Which rivets?

[massnspace]

Reworking a boom on a small, trailerable sailboat. Have to now reassemble it.

Situation 1: Cast aluminum end caps to anodized, extruded aluminum section. Loads on these rivets minimal.

Situation 2: Stainless padeye to anodized, extruded aluminum section. Loads on these rivets high.

The very first article I Googled (Practical Sailor) recommended NOT using aluminum rivets when fastening aluminum to aluminum. Said that the rivet WILL be a different aluminum alloy than the mast, and having that galvanic action between different aluminums is much worse than with stainlesses…..

Any opinions on this? Tef-gel between the metals?

Also…..I only own a standard type rivet gun. Have used it with aluminum many times, but not sure it will be strong enough to pull out stainless rivets?

Thanks

David
Seattle

[Briis]

Monel rivets might be the solution(?) since large SS rivets are PITA to work with.

Non-galvanizing (isolating) paste is a must in any scenareo you put different metals in contact, IMHO.

[JBP]

When replacing the gooseneck on my boat, I found the existing holes were slightly too big for 1/4 rivets so I had to switch to rivnuts. The tool was not terribly expensive and of course, he who dies with the most tools, wins!
I did not use tefgel because I used aluminum rivnuts, though I should have i suppose.

[Captain Bill]

I have done well with stainless rivets coated with Tef-gel. I found that 1/8 rivets can be done with a standard one handed light duty rivet gun. 3/16s are tough but I'm old and don't have the grip strength anymore for the light duty gun on 3/16 stainless. I got me a two handed one at Harbor Freight and it handled the 3/16ths just fine. I was replacing my gooseneck and had 48 1/4in rivets to do. I did two of them with a bit of trouble with the two handed tool as it was difficult to do the 1/4s, but again I was 70 when I did it and you may have better luck. The jaws gripping the mandrel also tended to slip on the 1/4 rivets. It might have just been the quality of the Harbor Freight tool or a combination of both. I took another trip to Harbor Freight and bought one of their air powered riveting guns. I already owned a compressor. The next 46 rivets were done in about 15 minutes. The slowest part was wiping off the excess tef-gel.

[boatpoker]

Originally Posted by massnspace

The very first article I Googled (Practical Sailor) recommended NOT using aluminum rivets when fastening aluminum to aluminum. Said that the rivet WILL be a different aluminum alloy than the mast, and having that galvanic action between different aluminums is much worse than with stainlesses…..

Practical Sailor should have taken a look at the galvanic scale before making such a silly statement.

[thinwater]

Originally Posted by boatpoker

Practical Sailor should have taken a look at the galvanic scale before making such a silly statement.

Wasn't me (chemical engineer).. But the galvanic scale is not the reason, it is more a matter of practical experience. In this application aluminum rivets would be fine, as would stainless. But Stainless and monel are sronger and thus prefered in most applications, as others have said. Avoid brass completely.

First, a trailer sailor is not like a cruising boat. It won't be in the water full time and will be rinsed with rain. Corrosion issues are less. Also, trailer boat spars are thinner; tapped machine screws are stronger in thick masts on cruising boats, but in thin spars rivets are stronger (I have done the testing).

Second, generations of beach cats were built with 316 and monel (better) rivets and they lived on beaches. You really don't need to over think it. Drill the old ones out with a 3/16" drill and replace them.

Finally, there may be some high load applications that require through bolts. However, rivets are as strong as other attachments on a size basis, as long as the load is in sheer. This is why many fittings spread the load with many fasteners. Perhaps a better pad eye design is needed. Pictures! if it is a sheet or vang attachment, is should be a strap.



If a hole is oversized, can you move it just a few inches? Locations may not be that critical. Simpler and more reliable than Rivnuts etc.

Do NOT use self tapping screws. They leave a point on the inside that can slice and snag wires and lines. They have a way of breaking and jamming. Either tap a machine screw, use a rivet, or through bolt.

---

If you post pictures you will get better answers.

[psk125]

Twenty years from now, when the galvanic action has done its thing, will it make a difference? Based on situation 1/ makes no difference. Situation 2/ use SS.

