The juiciest propulsion systems around power the new high-speed megayachts. 

Somewhere up in the stratosphere of the yachting scene are high-performance megayachts, lumbering along at 40-plus knots with geysers of water blasting out behind like jet contrails. The propulsion systems that push these 200-ton packages of aluminum and iron around at water-ski speeds have become a specialty item in themselves. In the diesel engine department, the Germans - MAN, Deutz MWM, and especially MTU - have acquired a good grip on this exclusive market.

On the other side of the transom are the drive systems, notably surfacing propellers and jet drives. Conventional underwater propulsion gear suffers from excessive bulk and inefficiency at speed. Props and struts add unwanted drag and draft, dulling the performance of planing hulls designed to glorify speed, maneuverability, and shallow water operation. Two companies have almost a stranglehold on drive technology: for jets, it's the Swedish firm, KaMeWa; for surfacing props, it's California-based Arneson Marine, Inc. More on these later.

American diesel manufacturers have not jumped on this particular bandwagon because it's a pretty small wagon, in marketing terms. You need massive engines rated at up to 3,500 hp, yet small and fuel-efficient enough to work well in the after sections of a yacht and give it a decent range. Most yachts don't need that sort of power, and American companies have targeted this slower but vastly larger market. Mega-diesels are boutique items.

Nobody does it better than MTU - an acronym for Motoren-und Turbinen-Union of Friedrichshafen (and Munich) GmbH, West Germany. Their 396-series diesels - which are frequently used in high-performance megayachts - are machined to infinitesimal tolerances, quality-checked with surgical thoroughness, and priced like a flawless diamond. A top-of-the-line MTU 396 diesel can run up to $500,000; the propulsion system on the 31-knot, 126' Atwood yacht, Time, (two MTU 12V396TB94s, with two KaMeWa jets) alone cost $1.3 million. And that was when the dollar was high. These engines are not for the faint-hearted or shallow-pocketed.

MOTOR WITH A MISSION 

MTU is an exotic hybrid. In typically German fashion, MTU was formed by competing diesel makers MAN and Daimler-Benz in 1969 as a propulsion "joint venture" for the production of quick, lightweight diesels for jets, tanks, fast passenger ferries, offshore vessels, yachts, and patrol boats.

MTU, almost wholly owned by Daimler-Benz since it bought MAN's 50% share in 1986, has become Europe's largest - at almost $2 billion in annual sales - producer of ultra-high-speed, high-performance diesel engines for special marine and industrial applications. Minority shareholders are the Maybach and von Brandenstein-Zeppelin families.

''MTU earned its reputation by keeping ahead of the demand for compact power."

Specializing in custom installations, with a minimum 1,000 hp per shaft, MTU's forte is designing and building complete propulsion systems of at least 1,500 hp per shaft for yachts 100' and larger. Marine engines are hand built "to order" at MTU's Plant Two in Friedrichshafen. The modern complex is situated along the shores of Lake Constance, on the site of Count Zeppelin's original dirigible field.

MTU engines have the highest specific power output for their size, making them a logical choice for a megayacht's high-horsepower, low-weight, and compact-design needs, according to John Moore, director of sales for MTU of North America. Power-to-weight ratios are in the 3.28 lbs/hp range. Customizing and microscopic tolerances put MTU's dollar-to-hp ratio atop the field.

MTU designates each engine by the number of cylinders first, the series number second, special add-ons third, and the design index last. Thus, the 16V396TB93 is a 16-cylinder engine from the 396 series (the number "396" equals the displacement per cylinder, in liters, times 100), "T"urbocharged, with external charge air and internal piston cooling (the "B"), and the design index of 3. The "9" applies to high speed yacht applications. (The 94s, or "04"s, are a more recent, more powerful model.)

MTU's marine diesel line includes three series tailored mainly for large and fast yachts, the 331/396 (657 hp to 3,433 hp); the 538 (1,630 hp to 4,120 hp), and the 956/1163 (2,950 hp to 9,924 hp). For lower power applications, Series 183 (173 hp to 591 hp at 2,300 rpm) was added in '87 when MTU assumed marketing responsibility for these small Mercedes-Benz blocks.

CORNERING THE MARKET 

At present, MTU has about 95% of the high-performance megayacht market, although Deutz and, to a lesser extent, MAN, are stepping up their marketing efforts. Published figures state that MTU has installed around 1,000 engines (1,156,000 hp total) on 500 yachts through October 1987. Sales are up for the second year in a row in this small-volume, pricey slice of the diesel propulsion pie. Noting the presence of MTU engines aboard six of largest megayachts displayed at the Ft. Lauderdale boat show, marine account manager Peter Schalk credits this success to MTU's triple-threat "system design/installation/product support" approach to propulsion solutions.

