What’s old is new again.
How many times have you heard that phrase when news-channel talking heads are referring to music, art, and clothes? (Like when bell bottoms made a comeback? Ugh.) But unlike with fashion, it sometimes makes sense to revisit the past with boats. At least Hatteras Yachts thought so shortly after it launched its 77 Enclosed Bridge Convertible a few years ago.
PMY was the first magazine to bring you test data on Hull No. 1 (“Big Dog,” March 2007). The boat was a behemoth, powered by twin 2,400-hp MTU diesels, and she made a solid 30 mph at a 2000-rpm cruise, while at WOT, she sprinted to an impressive 37.5 mph with a full 3,000-gallon fuel tank and six people onboard. This was a beast built for bluewater. But Hatteras has never rested on its laurels. So it looked into how it could improve the performance of the 77 by employing triple diesels, something it had actually done more than 17 years before on a series of megayachts.
In the late 1980’s, Hatteras introduced a number of motor-yachts ranging from 92 to 130 feet LOA. Now it takes a lot of horsepower to propel yachts of that size, and Bruce Angel, vice president of product development and design recalls that all the engines that were available at around 2,100 hp back then also weighed 20,000 pounds or more. Add massive running gear with 60-inch-diameter, multi-blade propellers and you have a heavy vessel and two engines working very hard to propel it.
So the engineers at Hatteras developed an alternative proposal: three 1,450-hp Detroit Diesel 16V-92 two-stroke diesels, which, in theory, would burn less total fuel and run at a reduced load at cruising speed while also reducing total engine weight. Their theory proved correct, and a total of nine triple-engine yachts were eventually built.
But times changed and engine manufacturers found ways to get more horsepower from fewer pounds. Today, 1,800-hp diesels from MTU and Caterpillar come in, at, or below 9,000 pounds—less than half of what they did 20 years ago.
When Hatteras recently took a liking to one such motor, Cat’s 1,800-bhp C-32 ACERT, it revisited the idea of trying triple engines for its current flagship convertible. Or as Angel puts it, “We went back to the future.”
After looking at archived engineering and tank-test data, engineers decided that three engines could benefit the 77-footer. They calculated that increasing total horsepower from 4,800 (with twins) to 5,400 (with triples), would improve the 77’s speed and operating efficiency while reducing both total fuel burn and engine wear and tear. There would be a 1,500-pound weight penalty with triples, but this would theoretically be offset by the fact Hatteras now saves 5,000 to 7,000 pounds per boat by using resin infusion, something not done on Hull No. 1. Indeed, in the standard hand-laid construction scenario, the glass-to-resin ratio was about 40-60; with the current infusion method it uses now, the builder sees the exact opposite ratio: 60-40.
Installing triple Cats would also enable Hatteras to shift the longitudinal center of gravity aft by a few inches. While that might not sound like much, Hatteras says that moving that much weight aft even just a bit lets this 175,000-pounder get up on plane at around 1300 to 1400 rpm. And during our test runs, her acceleration felt almost like that of an outboard-powered boat, quite a feat when you consider her size.
To further help planing, Hatteras modified the running gear. While the wing engines have the same wheels, the center one does not because it’s directly aft of a small keel. Water runs over this keel and into the prop faster than it does with the wing props. Hatteras found that adding about two inches of pitch to the center prop loaded it about the same as the wing props.
After hearing this explanation I couldn’t help but wonder: “Why didn’t they just use pod drives and save space—and perhaps some engineering and installation time?” Angel explains that Hatteras invests a lot of time and effort into maximizing a prop’s efficiency. Optimum propeller performance is at around 73- to 74-percent load, and each percentage point nearer to that number is worth about 0.4 knots. “That’s what we shoot for,” says Angel. He adds that the propellers available for current pod installations are limited by horsepower and torque and don’t offer an efficiency advantage over Hatteras’ props. However, when this obstacle is finally overcome, and there’s broader acceptance of multiple (more than two) propulsors and the close-quarter maneuverability that pods offer, he believes they will replace conventional propulsion systems.
I was also thinking, “Sure, the data shows that three engines can increase range, reduce load, and optimize overall running efficiency, but how do you fit them in a space built for two?” When Hatteras placed the center engine forward and dropped the wing engines back, it opened up 17-inch-wide walkways between the center motor and the outboard ones. Caterpillar also folded the Airsep canisters up and in a few inches on each engine, which widened room at the top. So when I walked into the engine room of this 77—named Bodacious by her owner and well-known big-game angler Pat Thomas —I was impressed that I could get around all three powerplants.
