An Interesting Pump Arrangement

This is the combination raw water / engine coolant pump on the Albin AD21 diesel. Most engines have a raw water pump, either direct drive or belt drive and then a circulating pump in the engine that is belt driven but with a metal impeller. This engine combined both of those pumps into a single pump with two impellers on a shared shaft with a wear plate in between. Rubber impeller pumps are a nonstop source of trouble for boaters, adding a second to an engine doesn't seem like a good idea and this isn't an idea that became popular. Some boats used to have gear pumps for water, all metal without rubber impellers. They exist for other applications, for example:

Might have been an issue of lowering initial cost but impeller costs add up and on some boats the impeller change is also very time consuming.