[team karst]

Monel is around 1/3 copper. I would tend not to use it on Al spars.

[wyb2]

Originally Posted by massnspace

…
The very first article I Googled (Practical Sailor) recommended NOT using aluminum rivets when fastening aluminum to aluminum. Said that the rivet WILL be a different aluminum alloy than the mast, and having that galvanic action between different aluminums is much worse than with stainlesses…..

This seems a little hard to believe. Looked up the article and the statement is short and not based on test data.

“ In fact, we observe the opposite to be true. The mast and rivet are different aluminum alloys, and it appears that the aluminum rivets are more active than stainless or monel in a marine environment.”

Anything is possible, but I’m not going to buy this one without more supporting info.

I also noticed this statement:

“The downside to rivets is their limited size range (only 3/16-inch is commonly used)... “

News to me, I have several sizes kicking around in my garage. Maybe they meant there’s a maximum practical size? That’s definitely true.

Originally Posted by massnspace

…
Situation 2: Stainless padeye to anodized, extruded aluminum section. Loads on these rivets high…

I went with aluminum rivets for a similar situation. I figured I would prefer the stainless hardware to cause the rivet to corrode, which can be easily replaced, than the stainless hardware and rivet to cause the mast to corrode. I coated the mating surfaces in 4200 UV, which also squeezed out from under each rivet as they were installed. Cosmetically this made more of a mess than I was hoping. Time will tell if it was sufficient for isolation. A thin isolation plate and tef-gel may have been better.

I’d suggest you make sure to get aluminum rivets with an aluminum mandrel. Al rivets with steel mandrels are common, and it seems to me the ball left in the rivet will leave rust stains running down from each one.

[Cheechako]

Originally Posted by massnspace

Reworking a boom on a small, trailerable sailboat. Have to now reassemble it.

Situation 1: Cast aluminum end caps to anodized, extruded aluminum section. Loads on these rivets minimal.

Situation 2: Stainless padeye to anodized, extruded aluminum section. Loads on these rivets high.

The very first article I Googled (Practical Sailor) recommended NOT using aluminum rivets when fastening aluminum to aluminum. Said that the rivet WILL be a different aluminum alloy than the mast, and having that galvanic action between different aluminums is much worse than with stainlesses…..

Any opinions on this? Tef-gel between the metals?

Also…..I only own a standard type rivet gun. Have used it with aluminum many times, but not sure it will be strong enough to pull out stainless rivets?

Thanks

David
Seattle

There's not much worse than SS with aluminum, I doubt ANY aluminum rivet will be worse corrosion wise than SS.... from my experience anyway....

A hand gun does SS or Monel rivets fine.

[thinwater]

Just like an anchor thread.


I suggest going to the local dinghy and beach cat park and observing what has worked. It will be 95% stainless rivets. A common hand gun will work with 3/16" SS rivets but will take two hands.

[Cheechako]

One problem with aluminum "pop" style rivets is the 'pull pin' the part that breaks off, may leave part in the rivet and that part is steel. Corrosion.
Not sure if all SS ones have SS pull pins or steel...?

[thinwater]

This might be interesting reading for some.


https://www.mcmaster.com/products/rivets/rivets~/
Lots of descriptions and specs. For example, the SS rivets are generally about 3x as strong as the aluminum rivets. Sometimes it matters, sometimes it does not. Strength also depends on the spar strength.


For some dinghies, and especially beach cats, it is vital that mast rivets be of the sealing type. The open type will allow the mast to flood when capsized, making the boat very hard or impossible for the crew to right. Sealing of the cap is also crucial (these boats don't have internal halyards for this reason).

[Pauls]

The rivets being discussed here are the cheap, hardware store available class of rivet. They are very limited in strength. And they fail badly in a high cyclic loading because they lack a good retention of the rivet mandrel, ie the mandrel becomes loose and the rivet strength is significantly reduced.

For high loadings you want to upgrade to a higher class rivet. McMaster Carr has a bunch of the cheap, household type rivets. If you scroll down they have a selection of high strength rivets. Compare the rated shear and tensile strengths. The high strength rivets, in aluminum, are about double. Cherry is a common manufacturer of these. There are a number of different alloys available.