| What
better time than now to talk about fuel efficiency? With the cost of a
gallon of unleaded hovering around $3, an 80-mile round-trip out to the
banks in a boat that gets 2 mpg at cruise will run you around $120. At
3 mpg, though, you’ll burn only about $80 worth of fuel. Multiply that
by 10 trips a year, and you’re looking at a $400 savings on fuel. Now
more than ever, small changes in fuel efficiency add up over time to
big differences in the cost of going fishing. Probably the biggest and most obvious factor in fuel efficiency is your initial choice of power package. The vast majority of recreational boats are offered new with a choice of at least a couple and sometimes a dozen or more power options. For the purposes of this discussion, though, we’ll consider outboard-powered trailer boats from 23 to 26 feet. There are several reasons for this. First, there’s a surprising amount of performance data related to these boats available from outboard manufacturers. Second, the range of power options for boats in this class is wider than for perhaps any other, since many models run just fine with either single or twin engines. Third, these are simply very popular boats among Southern California saltwater anglers. Horsepower vs. Mileage One basic assumption most of us make is that a higher-horsepower engine delivers better performance but worse fuel economy than a lower-horsepower engine. That’s certainly the general rule for cars – a little 140 hp four-cylinder will get better mileage than a 300 hp V8, all else being equal. Usually it’s true with marine power too, but things are more complex than that. Let’s look at some numbers for the Regulator 23 CC, a 23’4” x 8’4” deep-V center console built in North Carolina. Yamaha offers “performance bulletins” for the 23 CC rigged with a variety of different engines. Rigged with a 300 hp HPDI direct-injection two-stroke, the 23 cruises most efficiently at 26 mph and 2.40 mpg. With a 250 hp HPDI, the same boat cruises best at 24.3 mph and 2.86 mpg. Top speed with the 300 is 46.9 mph; with the 250 it’s 43.4 mph. So the more-horsepower-equals-more-speed-but-worse-economy rule holds true here. Another general rule demonstrated by this comparison is that as horsepower increases, so does the optimal cruise speed. This bears a little more consideration. If you look at graphs of fuel economy versus speed you’ll notice that for most boat-engine packages, economy drops off steeply on either side of the optimal cruise speed. So it’s important to compare not just mileage at optimal cruise speed, but also what that optimal cruise speed is. A 36-foot offshore center console with triple 250s that tops out at 75 mph might cruise most efficiently at 50 mph, usually a bone-breaking pace on the open Pacific. On the other hand, a 21’ skiff powered by a 150 might cruise best at a pokey 23 mph; push it up to 30 on a smooth day, and your mileage goes down the tubes. If you want to cruise at a more typical 26 or 28 mph, a 200 might offer better “real-world” mileage. Twin vs. Single Now on to the old twin vs. single question. Yamaha also tested the Regulator 23 with a single 225 hp four-stroke, twin 150 hp four-strokes and twin 115 hp four-strokes. Performance was virtually identical between the T115-rigged boat and the 225-rigged boat – optimal cruises of 26.3 and 25.5 mph and mileages of 3.06 and 3.07 mpg, respectively. Top speed was nearly identical, too — about 43 mph. The only real difference was that the T115-rigged boat was over 700 lbs. heavier as tested, of which only 218 lbs. was engine weight. Translation: the twin 115s probably would beat the 225 in both economy and top end without the extra 500 lbs. to push. But twin 115s, for whatever reason, aren’t a very common configuration. The usual choice you face when buying a 23- to 26-footer is between a single 225 or 250 and twin 150s. The differences here are more apparent. Optimal cruising speed increases by about 2 mph with the 150s, mileage drops to 2.67 mpg, and top end is almost 50 mph. The importance of top-end speed is a personal matter, I suppose. For a saltwater fishing boat, it seems to me like an insignificant concern. After all, how often can you really go 50 mph? In my mind, cruising speed and efficiency are vastly more important characteristics. There’s no question it’s reassuring to have two engines instead of one when you’re 50 miles offshore, but between the higher initial cost, the higher maintenance costs and the worse mileage (about 5 extra gallons of fuel burned per 100 miles at cruise) of the twin 150s, that peace of mind won’t come cheap. Four-stroke vs. Two-stroke Mercury’s testing of 135 hp and 150 hp Optimax models and 135 hp and 150 hp supercharged four-stroke Verado models on a Boston Whaler 190 Outrage, an 18’10” x 8’0” modified-V skiff, offers a couple of other interesting insights. First, the Verados delivered 2 to 4 percent better economy at optimal cruise than their two-stroke counterparts, but optimal cruise for the Verados was substantially slower than for the Optimaxes — 22.8 vs. 26 mph for the 150 hp engines. Generally, in fact, four-strokes reach their maximum efficiency at lower rpm than two-strokes of the same horsepower fitted with the same props. The other side of that coin is that four-strokes are usually more efficient at trolling speed than two-strokes. The Optimaxes also had a lot more “pep” than the four-stroke Verados — the 150 Verado took 9 seconds to hit 30 mph from idle, while the 150 Optimax took just 5.6. On the other hand, the Verados were far quieter across the entire rpm range. Noise level for the 150 at cruise was 81 dB, compared with 98 dB for the 150 Optimax. At 2,000 rpm, about trolling speed, the difference was a full 20 dB. The importance of acceleration and noise level, like top-end speed, is debatable. Certainly, there are some situations, such as negotiating the surf, in which quick acceleration is a very real safety factor, but not many of them. The value of a quiet engine depends, I guess, on whether you prefer to actually hear what your crew has to say or just nod your head and ignore them. $4,500 at Current Fuel Prices It probably goes without saying that these tests aren’t gospel. Conditions differ, props differ, and, of course, drivers differ in their ability to properly trim a boat. Moreover, engine technologies vary quite a bit; the “power curves” — the relationship between rpm and horsepower — for engines of the same horsepower from different manufacturers may not look at all the same. For instance, Mercury’s supercharged, small-displacement Verado engines don’t share the same performance characteristics as Honda’s or Suzuki’s or Yamaha’s more “conventional” four-stroke engines. Hull designs obviously factor heavily in the equation, too. The important point here is simply that there’s a lot more to the choice of power than there appears to be at first. If you’re shopping for a new boat or repowering an old one, do your best to track down performance data for various configurations on the hull you’re interested in. Consider not just optimal cruise mileage, but also the speed at which it’s reached and how quickly efficiency decreases to either side. The differences in economy between various power packages may seem insignificant at first glance, but here’s another way to think of it. In the case of the Regulator 23, the difference in fuel usage between twin 150 hp four-strokes and a single 225 hp four-stroke at an approximate cruise of 28 mph is on the order of 1.5 gallons per hour. If you figure, conservatively, that you’ll put 2,000 total hours on the engine(s), of which perhaps 1,000 will be at cruise speed, that’s an extra 1,500 gallons of fuel – or around $4,500 at current fuel prices! |