b If the filter is clear and water still fails to circulate, check the sea water pump impeller. Find the pump by following the pipe up from the filter. It will probably be mounted on the engine and will look like the one illustrated below. On most units, the impeller is accessed by removing the end plate. It can be withdrawn by levering it out with two screwdriver blades, one either side. Replace it if there is any sign of damage. If the ship’s spare impeller comes as part of a kit from the manufacturer, this will have a spare gasket for the end plate. If not, try to save the original or, failing that, use instant gasket from a tube - all vessels should carry this.


c If the filter is clear and the pump in good condition, check for an air lock. These can occur without warning, especially on sailing craft after an energetic spell under canvas. Find a joint in the sea water system near its highest point and break the union with a spanner (wrench) or unfasten the hose clamp. Start the engine. If an air lock is present, the pipe will sputter for a while, then sea water will begin flowing. Remake the joint while the engine is running and that will temporarily solve the issue. This is a last-ditch and messy solution. Try to divert any escaping water from the engine’s electrics, and be extremely careful not to get clothing caught in belts or other moving parts.


A typical sea water strainer Impeller


A typical seawater pump


32 | MANUAL OF SEAMANSHIP


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