In the end I replaced the outlet one with a 1 1/2 inch TruDesign plastic ball-valve and I am very pleased with it. It was impossible to unscrew the original, which was nickel-plated brass (not DZR) and when I tried the brass outlet elbow just crumbled to pieces due to dezincification damage.

    So I had to grind the old skin-fitting off from the outside which was actually very easy. The original skin fitting is brass but it is quite thick and there was minimal dezincification. I replaced with a marine bronze one.

    I’ll replace the three nickel-plated brass ball-valves under the sink next year, hopefully without having to replace the original skin fittings.

    I also replaced the seawater inlet ball-valve on the saildrive for bronze one.