Plastic or metal?

I was surprised to read in the ‘How it Works; Hull Fitting’ article in your November issue that Callum suggests that ‘….metal skin fittings MUST be used for an inlet…..’

I don’t think this is correct. Plastic skin fittings can be used for both inlets and outlets, and they operate quite satisfactorily below the waterline. Incidentally I had correspondence with you and Vyv Cox about mixing materials on skin fittings and valves last year. Tru Design confirmed to me that it is fine to mix either DZR or bronze fittings with their plastic components. Obviously do not mix DZR and bronze together on the same system.

I have installed several plastic valves and skin fittings on my boat with some parts in bronze where the Tru Design product range did not have the right part available. Incidentally, plastic right-angle bends are heavily restricted because of the mould tooling that has to be used, and metal bends have a much better flow capacity.

John Francis Callum Smedley writes: John makes an interesting point. I wrote my book, Diesels Afloat, from a commercial vessel perspective as much as from a cruising yacht point of view. On fishing vessels, plastic skin fittings can only be fitted above the waterline and never in any machinery spaces, which includes bilge pump and galley areas. If a yacht is not commercially coded, the owner can fit whatever they like, and I’m aware of plastic hull fittings being used below the waterline quite successfully. However, in my opinion, only metal should be used below the waterline, especially in any machinery spaces, due to the risk of fire and melting. Plastic fittings can also be more brittle, which means that if a vessel is grounded they can be damaged in a very different way to metal, which just tends to deform.

Refurbishing a heat exchange manifold on a marine diesel engine. The level of corrosion due to electrolytic action between the alloy manifold and bronze fittings was not noticed until the engine was removed from the yacht