Copper nickel

I work at a power plant that has to deal with sea water for cooling. Although piping systems are quite a bit different than a boat hull, our exeriences are somewhat informative.

We use a lot of 90-10, 85-15 and 70-30 Copper Nickle. Physical strength is OK (not quite as strong as a good steel), corrosion resistance is real good, workability (including welding) is no problem, but we do have problems with erosion (not likely to be a problem in a hull). Copper containing materials at our location also tend to be naturally resistant to bio-fouling. I recent years, we have been selecting other materials when we get a chance. Stainless steels with extra molybdenum have better strength and good corrosion resistance (pitting is a big problem with regular stainless). Because the industry has failed to push hard enough to get reliable stainless products for seawater service standardized, the extra moly versions tend to be harder and more expensive to get.

 

Most of the copper alloys have a problem of erosion in pipes, the worst being the pure copper. In England it has been a long fashion of heat ex-changers in copper which failed because of erosion with a stubborn regularity (that was as reliable as the 60's English motorcycles and Triumph cars...) The solution was a 90-10 copper nickel with a careful design to keep the water speed in the pipes under certain limits.

The main problem using copper nickel for a hull is these alloys are a so cathodic (about 0.35 V) that you're condemned to use only bronzes for all the fittings, screws, etc. The best stainless steels will be at the the extreme limit ( about 0.50 V so a differential of 0.15 V), and all the "less" noble metals will be dissolved by the galvanic corrosion like sugar in hot coffee. The price is unfriendly for making a boat, as the boat will be heavy thus using a lot of metal.

Marcel Bardiaux conceived and built in 1966 INOX, a sail boat entirely in SS (even the masts!!! I think it was the first yacht in SS, a few others has been made in the 80's in France) and the sea trials were a nightmare. The slightness difference of SS alloy and pitting galvanic corrosion started, so Bardiaux had to remake all the rivets, fittings, screws, bolts washers etc... in the same alloy as the hull. When Bardiaux fixed methodically all that, the boat became very durable and has survived to his owner.

The SS alloy really usable without too much worries for a hull boat is the 316L Mo. Very expensive, not truly very strong compared to its weight, an horror to cut and machine but happily very weldable although with a lot of distortion. Only compatible with the same SS alloys and some bronzes. Finally it is totally uneconomical as it's heavy and using also a lot of metal.

Titanium (I saw a few tons worked for the submarines at the Arsenal de Brest) is very nice with a good ratio weight strength, but it's very hard to work and welding is delicate (euphemism...) with argon back purge and other technical niceties. The price is lets say high, plus the expensive work; that put the "cooked" carbon/nomex in the rank of cheap material.

Finally, after examining closely all the facts, for a metal yacht aluminum is a clear winner. Light with a good ratio strength/weight, easy to work, welding is well mastered and the price is "affordable": being light (2700 kg/m3) you get more material for making a hull per kilo than cupronickel (8900 kg/m3) or SS (around 8000 kg/m3).

Using an expensive heavy and not so strong material for a yacht hull doesn't seem to be a very good idea (it's not the same thing for some work boats having very different requisites...)

 

I've been thinking for a long time that CuNi would be a great material for a hollow fin keel structure on sailboats. No corrosion, minimum fouling, compatible with lead the lead ballast poured or placed inside it, and save to fasten to the canoe body of the hull with bronze or Monel keel bolts. The extra costs would be a mere fraction of a full CuNi hull, and there could even be some small savings in maintenance and fuel consumption.

 

Unfortunately it fouls very quickly, Unpainted it also corrodes, albeit slowly. It's hard to work with, and it requires specialist welding. But if you paint and antifoul then you may as well put an anode on each side and build it out of steel. IMO