wrote

If I push my engine hard for any length of time, as on the upper
reaches of the Llangollen in the tunnels, it develops an air lock in


the top of the skin tank and overheats.

??



Skin tank=cooling tank/heat exchanger built/welded onto "skin" (boat metal
plate in contact with water). Closed circuit: heat from engine is lost by
transfer to water surrounding the hull.
Takes the place of conventional air cooled radiator, or using water from
canal/river/sea for cooling.

hth