Cupro-Nickel Sheetmetal over Glass-Ply?

Does anyone know whether it is possible/practical to attach cupro-nickel sheet-metal to a fiberglass-plywood hull for anti-fouling purposes? If so, what is the minimum thickness recommended, and how do you fasten the sheet-metal to the hull? My question is academic as I don't have a specific need, but I just wondered if it would work.

 

Well, after seeing what happens to cupro-nickel coins that have been buried in sand at surf beaches ( yes, the metal detector scroungers at work ) I'd say it might corrode pretty badly, quite quickly.

 

we had copper foil on our 38' -8 knot FRP cruiser. 20 problem free and growth free years and counting. It was held with a fitted adhesive... sort of like Mac-Tac.

 

If you are referring to Monel metal, it has great antifouling properties and corrosion resistance.

 

Agreed, monel is highly corrosion resistant. In fact, I know of working vessel hulls with no paint or anti fouling on them of this material. It's costly as sin too.

Sheet metal skins over plywood have inherent flaws as a concept. The bond line will be the issue and monel isn't going to fair any better in this regard, even if it does have excellent corrosion resistance. Any fasteners, other then monel just aren't practical, so adhesive (epoxy) would likely be the only attachment choice. It ranks above copper on the galvanic scale, just below silver so it's pretty noble stuff.

 

But, strangely, cupro-nickel coins, which are only 25% nickel, though, can't stand up to salt. Verdigris galore.

 

In another thread on the topic of cupro-nickel jehardiman said

So maybe the problem with coins corroding is that they were buried in the sand?

 

JSL, do you know if your foil is straight copper, or if it is a copper/nickel alloy? I understand that straight copper works okay as an anti-fouling in colder waters, but that it doesn't do nearly so well as cupro-nickel in warmer waters.

Can you recommend a supplier for the foil you used? Do you know how thick the foil is? Have you had any problems with delamination?

 

As I understand it, cupro-nickel typically is 70%-90% copper, while Monel is 60% to 70% nickel. I gather that nickel is more expensive than copper. Do you know if Monel has better anti-fouling properties than cupro-nickel?

 

PAR, would you further explain the bond line issue? Do you know if what jehardiman said (above) is true? If cupro-nickel sheetmetal was mechanically fastened to the hull, and no attempt was made to adhere the metal to the hull, would the inner surface of the metal corrode because it wasn't actively flushed with seawater?

If the sheet-metal/foil is applied over plywood/fiberglass, wouldn't the metal be isolated electrically from the rest of the boat, and therefore electrolysis not be a problem?

 

A plywood hull is made of flat panels, so laying sheet metal over should work. Maybe vacuum bagging it?

 

Monel comes in several alloys, so you have to compare apples to apples. The most common alloy is 67% nickle, while the other versions rely on different percentages. This is because of the workability of the metal, which is difficult to say the least. It doesn't machine well (without help) and tends to work harden, so you'll see monels with aluminum and other metals in it and it's annealed as best as it can to work.

Monel is used in the industry currently and you probably don't realize it. The wire used to seaze anchor pins is usually monel, raw water strainers usually have a monel basket, of course underwater piping, keel bolts, shafts, etc.

Like anything permitted to live in dormant water, it'll develop growth.

Fasteners don't seem the best approach, as eventually, the expansion rates will egg out the fastener holes and moisture will get behind the sheet, trapped on the plywood surfacve. An adhesive approach would be best, though I'd constantly worry about the seams, so I'd consider Sil-Fos on the seams (to keep the heat down), so the metal shell becomes a water tight homogenous skin, that is epoxied to the plywood substrate. Unless you happen to have a pallet of this stuff laying around for free, I can't imagine using monel in this fashion, but yes, you'd have a pretty bullet proof shell afterward.

 

Copper-Nickel Hulls

There was some shrimpers constructed of copper-nickel long ago
http://www.copper.org/applications/marine/cuni/copper_topics.html#coppermar

And another discussion on this forum
http://www.boatdesign.net/forums/metal-boat-building/copper-nickel-37691.html

 

I knew I had made a few postings on this subject,...just found another
Copper-Nickel, Seawater Applications
http://www.boatdesign.net/forums/materials/copper-nickel-seawater-applications-11633.html

 

From what I remember (this was 20+ years ago) - it was "Permafoil' from the UK. There were 2 grades 80/20? CuNi for slower boats/colder water and 70/30 CuNi?? for faster boats/warmer water. Thickness was about 18mil???. It was applied on a sealed hull and the sheets were peel-stick-overlapped using a roller. Hard work and had to be done with care since after 24 hours the adhesive cured and the only way to remove was heat gun or grinding. I was a bit skeptical about the bonding strength but even before curing a test showed about 80 psi bond.
Not sure if 'Permafoil' is still about. Like any metallic material, you have to do your research on corrosion, bonding, etc.
Why did we use it? Antifoul paint was going through the 'tree-hugger' phase and quite ineffective.... at mid season our hull had too much 'culture'. Fed up, we regressed to the 'old' days of copper sheathing