How do you find the most efficient cruising speed on your powerboat? It's actually pretty easy. To see how it's done, watch this quick demo video.
No matter what type of powerboat you have, the process works the same way as it did with our Bayliner Element XR7 test-boat. Just remember, you do need a way to measure those key factors: speed, fuel burn in gallons per hour (GPH), and engine RPM.
We recommend using a GPS to measure speed, since it's far more accurate than the average boat speedo. Most modern marine engines, including the Mercury 150 Four-Stroke on that Bayliner, will either show you fuel flow in GPH on a gauge at the helm, or you can plug in an engine monitor. Same goes for RPM. If your boat doesn't have one or the other (or doesn't have either) you can stop in at your friendly local dealership or marine mechanic, and ask them if you can borrow a monitor (or shop tach, as needed) for an hour or two.
If neither of these option are available, you can always go the analog route when measuring GPH and RPM. For GPH, you'll need a clear container with valves and fuel barb fittings on either end, and a vent in the top. Measure out a tenth of a gallon into the container, and make a mark to indicate the fill-line. Disconnect your boat's fuel line down-stream of the bulb and attach your container with both valves open, then pull off the dock and get the boat up to speed. Once you're steadily running at the desired RPM close off the "in" valve on your container, put your phone in stopwatch mode, and measure exactly how long it takes for your engine to suck down a tenth of a gallon of fuel. Then do the math to figure out how many GPH you're burning.
Here's an example: say it takes 30 seconds to burn that tenth of a gallon. Multiply the 30 by 10 to account for burning a full gallon instead of just a tenth, and you get 300. In other words, it takes your boat 300 seconds to burn one gallon of fuel, at this specific RPM. There are 3600 seconds in an hour, so if you divide 3600 by 300, you get your GPH—in this case, 12. Of course, you'll have to do this at each RPM increment, to figure out where your boat gets the best mileage.
What if you need to measure RPM in an analog manner, too? Get your boat on plane, then divide up your remaining throttle settings into quarters (IE, on plane plus 25-percent; plus 50-percent, and so on), and use masking tape or a grease pencil to make marks at the helm. Then take a measurement at each mark, and you'll know where to set your throttle to find the most efficient cruise.
Remember, this method doesn't work with diesels, which may use fuel for lubrication and have a return line that feeds un-burned fuel back to the tank. And with very small outboards (20 HP or less) it can take quite a while to burn out even a tenth of a gallon of fuel.
Does all of this sound like a real pain in the kiester? If you don't have GPH and RPM at the helm, it certainly can be. But remember—invest an afternoon determining your boat's most efficient cruise, and you'll save fuel money all summer long.