Re: water flow GPM per HP of an engine?
Your guess of 3-5 GPM is probably pretty close. However since his engine is an older raw water cooled engine, I would suspect scaling or clogging of the cooling passages is more likely the cause. An oxalic acid flush may help. When I changed the head gasket on the Universal 5411 on my previous boat, I found 5 of the 8 engine passages completely plugged. Flushing had not been able to open them.