Re: water flow GPM per HP of an engine?
Sorry, no formula on this one because engines have thermostats. It isn't hard to figure out how much power goes out the cooling system since it is typically equal to the amount of shaft power the engine produces (another roughly equal part goes out the exhaust). Then, you can assume an inlet and outlet water temp and you can calculate a flowrate. The trouble with this is that the flowrate is actually very small and the inlet water temp and the power output of the engine fluctuate based on the usage. To deal with this, they install a thermostat so that the maximum water flow exceeds the cooling needs of the engine in the worst case scenario and the thermostat closes down to deal with other situations. Most of the water that you see in an exhaust is actually going through the bypass loop to keep your pump happy except under really high throttle situations. If the flow of water gets to be so low, then this will make the engine overheat but up until this point, the thermostat will just open more and take care of it.
If you really suspect that the problem is with the flowrate, I would recommend finding another person with the same engine and doing a comparative bucket test at the same engine rpm with the engine cold (the thermostat will be closed so the flowrates should be the same). Just time how long it takes to fill a couple of gallon bucket and compare.
Can you see the thermostat opening on the temp gauge? Most real gauges will let you see it open and close if you watch really carefully.