Between your engine and your generator, your houseboat can really guzzle up a lot of gas. If you’re ready to hit the lake hard this summer but you want to save a little money, here are some expert tips from our friends on the forum to get you through the boating season with a wallet that’s a little bit heavier!
Amelia: Just a few guesses, to start things off, anyway:
Go fairly slowly. The gas consumption goes way up if you insist on fire-walling the throttle. There's a sweet spot where time/distance/fuel burn intersect, and a little experimentation will help find it.
Keep the hull clean. If you're hauling a load of algae, barnacles, etc, you'll pay for it in both speed and gas consumption.
Use zero ethanol gas, if you have gasoline engines. I'm told that they run a lot better and more efficiently without all that corn glopping the innards up. (You may want to verify this with somebody who knows what he's talking about. Our car gets nearly 25% better mileage with E0.
BananaTom: Get SeaTow or Boat US and "breakdown" when you are the farthest from the dock for a tow back. Haha. Use Sea Foam every now and then, to keep it clean, all jokes aside.
easttnboater: I keep my engines in tune mostly because I like to fiddle with engines. But, I am pushing over 50,000 lbs with two V8 engines—the mileage is never going to be that good.
42gibson: we have two 454's and at 2000 RPMs and the generator running (10 mph) we get 1.25 mpg. It isn't great but I knew it never would be, it’s a boat.
Endurance: Every boat has a "sweet spot" that maximizes speed and minimizes fuel burn. I can think of at least three ways to find your best speed for maximum speed and minimum fuel burn.
The first way is the easiest, but it takes a gauge that will probably cost more than your annual fuel budget. You buy and install a meter called a Floscan. If you get their upper end gauge, it can read speed by paddlewheel or GPS. It also gives you an instant readout on gallons (or liters) per hour. Because it "knows" both speed and fuel burn, it can also give you an instant read on miles per gallon (or liters per kilometer). You just adjust your throttles until the Floscan gives you maximum miles per gallon then leave the throttles alone for the rest of the trip.
The second easiest way takes a medium amount of technology. You can buy a more basic Floscan that reads only fuel burn rate. But you might not even need to do that. Many boats these days come with some smart electronics built into the tachometer or similar engine management software. On my boat, it is the Mercury Smartcraft tachometers. If you are lucky enough to have this on your boat, you just scroll through the menus until you get a gallons per hour readout. I made a chart of my fuel burn rates at various engine RPMs. Naturally, as RPMs increase, so does fuel burn. But as you increase RPMs, speed also increases. To figure out what's the best speed for maximum miles per gallon, you also need to make note of your speed at the various RPMs where you noted fuel burn. If you have a GPS, you can use that to measure your speed. If not, you can download an app on your phone that will give you speed. I used a free app called GPS Essentials for my Android phone. Apple probably has something similar for iPhones. You then just crunch the numbers at various RPMs to find the one that gives you the highest miles per gallon.
The third way is the most basic, but it takes some time. If you have a similar destination you visit from time to time, you just do it at different engine RPMs each time you do it and keep track of how much fuel it takes to make the trip. The RPM that uses the least fuel is the winner.
If these methods are too much effort for what you have in mind, just know that for most boats with gas engines, you will get your best miles per gallon between 2,000 and 3,000 RPMs. Diesels do better with torque and don't like high RPMs. For diesels, you probably want to be around 1,500 to 2,000 RPMs. Like all rules of thumb, this is an oversimplification, but it is at least a starting point for the more in-depth methods I outlined above.
For every boat I have operated, operating at wide open throttle is akin to driving along throwing twenty dollar bills into the lake. At higher RPMs, fuel burn increases exponentially. Speed increases with higher RPMs, but the rate of increase usually falls short of even a linear rate of increase. Just like driving a car, an easy hand on the throttle will save you fuel and wear and tear on your engines.
Frantically Relaxing: If you have a displacement boat—one that won't plane—reverse the “pleasureboat” thinking on propping. Easttnboater mentions pushing around 50,000 pounds with a pair of V8's—Our boat weighs 36,000 pounds and is pushed around by a pair of 120hp 4-bangers. And whoever owned it before me (or maybe the factory) propped it with a ridiculous 16-by-9-inch props, which let the engines hit the recommended RPM range of 4000 to 4400...That's all fine and dandy except for the fact the engine is swallowing as much fuel as it can by design, all to go 10 mph. It would do a 7mph cruise at about 3100 RPM, which with these dinky engines would get me close to 2mpg. Another issue with a 9-inch pitch prop is maneuvering around the harbor, just not a lot of push. So I changed them out, ordered a set of 15-1/2-by-11's at the boat show. A couple of weeks later I get a phone call; they can't get the 11's for several months, would I take a set of 13's with a bit of a back-tweak to near 12—? Okay, why not...
Turns out they worked out great! The steeper pitch makes the boat actually DO something while in the harbor now. My 7mph cruise RPM dropped from 3100 to 2400 (see the pic). My max speed with this prop is 9.7 mph and the engines only hit like 3400 RPM. Cruising at 2400 RPM brought our mileage to around 2.5 mpg. While I don't drive at WOT, if I did I'd get better mileage because the engines can't swallow near as much gas at 3400 RPM as it can at 4100...
Bottom photo credited to Frantically Relaxing