One of the weaknesses of rubber impeller raw water pumps is their need for yearly replacement of the impeller and occasionally new seals. For years we sailed a Concordia with a Graymarine 91 gas engine which was equipped with a bronze gear raw water pump. The seal was a simple fiber packing. The pump functioned perfectly for years without failure and since it was positive displacement never lost pumping capacity over time. My question is why rubber impeller pumps are so commonly used? The 365's because of the tight space around the pump make the job of maintenance inconvenient to say the least. Have read several posts here about rubber impeller flaps becoming lodged in the exchanger and causing overheating. Seems to me that these rubber pumps are a poor choice for all-important engine cooling. Thoughts?
What was the material of the blades for the Graymarine? or are you saying it was a piston? In which case the piston is less efficient and considerably harder to overhaul, as well as more prone to seizing due to corrosion or galling when not in use.
At a guess, its because of priming, if you are talking about metal gear pumps. They need fluid inside them or they will not draw. For a pump below the water line it would probably be fine.
Gears were bronze & location was below the waterline. Made by Oberdorfer. Looked like this.
As SVJourney said problem with a gear pump is they are not self priming, they must be quite low so that heeling or wave action cannot momentarily drain the intake. Once air is introduced it will need to be bled. I gave up using one on my water cooled refrigeration because of this.
If you decide to try one make sure of the flow rate also.