Stainless steel and aluminum

[Alan Peck]

Sorry this is not a wooden boat question. But at least it is a boat question.

I am fixing up my old aluminum fishing boat. The plywood reinforcing at the top of the transom is rotten. The original is fastened in place with large aluminum rivits. I don't know where to obtain or how to work with aluminum rivits. It would be easier for me to use stainless steel bolts. Is this okay, or is there a problem in mixing aluminum and stainless steel from a corrosion standpoint? I don't think so, but I am not sure.

Thanks

[sdowney717]

All marine outboard sterndrives are cast aluminum housings and use SS bolts to hold it all together. And this is under the water. Of course there is a zinc to help.
There would only be a potential problem in salt water and anyway if you could seal it up with a varnish or something to keep out water why not do it.

[Roger Cumming]

Aluminum and stainless get along just fine, and not being underwater, there should not be any problem. Aluminum pop rivets would work, too, if you can borrow someone's pop rivet tool. It may not be worth buying one to drive a few pop rivets.

[Klaus]

Well, aluminium and stainless might get on well where you live but this is not the case here on the West Austrailan coast. The mast and booms of my boat are anodised and I thought this would insulate the stainless fittings but it is not so. The dreaded white powder corrosion started in just a couple of seasons. I pulled every ss fitting, made a teflon washer where that was practical and also smeared lots of 'Duralac' corrosion preventer onto the ss fasteners before tightening them.
This and frequent inspection should do the trick, doing nothing would set up a time destructer, freezing up any fastener so it would break before it could be undone.
Klaus

[Tom Lathrop]

Aluminum and stainless are death in salt water unless there is some protection to keep them apart. Aluminum is sacrificial to the stainless and turns to a white powder around the stainless screw or bolt. That is why stainless fittings on aluminum masts are bedded in some insulating material like 3M101. Anodizing the aluminum is a big help but not a guarantee. The anodized oxide is normally electrically non-conductive.

Aluminum pop rivits are not that hard to find.

Klaus types faster than I do, but the story is the same.

[ 11-15-2004, 10:09 AM: Message edited by: Tom Lathrop ]

[kc8pql]

Aluminum Pop rivits are easy to find. Your local Big Box will have them. Anyone who says aluminum and stainless are compatable never tried to rebuild an old aluminum mast. They are far apart on the galvanic scale.

[maa. melee]

Since your boat has aluminium solid rivets, I suggest you replace them with the same things. They are sold at almost every hardware store or can be bought online for cheap. You can also buy the installation kit too or make your own...All you need is a block of steel small enough to put in your palm with a small indentation (usually made with a drill bit the same diameter of the head of the rivet and drilled only deep enough so the bit doesn't drill into the steel past the angled point. Then alls you do is put the rivet in the holes, push the block against the blunt end of the rivet and give the head of the rivets a few hits with a steel hammer. Instant security. Hope this helps.

[Bob Smalser]

Not my area of expertise, but I have been down this road before and here in naval shipyard country there is a whole lot of aluminum hull experience.

Here's what I've saved from professionals on fastening aluminum in high-strength marine applications:




A: Try SS Solid Stem pop type rivets.

Q: "I thought SS rivets weren't such a hot idea because of nobility?"

A: The aluminum rivets have very little strength in small sizes. With the proper application of a barrier compound ie: Alumaplast, it works out....the lesser of two evils.

Only 2 alloy groups of Aluminum should be near salt water, the 5000 series for plating and the 6000 series for structural. NAPA pop rivets for automotive applications are neither.

Pacific Offshore Rigging and other fabricators use SS rivets to secure fittings to aluminum spars. They have in business for a long time and take care of some pretty big boats successfully.

I buy my rivets from their supplier. And don't use one of those homeowner-grade install tools. Look for one of the big industrial units through Graingers.

Solid Stem means that the break away piece just leaves a bit of a nub, the rest of the SS stem staying inside the rivet.



[ 11-15-2004, 10:46 PM: Message edited by: Bob Smalser ]

[Tom Lathrop]

Bob,

You may be right but, from the original question it seems that the problem is not with the hull of the boat but with a plywood reinforcing member across the transom. This was probably intended as a stiffener and not as a critical strength part. Almost any method that secures the plywood in place would be sufficient in this case. 5200 or PL might be the best way to do it and just skip the corrosion issue.

Mast fittings and and an aluminum hull in the water are two very different environments but fastenings on both should be treated with an insulating material. I have had stainless screws in aluminum weld tight after only a single use in salt water before I learned not to do this.

[Bob Smalser]

As I said, Tom...this ain't my lane. I don't know, either.

I'd sure be asking Dave Fleming off-line, tho, as aluminum replaced wood in the middle of his shipwright career.

[Dan St Gean]

So bed the ss components in a compoiund of some sort...but what to do with the fasteners? :confused:

[kc8pql]

Use Tef-Gel. It's an anti-seize corrosion inhibiter. Works great. Jamestown has it. West sells something called Tuf-Gel. Not the same thing. It's much thinner. I wouldn't trust it.

[Alan Peck]

Thanks everyone for the information. I think I'll go with the stainless steel fasteners and use the tuf-gel.

I don't think I'll have any problems as the boat is mostly used in fresh water and only occasionally used in salt.

Also, it will be easy to inspect.

As soon as I fix this, its back to working on the wooden boat.