said for facts!

At the end of the day, no one should ascend a mast without a safety harness and should the mast steps fail, the safety harness should substitute. The thought of the radar scanner breaking free is something else.