Gas mileage

I have a 96 V21 with the same year 150 johnson. The boat gets very poor gas mileage. I'll go on a 10 mile day and burn 25 gallons or so. speed wise it's great out of the hole but cruises at about 22 knots at around 4300 rpm. kind of disappointing. Does this sound normal for the engine. I'm more concerned about the gas. Could It be the wrong prop maybe? any insight would be appreciated.