The burn rate on a pair of 427 motors running at a comfortable plane, which on my boat is somewhere around 2800 rpm, is around 24 gal per hour (12 gal per hour per motor). At that speed with a 25” pitch equivalent in a 3 or 4 blade, the boat will be moving around 26-mph in theory before slippage, and with slippage added it’s probably running a a speed of around 19-mph. The motors are running right near their peak torque levels (which is 2900 rpm for a 427) and they’re loafing.
Running a 120 mile route, will take around 3 hours out and 3 hours back, if you maintain that speed. That’s around 144 gal of fuel. At $4.00 the cost for the day is, surprise, nearly $600 (actually $576). If you back down and run slower, you may actually run more fuel per mile, because running right on the top of the torque curve seems to be the easiest on the motor and probably the most efficient speed. I like to get the boat up on a plane and back off until the break across the bottom of the transom is still nice and clean. The boat seems like you could run forever at that speed.
I used a slippage factor of 25% just for a plug number, but it isn’t too far off. The resulting mpg comes in around .83 miles per gal. In reality, I think I can do a little better than that, but never really tested it too accurately with the 4-blade installation. That’s something I’ll do in the upcoming season, because we have a 50-mile run from Commodore Yacht Club to Cedar Creek Yacht Club we like to make. Incidentally, we’ll be making this same run in the 327-powered Sea Skiff too, at much faster speeds and much lower costs.
We’re like you, with regards to cutting down on the travel distance. We have some really sweet (and remote) anchorage locations on the Cumberland that are within 20 miles from our marina, so that helps with the fuel burn. The cost of fuel is one prime reason you see so many boats just sitting there at the marina all season long. Up on Lake Erie, if you’re going to run from Erie to Port Dover, for instance, be prepared to spend the bucks, because Canadian fuel isn’t any cheaper on that side.
I'm guilty of just putting gas in the tank and running without really checking the consumption closely. We can go out for quite a few runs on a fill up, and we run at varying speeds depending upon whether or not we're trying to reach a destination, or just doing a cocktail cruise. When we do the latter, we're generally loaded with an additional 1500 pounds of people, food, ice, and beverages, and we tend to run below planing speeds.
My numbers are based upon a 38' Express hull, 427 power, 2-2/2:1 gear reduction, and 4 blade 23x24 props. As noted, these are a bit over-pitched, and I may eventually have them repitched to 23x23 beause they have such an enormous bite.