Water pumps
Almost all boats, whether directly cooled or indirectly cooled by fresh water. In addition to cooling the engine the water cools the exhaust system, usually made from rubber and plastic materials that could not withstand direct exhaust gas temperatures.
The type of pump used is a positive displacement type, whose body is commonly made from a copper alloy such as brass or bronze, with a rubber impeller. The pump is driven directly by the engine in some designs, or indirectly via a belt in others.
The type of pump used is a positive displacement type, whose body is commonly made from a copper alloy such as brass or bronze, with a rubber impeller. The pump is driven directly by the engine in some designs, or indirectly via a belt in others.
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This water pump (Below) is directly driven water pump on a Beta engine. The location of the pump is excellent in this design, ideally placed for maintenance of the impeller. 
Below a failed impeller from a Perkins narrowboat engine,
The impeller was very worn, worn face plates can often cause the pump not to prime, so that it ran dry for a time, this was the cause of the failure. Reversing the plate, possible with some designs, restores flatness on the impeller side. Since this was done the pump has performed faultlessly. 
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Component parts of a typical water pump
2. Pump body 3. Impeller 4. Drive pin 5. Gasket (sometimes an O-ring is substituted, located in a groove in the body) 6. Face plate. Wear of the plate can lead to priming problems. Reversal of the plate is sometimes possible. 7. Screws. Can be replaced by knurled nuts in the Speedseal design. 8. Shaft 9. Seals 10. O-ring. Optional, sometimes an O-ring is used as a flinger, there may be a purpose made plastic flinger or in some kits no such part is included. 11. Screw 12. “New moon” plate or cam. This causes deformation of the impeller blades, varying the cavity size. 14. Washer. 
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