Re: water flow GPM per HP of an engine?
I believe my Yanmar 3GM 22.5 hp raw water cooled is supposed
to circulate 4 gallons per minute @ 1000 rpm's and 8 gallons
per minute @ 2000 rpm's.
Test that may be helpful is to run motor for 30 seconds collecting
exhaust water into large bucket and measuring amount of water.
Then at same rpm's do same but with hose exiting water pump directed straight into bucket and measuring again. Will give you
idea of degree of blockage if any.
Suggest checking out basics first such as any blockages at intake, seacock, strainer, shells/barnicles in water pump and as suggested
checking for blockage in mixing elbow.