Tips for Improving Your Boats Fuel Efficiency

The terms “fuel efficiency” and “boat” hardly belong in the same sentence. The pinnacle of economy for recreational, planing boats is in the neighborhood of 10 mpg, and that’s for lightweight, efficient hulls with small or mid-range outboards cruising at something like 20 mph. At the other end of the spectrum, big sportfishers and large, multi-engine outboard boats get less than 1 mpg, crossing the line between miles per gallon and gallons per mile.

Considering gas prices these days, though, it’s worth trying to make the best of a bad situation by taking some simple steps to improve your boat’s fuel efficiency. The gains aren’t likely to be great, but over the course of a season at $5 to $7/gallon, even a few tenths of a mile per gallon can add up.

Lighten Your Load

One means of improving mileage — which should be obvious but is too often overlooked — is reducing your boat’s load. Short of leaving your brother in law at home or swearing off live bait, there are several ways to do this.

For starters, get rid of excess junk — cleaning supplies, the box of 10-oz. torpedo sinkers you bought for that grouper trip three years ago, your old weight belt, the marine grill and two bottles of propane you haven’t used since the weekend after you bought them, and so on. Each piece of extraneous gear may not weigh much by itself, but the weight adds up.

"Gasoline weighs around 6.2 lbs. per gallon...you don’t want to risk running out of fuel for the sake of mileage, but with a little common sense and forethought you can often reduce your fuel load significantly. "

You can also reduce weight, of course, by carrying less fuel and water. Gasoline weighs around 6.2 lbs. per gallon and diesel a bit over 7 lbs. That comes to about 310 lbs. for 50 gallons of gas or 350 lbs. for 50 gallons of diesel. Water weighs 8.3 lbs. per gallon, so your 20-gallon freshwater tank is good for 166 lbs. and your 40-gallon live well is almost 350 lbs.

Obviously, you don’t want to risk running out of fuel for the sake of mileage, but with a little common sense and forethought you can often reduce your fuel load significantly. The same goes for water — if you don’t need it, don’t take it. And remember that many live wells with high-speed pickups fill by themselves when you’re running, even with the pump off. Close the seacock or another valve if you won’t be using the well.

Pick the Right Prop

Your best chance for a major improvement in fuel economy is changing your propeller(s). Many boaters don’t realize just how much difference a prop change can make; in some cases, even changing to a different model of the same pitch and diameter can drastically change a boat’s performance characteristics.

"Many boaters don’t realize just how much difference a prop change can make; in some cases, even changing to a different model of the same pitch and diameter can drastically change a boat’s performance characteristics. "

Propeller selection is a topic of its own, but suffice it to say here that there are two dead giveaways that you’re running the wrong prop(s). If, when you pin the throttle with a normal amount of gear and people on board, your engine RPM doesn’t reach the minimum WOT (wide-open throttle) RPM recommended in the owner’s manual, your boat is likely “overpropped,” meaning your prop has too much pitch. The effect is like riding a bike in too high a gear. Conversely, if your engine RPM exceeds the maximum recommended WOT RPM, your boat is “underpropped,” like a bike in too low a gear.

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Work on Balance

It’s not just the total weight of crew, gear and fuel that affects economy, but also how it's distributed. If you have to trim your engines or drives way up to reduce your wetted surface or way down to manage chop or prevent porpoising, you’re burning more fuel than you need to. The same goes for trim tabs — if it takes a lot of tab to keep your bow down in chop or prevent porpoising on smooth water, you’ve got a balance problem. Likewise, using trim tabs to keep your boat running level is inefficient.

"The easiest way to adjust weight distribution is to move crew around...In lieu of that, move heavy, easily portable gear like coolers, ice, tackle boxes/bags, emergency water jugs, extra fluids, tool chests and kill bags. "

The easiest way to adjust weight distribution is to move crew around, but that’s not always an option. In lieu of that, move heavy, easily portable gear like coolers, ice, tackle boxes/bags, emergency water jugs, extra fluids, tool chests and kill bags. In the longer term, you may also want to consider moving batteries to a different location.

Remember also that weight distribution changes significantly on many boats depending on the amount of gas in the tank(s) and water in the live well(s). If conditions allow, it’s possible to meaningfully improve mileage simply by moving a couple of people from the stern to the bow or vice-versa.

Pay Attention to Nuances

If you have a fuel-flow gauge, another easy way to reduce the amount you spend on fuel is simply to devote a half hour to learning the nuances of your rig. Every hull-engine combination performs differently, and every one has an optimal cruising speed and attitude. The best place to figure it out is on a sizable piece of relatively flat water without a lot of other boat traffic.

Start out making long, straight runs, preferably at right angles to the wind direction, at about the speed you normally cruise. Fiddle with trim until you maximize your mileage, and then speed up or slow down by a couple of miles per hour. If you get better mileage at that speed, adjust again in the same direction. If not, adjust back in the other direction. Keep at it until you find the combination of throttle and trim that produces the best fuel efficiency.

"fuel efficiency does peak at some speed, on either side of which it decreases. If you can cruise at or near that point the majority of the time, you’ll burn less fuel. "

Remember that this is by no means an exact science. The most efficient speed and trim combination for any given boat and engine combination varies with factors like load, weight distribution and wind and water conditions. On the other hand, fuel efficiency does peak at some speed, on either side of which it decreases. If you can cruise at or near that point the majority of the time, you’ll burn less fuel.

Small Gains

While the right prop(s) can make a big difference in your economy, the kind of gains achieved by getting rid of 100 lbs. of unneeded gear or fiddling around with trim and throttle can seem insignificant. In other long run, though, they do add up — especially if you use your boat a lot.

If you can improve your average mileage from say 2.0 mpg to 2.2 mpg (fairly typical for a 25-foot, outboard-powered fiberglass boat), you’ll burn about 3.4 gallons less during a 75-mile day on the water. At $4/gallon at the gas station, that’s $13.60. At $6/gallon at the fuel dock, it’s $20.40. Multiply either of those numbers by say 20 trips a year, and you’ve saved either $272 or $408. And that counts.