It's a well-known fact: Single-engine boats are more efficient than twin-engine rigs. So it only makes sense that these new hopped-up, triple-engine outboard boats are cash-burning gas hogs, right? Pro-Line BoatsA Pro-Line 35 Express, rigged with triple Verados.

Wrong. After several years of efficiency tests on triple-engine boats, including many models that were previously tested with twins, the results are in: Triple-engine boats are commonly at least as fuel efficient as twin-engine rigs of about the same size and weight. What's more, they can even be more efficient than twin diesels in the same boat.

The first time I noticed this surprising phenomenon was after testing a Fountain 38 Sportfish Cruiser rigged with triple 275 HP Verado outboards. The Verados are Mercury's latest and greatest: supercharged four-stroke in-line six-cylinder engines with 158.5 CID. On our test boat, they were swinging 15½-by-24-inch lab-finished, four-bladed stainless-steel props through 1.85:1 gear reductions. The boat had a quarter tank of fuel, empty water tanks and two people onboard.

Driving the boat was Reggie Fountain himself. With a wide grin, he told me I'd be surprised at just how fuel efficient this boat would prove to be. He was right. Here's what happened as we increased RPMs from 1000 to wide-open throttle (See below chart):

Obviously, if you run this rig around fast enough to merit a ticket on the interstate, your fuel economy won't be all that great. Even at its worst, however, 0.8 mpg isn't incredibly bad for a 38-foot boat. And at its most efficient speeds, from 27.6 to 35.3 mph, the triple-engine Fountain earned 1.8 mpg – extremely good for a boat of this size.

How do these numbers compare to a similar twin-diesel rig? The same boat with twin 315-HP Yanmar in-line-six, 254 CID engines, swinging 15¼-by-24-inch, four-bladed stainless-steel props through 1.5:1 reductions, was tested with three-quarters of a tank of fuel, no water and two people onboard.

Traditional theory and common sense tell us that the twin-diesel, stern-drive rig should be a more efficient machine. But look at the numbers (broken down to 300 rpm increments, since the diesel doesn't spin nearly as fast as a gasoline motor):

Pro-Line BoatsThe Same Pro-Line with twin Verados. As you can see from the lagging mid-range numbers, this boat was propped for top-end performance. So let's add a freebie tenth of a mile per gallon into its column, generously assuming a prop-swap can get you that much of an advantage.

Still, in the best-case scenario, the twin diesels cruise at 1 MPG. Of course, top-end is significantly slower than the triple-outboard rig, and if you run the triple boat at the diesel boat's top speed, you'll still be getting at least 0.2 more MPG. There's no doubt that the triple-engine rig is, in this case, clearly and significantly more efficient than the twin diesels.

Now let's see how the numbers stack up against a twin 275-HP Verado Fountain. Unfortunately, I don't have numbers that I can vouch for on a 38 with twins, but I do have them on a 33-foot Fountain Sportfish Cruiser. This boat weighs nearly 2,000 pounds less than the 38 but shares the same Fountain super-efficient twin-stepped running surface. So with identical power plants, one less engine, the same bottom and a weight advantage, this twin-engine rig has got to be more efficient, right? Wrong again – check it out:

Fountain PowerboatsThe 38 rigged with triple Verados can cruise at an astonishing 1.8-mpg! As the numbers prove, the triple-engine rig is, again, significantly more efficient in nearly every portion of the power band. True, the twins do better when you're putting along in a speed zone. But at cruising speeds, the triples beat out the twins across the board, tie the twins at the upper end of the power band and drop just 0.1 mpg below at wide-open throttle – while moving the boat some 30 MPH faster.

So who's going to spend the least amount of cash on fuel this year: the twin-diesel, twin-outboard or triple-outboard boater? Though it seems incredible, the one with triples on the transom wins out, even though that boat is bigger and heavier than the one with identical twin power plants.

HAPPY HYDRA

Now let's look at a pair of Hydra-Sports 3300 Vectors. One was rigged with twin Yamaha F250 V-6 four-strokes with 204.6 CID and 2.0:1 gear reductions, and one was rigged with identical triples. The twin boat was spinning 15¼-by-19-inch, three-blade stainless steel props, and the triple boat was spinning 15-by-21-inch, three-blade stainless steel props. (I didn't test these personally; the performance information was gathered by Yamaha techs, who are known for reliable data.) First, look at the numbers for the twin-engine rig:

And, the triple-engine boat:

These results paint a different picture: Once again, the twin-engine rig does better at very slow speeds, but in this case, the twins also pull off a noticeable advantage through most parts of the power band. But there's one startling and very important exception: At the most efficient cruise – where you and I will be running our boats most of the time – these two rigs tie for efficiency, at 1.4 miles to the gallon.

In this case, you can make the argument that the twins eke out a win since they enjoy an advantage elsewhere in the power band. But don't score one for the twin rig too quickly; there are more facts to consider.

Note, for example, that the triple-engine boat's most efficient cruise is at 30.5 mph, while the twin-engine boat's most efficient cruise comes at a notably slower 28.5 mph cruise. Now recognize the fact that the twin-engine rig is spinning more RPM to go this fast (4,000 versus 3,500), which means the engines are always working harder to get the boat moving at this slower speed. That, in turn, can potentially lead to a shorter life span or more maintenance requirements. Score one more for trips, or call it even at the worst.NavicoWhen you feel the need for speed, triples is obviously the way to go-but when it comes to efficiency?

LOOKING AT AVERAGES

In the above cases, the triple-engine rigs come out ahead. But is this always the case? No. In fact, a Contender 33T rigged with twin Yamaha F250s is consistently about 0.3 mpg more efficient than one rigged with triples, throughout the power band. And a Pro-Line 35 Express rigged with triple Mercury Verados showed nearly identical efficiency numbers, with a 0.1 GPH advantage at cruising speeds. So while three can be less than two, it isn't necessarily so, depending on the type of boat you're talking about.

To find out which rig offers the best overall efficiency across the board, I averaged the fuel burns at cruise for all the boats I've tested over the past three years that are 34 to 38 feet (the size range that usually includes both twin and triple rigs). I included only boats rigged with 250 to 300 HP and did not consider those rigged with Yamaha's 350 hp four-stroke, since there are no boats that have been tested (not yet, anyway) with triple engines of this model. All averages were taken at the most efficient cruising reading, which usually came in the low 30s for triple-engine boats and in the upper 20s for twins.

The average fuel economy of all of the twin-outboard boats was 1.28 mpg. The average of the triple rigs tested during the same time period: 1.30 mpg. Not much of a difference, for sure, but the triples do come out ahead.

Does that mean we should all run right out and buy triple-engine boats? Well, I don't know about you, but I consider adding about $20,000 to the cost of a new boat a pretty steep price to pay for a 2 percent advantage in fuel efficiency. Think of it this way: If you run your boat 1,000 miles this season, you'll save about 15.4 gallons of fuel. At $4 a gallon, that's $61.60. At that rate, it'll take 325 years for your fuel savings to pay for that third motor. So speaking from a strictly financial viewpoint, twins may be the better bet. Maybe.

Why not for sure? There are several reasons. In virtually every case the triples were spinning fewer RPM than the twins to get to their most efficient cruising speed. That adds longevity and reliability. Now think of the practical advantages of triples. In all cases, their best cruising speed is faster than the twin-engine boat's best cruise. In all cases, top-end is faster, too. And remember that most twin-engine boats in this size range can't plane on one motor. That means that if you have mechanical problems with one of your power plants, it's a long putt home. But if you're rigged up with triples, simply tilt up the troubled engine and you'll have no problem running home at a reasonable cruising speed.