The three engines’ large exhausts, which you do have to step over to get to the walkway, feed into two common chambers on centerline and then exit underwater at the stern. The front of the center engine ends at the forward ER bulkhead, but all in all, key access points on all three motors are easy to reach. And at 5'7" tall, I had standing headroom.
I also wondered about how all those ponies get enough air to achieve their maximum potential, especially since there are no hull-side vents. It turns out combustion air enters under the cockpit gunwales and is fed to the engines via two 20-inch fans, one in each aft corner of the ER. When the engines are shut down and it’s time to cool off the space, one of the fans can be reversed to suck out hot air while the other blows cool air in.
All in all a cool setup, but only a sea trial would show if three engines would outperform twin diesels. So on a sporty May morning the 77 sped out of the inlet near Beaufort, North Carolina, with Thomas, several of his friends, a few Hatteras reps, and yours truly, to see what Bodacious could do. In the six- to eight-footers she easily made 25 knots while no one on the enclosed flying bridge even flinched at the spray coming up and over the boat. The only other vessel I saw nearby was an aircraft carrier. (That kind of says it all.)
I already knew this hull could handle big seas, but to get speed numbers we’d need to get into protected water. So after dropping off Thomas, it was time to collect data. She cruised effortlessly at 35.7 mph and 2000 rpm with only 66 percent load on three motors—they were working, but not too hard. At this speed, total fuel burn was 200.9 gph and she made 0.18 mpg. If the burn sounds high, remember that’s for three engines. By way of comparison, cruise speed on the twin-engine 77 was 30 mph with a total fuel burn of 182 gph, but efficiency was just 0.16 mpg. I later calculated range at that 2000-rpm cruise speed to be 464 statute miles, about 20 miles more than the twin-engine 77 at the same rpm. In addition, the triple-engine Hatteras did all this while carrying 100 fewer gallons of diesel (3,000 vs. 2,900). At WOT, my test boat made 41.5 mph while consuming 278 gph, giving her a range of 389 miles and earning 0.15 mpg. By contrast, the original 77 had a top hop of 37.5 mph while burning 240 gph, and a range of 422 miles (0.16 mpg).
According to my data, between 1500 and 2250 rpm, the triple-engine 77 has the edge on fuel efficiency versus speed (see “By the Numbers,” this story). But at the very bottom and top of the rpm range, the twin MTUs edge out the three Cats in performance. Taking into account that this boat will spend the majority of her running time in this middle-rpm range, the three motors make a strong case in this application. And when it comes to cost, Hatteras told me that the difference between the twin MTUs and the triple Cats is “not very far apart,” although it didn’t provide hard numbers, as per company policy.
The bottom line is that our test shows that triple engines are a viable option in the 77. Admittedly, from a maintenance standpoint, there’s another motor and set of running gear, plus all that other fun stuff to deal with, but in exchange you get another level of redundancy and thus security. You can also run the motors in myriad configurations for fishing or to get home should one (or even two) go down. When you add the safety factor to the enhanced operating efficiency, using triple diesels for your 77 proves that three’s company, not a crowd.
2/1,800-bhp Caterpillar C-32s; Cat electronic engine controls; choice of satin or high-gloss cherry, African mahogany, or maple veneer for bulkheads; Pompanette helm seat; 32" plasma TV on flying bridge; Cruisair 118,000-Btu A/C; 2/27.5-kW Onan gensets; ram-air ER induction system w/ 2/20" axial-vane fans; transom door; 2/65" fishboxes; 10-cu-ft. bait freezer
3/1,800-bhp Caterpillar C-32 ACERTs; teak cockpit sole; 1,400-gpd Sea Recovery watermaker; Eskimo ice maker; American Bow Thruster bow thruster; 2/Underwater lights; 4/VEI 15" helm displays; Furuno NavNet system; Northstar 6100i chartplotter; 2/Icom VHFs; Fisher Paykel dishwasher
Cabin: 1 master, 4 guest
Test Boat Specifications
- Props: 3/1,800-bhp Caterpillar C-32 ACERT diesels, ZF marine gears w/ 3.5: 1 ratio, 45x81 6-blade Nibral props
This article originally appeared in the August 2009 issue of Power & Motoryacht magazine.