16/09/2015 at 1:59 pm #20352
Looking at the mast fittings after returning from the excellent International Rally on Lough Derg, well done Monica and the UKWA committee, I discovered a sheared rivet and another about to go on my mast spreader bracket. Boat was built in 1979/1980. Its an old-style bracket in good condition with 2 rivets either side of the mast, no corrosion underneath the bracket, and I intend to replace all 4 rivets.
Rang my “local” (50 miles) chandler and they tell me they have stainless steel rivets in stock. Is there a difference between “stainless” rivets and “Monel” ones, or are they one and the same thing?
Which should I use? Any one, any idea what the correct diameter and grip length is?
Thanks in advance for any replies.
Dave.16/09/2015 at 7:25 pm #20353
I replaced my old spreaders earlier this year and as far as I can remember I used m5dia x 8 mm length and they were monel.
I have always considered monel superior to stainless as I believe the corrosion problem is slightly less using the former. Usually I put a little sika on the rivets. Hope that helps.
Regards, Pete24/09/2015 at 9:26 pm #20385
Why not use aluminium pop rivets?
The spreaders only push against the mast and do not load the rivets at all. All the rivets need to do is keep the spreaders in place. Only if you are a very heavy helm and/or crew you may want the spreaders to pull rather than push top minimize mast bend. In that case the spreaders mainly pull the bracket that fits around the mast. The bracket transfers most of the forces to the front of the mast. Again the rivets are only lightly loaded.
Having said that, yes Monel has better galvanic properties than stainless steel with an Aluminium mast, but Aluminium rivets have no galvanic issues at all. Sika does not prevent galvanic corrosion at all unless the parts are fully electrically isolated from one another. Sika alone cannot do that. There is however a solution based on Galvanic physics. You may want to check out a British product called Duralac.
On the Internet the typical characteristics of Monel and Aluminium rivets are available.
Alu 4.8: Shear strength: 2620N Tensile strength: 3500N
Alu 6.4: Shear strength: 3780N Tensile strength: 4000N
Monel 4.8: Shear strength: 3300N Tensile strength: 3750N
SS 4.8: Shear strength: 4400N Tensile strength: 6100N
As you can see a one size larger Aluminium rivet is already stronger than a Monel one, and it comes close to an SS one. Any of the above nails well exceeds the forces that are ever put on the spreader mast connection on a Wayfarer.
Then there is another matter: The pin. Note that the head of the pin remains in the rivet unless you are using a very special type of rivet. With Aluminium it is not terribly important of the nail’s head starts to rust. However with SS a rusting head will corrode the entire nail. Pop rivets are usually available with SS nails and steel nails. If you decide for SS rivets, get the ones with SS nails in them.
With Monel and Alominium rivets bleeding (rust) can be easily prevented by a little dap of Sika in the rivet hole, over the nails head.06/11/2015 at 9:01 am #20488
Apologies Swiebertje, for the tardy reply. In the end, I used 4.8mm Monel rivets, as the hole diameter was not enlarged. So why enlarge a rivet hole unnecessarily? Time enough to do that when/if it becomes too oversize for a standard size rivet.
Interesting info regards the shear and tensile strength of the various rivet materials and sizes.
You must be logged in to reply to this topic.