How to Get Better Fuel Economy From Your Boat

If you have live mpg or fuel flow, Mercury's method is simple: get on plane, let the trim settle, then change the throttle in small steps and watch for the highest mpg. Without automatic trim, adjust trim and throttle together in small amounts. Yamaha notes some owners find their boat is actually most efficient a bit faster than they thought, while others save a lot by dropping a few hundred RPM.

Mercury describes this approach:

• Start with a full tank. Run at your first test RPM (just on plane) for a set time, such as 10 minutes, half in each direction, and average the GPS speeds.
• Refill and note how much fuel it took. Multiply to get gallons per hour.
• Repeat in steps of about 500 RPM up to full throttle.
• Divide speed by gph for each step to get mpg. Keep trim and trim tabs the same for every run so you compare fair numbers.

Discover Boating's first tip is simply that slower speeds use less fuel. Mercury notes the most efficient speed overall is usually slow displacement speed, before the boat gets on plane; once on plane, most outboard boats find their best mpg somewhere in the 3,000 to 5,000 RPM range, and efficiency falls steadily above that. Wide-open throttle is almost never the efficient setting. Avoid running in the "plowing" zone just below plane, where the bow is high and the boat is working hard without going fast. BoatUS adds that a planing hull is usually most economical just after it comes up on plane: it takes a fair bit of power to get over the hump, but once you're on plane and trimmed, you can often ease the throttle back a little without losing speed.

A fouled bottom is like pushing a carpet through the water. BoatTEST explains that barnacles, grass, tube worms and slime quickly add drag, and recommends choosing antifouling paint that suits your home waters. Clean or haul the boat on a schedule, and keep the prop, trim tabs and intakes clean too (see the hull cleaning and bottom paint guides).

A dinged, bent or wrong prop wastes fuel. Discover Boating says that if your boat is slow out of the hole or lacks top speed, the prop may be wrong. Mercury notes the prop that gives your boat the best top speed is the most efficient at WOT, but may not be the best at cruise, so ask your dealer about props that suit how you really run. Your engine should reach its WOT RPM range from the owner's manual with a normal load. BoatUS estimates a dinged and bent prop can cost you around 10 percent of your fuel, and prop shops can measure how far out of spec it is and repair it. See the propeller guide for checks and repairs.

Mercury's basic sequence: trim fully in (down) at idle and while getting on plane so the prop bites, then trim out a little at a time once you're at a steady speed. Trim back in for turns and when running into a chop. Trimming out lifts the bow and reduces the wetted hull, which cuts drag. Too much trim lets the prop ventilate (grab air), so RPM rises and speed falls. With a live mpg display, trim up slowly and stop where mpg peaks. Discover Boating notes trim tabs used correctly reduce drag, especially while getting on plane; once on plane, raise them until the boat runs level. A Boating Magazine racer put it simply: a boat set up so it doesn't need much trim is more efficient.

A few more trim checks:

• Look at your wake. BoatUS's technical editor notes a well-trimmed boat usually throws the smallest wake at a given speed. A big, plowing wake means you're pushing water and burning fuel.
• Sterndrives: you can't see the drive, so use the gauge. Mercury's advice is to trim up gradually as you accelerate and stop just before the bow starts to bounce (porpoise), which is usually the most efficient point.
• Don't use trim tabs to hold up a badly loaded boat or level a list. BoatUS says that adds drag and can hide a dangerous load; move or remove the weight instead.

Extra weight means the hull sits deeper and drags more. Discover Boating has watched owners clear out around 250 pounds of things they didn't need. Ideas:

• Empty lockers and keep only what you use, plus required safety gear.
• Don't carry full water or holding tanks if you don't need them.
• Carry the fuel the trip needs plus a healthy reserve, not always a full tank on a short outing (gasoline weighs a little over 6 pounds per gallon).
• Spread weight evenly and avoid loading the bow.

Discover Boating suggests keeping time idling at the dock to a minimum. Warm up as your manual recommends, then go. Mercury's service team says a fuel-injected engine usually needs only a minute or two to reach operating temperature (longer in very cold weather, and a few minutes more for carbureted engines), and that warming up much longer just wastes fuel. On diesels, follow the maker's guidance for warm-up and avoid long periods of very light load; ask your dealer what's right for your engine.

A healthy engine burns fuel efficiently. Follow the maintenance schedule in your manual:

• Spark plugs (gasoline) and fuel filters, including the water-separating filter.
• Clean, fresh fuel. Mercury recommends E10 at most and treated fuel for storage; water or phase-separated fuel hurts performance.
• A clean air intake and, on diesels, clean air and fuel filters.
• The correct engine oil and a cooling system that keeps temperature normal. Discover Boating adds: use the gasoline grade your engine maker specifies.

If your engine has no fuel data, an aftermarket fuel flow sensor or engine data gateway can show gph and mpg on a chartplotter. On gasoline boats, adding a sensor means opening the fuel line, which is a job for a qualified technician using marine-rated parts. Even without one, a fuel log (step 12) gives you most of the benefit.

On twin-engine boats, engines running at different RPM or trim waste fuel and cause vibration. Sync them (see the twin engine sync guide) and compare each engine's fuel flow at the same RPM: one engine burning clearly more than the other is worth a check.

Each time you fill up, note gallons, engine hours or trip distance, and conditions. Over a season you'll see your real average, spot when economy slips (often the first sign of a dirty bottom or prop damage), and know your safe range. If economy suddenly drops, work through the boat-is-slower diagnosis guide.

Don't let the fuel gauge be your log. BoatUS points out that boat gauges are notoriously inaccurate: odd-shaped tanks built into a V-hull can read half full when three-quarters is gone, readings change between rest and planing, and sloshing makes them jump. Track gallons burned instead, and remember that a headwind, an adverse current, rough water or a fouled bottom can cut your range well below your calm-day numbers. Running out of fuel is one of the most common reasons boaters call TowBoatUS.