From a engineering standpoint, the minimum flow rate required for a 9000 btu a/c with a 10 degree delta
T (conservative) is 2 GPM or 120 GPH.
A march 500 GPH pump probably only puts out 300 ish GPH at the pressure head
needed. You have to plot in on the March Pump curve available here:Magnetic Drive Pumps | Mag Drive Pumps | March Pump
Select the DC pump curve.
A 16,000 BTU unit would need a minimum of 200 GPH or 3.2 GPM.
A 24,000 BTU unit would need a minimum of 300 GPH or 4.8 GPM.
For small marine
A/C, assume 5 to 8 feet total system head
. You find the pump that is correct where the pump curve is to the right of the intersection of head / GPM and pressure. Do not select a pump with a curve to the left of the system design point.
If your in 90 degree water a lot, double the flow rate. The 10 degree Delta
assumes 85 degree F max condenser supply temperature.
If you need a real pressure drop calc on a system, drop me a PM