Re: Bilge pump non return valve.
It's not quite as simple as no check valve on centrifugal pumps. A great many centrifugal pumps MAY use a check valve (or foot valve, as they are called when installed directly into the pump outlet. Most sump pump and condensate pump installations use a check or foot valve.
It's just that the typical 12 volt bilge pump for small boats has very low head capacity. The tradeoff is between flow and head, as it is on all centrifugal pumps. Higher head = lower flow, and vice versa.
s/v My Sweet Girl!
Catalina 30 (Mark I)