Ken Fawell asks:

This is a picture (above) taken inside my yacht.

Apparently, the coating on the stern tube has been worn away by the black vibrating water hose. Can your experts advise what (if any) action I should take to fix this?

Vyv Cox replies:

It appears your stern tube is copper or a copper alloy, based upon the green corrosion product that has been revealed by the wear.

This was originally covered with a GRP layer with flowcoat on the surface.

I think the best treatment is to abrade carefully, taking the green corrosion product back to bright metal and removing the shiny surface of the flow coat.

Then repair and make good using polyester or epoxy resin with glass mat. Epoxy has better adhesion to old GRP, but it is more difficult to wet out chopped glass mat with it.

Woven rovings are easier to wet out, but are less able to cover three dimensional curves.

Flowcoat is gel coat with added wax to prevent stickiness, but a layer of paint should be sufficient for a small repair like this.

Thrust plate and Aquadrive set-up in Vyv Cox’s own boat. Photo: Vyv Cox.

The photo above is of the thrust plate for my Aquadrive. The plate is glassed in using chopped strand glass and epoxy resin.

I didn’t bother with flow coat for this – instead, it is finished with an aerosol tin of cream paint to match the surrounding glassfibre.

It should be quite possible to make a repair like this to your stern tube.

I’d suggest you support the hose with a suitable bracket and clamp in future so it can’t vibrate against the stern tube.

Vyv Cox is an expert in metallurgy and mechanical engineering.

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