Schalk feels that "selling horsepower with a system to handle it" is the way to go, particularly with MTU's notoriously heavy breathers. "Yacht installations, by the custom nature of their construction, will always see problems, especially with the air and exhaust systems required for mega yacht output." Forty-five MTU engineers are assigned specifically to marine applications.

MTU earned its carpeted engineroom real estate by keeping ahead of the marine industry's demand for compact power. The evolution of today's popular 396 series - from an early '70s introduction of a lightweight 107-hp-per-cylinder NATO Leopard tank engine to the triple-turbo, 12V- and 16V396TB94's, rated at 214-hp-per-cylinder, which came out in 1985-86, parallels the international trend.

The 12-cylinder TB93 chromed-headed MTU engine delivers 1,960 hp at 2100 rpm. 

MTU's stated goal for the newest 396-04 series was to optimize engine energy in the cylinder heads and pistons for improved firing. Using one-cylinder research engines to experiment with various injection systems in conjunction with combustion-chamber shapes and air motion, MTU focused on shortening the duration of combustion. Research was supported by MTU's ability to design and produce its own injection and turbocharging systems.

The lightweight 396-04 engines develop a whopping 50% more output per cylinder than the original 331s, in the same envelope, plus an inch or two for the turbos. MTU's expertise with alloys helped them develop innovations like the three-walled aluminum exhaust manifold, which keeps weight and engine room temperatures down simultaneously. (Hot parts of the turbochargers and exhaust manifolds are also waterjacketed.)

On the 04, a reduction of up to 15° crankshaft rotation reduced fuel consumption by about 5%. Shorter combustion duration helped reduce nitrogen oxide emissions and exhaust noise levels while it improved maximum compression pressure.

A broad torque range - necessary for fast vessels - is provided via single-stage sequential turbocharging employing two sensors. A charge-air pressure ratio of 3.8:1 is achieved by use of milled titanium compressor wheels in the MTU-manufactured turbochargers.

LONG-LASTING

Operational reliability, the overall abiding promise of the German diesel maker, hasn't been compromised in the quest for horsepower. Output isn't figured for big numbers, as MTU fully expects that after ten years every engine should still measure up to its factory output rating. A nice round number, ten years - or 6,000 hours, if you prefer - is the target for a 396-04's first scheduled overhaul.

Quality control is an obsession at MTU. As such, it represents more than 30% of manufacturing time per engine. MTU doesn't bolt two V-8 crankshafts together to make its V-16, like DDA. Connecting rods, matched and numbered from the casting stage, are matched carefully prior to installation.

A digital electronic governor, developed by MTU and field-tested for more than three years, is a strong selling point. All 396-04's have MTU's full microprocessor-based control system which governs, monitors, and controls every shipboard propulsion element, including gas turbine "boosters".

Low fuel and oil consumption derive from engineering standards that call for cylinder cutout, charge transfer and sequential turbocharging systems. On the 396-04, a reduction of up to 15° crankshaft rotation reduced fuel consumption by about 5%. Reduced combustion duration helped cut down nitrogen oxide emissions and exhaust noise levels while it improved maximum compression pressure.

Always pushing known power and longevity limits, MTU continues investigating cylinder and bearings "hot parts." Ceramics work in conjunction with German universities is ongoing. According to MTU Marketing Manager Dr. Josef Fischer, MTU is committed to the marine high-performance market, and an even faster engine series is a only a few years away.

In the meantime, yacht demand for the 396 engines continues to grow. Twin 12V 396 TB93 MTUs drove "Virgin Atlantic Challenger II" to a record-breaking transatlantic crossing averaging 50 mph, despite severe fuel quality problems.

Of the new 396-04 series, MTU's 12V TB94 not only has the same output as the TB 16V TB93, but it's housed in an easier, more compact system with the electronic governor. The first 04's were installed in the 126' Time and an equally-large Baglietto.

KAMEWA'S WATER ROCKETS 

Propulsion, as well as raw power, sets the new breed of megayachts apart from their slower, more serene, transatlantic sisters - equally large but deeper-draft yachts capable of cruising long distances at a stately 8 to 13 knots. Recently launched transatlantics can make 20 in comfort.

At speeds over 25, drag and cavitation caused by conventional prop, strut, and shaft propulsion reduces efficiency markedly. Owners of 30-mph planing yachts are naturally drawn to water jets and surface-piercing drives for their ability to deliver shallow water performance and blistering speed with a high degree of comfort.

Water jets, as their name implies, use an impeller to draw water through a huge inlet on the underside of the hull. Sped by the impeller pump to about two times boat speed, the water is immediately channeled against a stator to stop its rotation, then discharged through a nozzle at the transom to produce thrust.

Steering is generated by rotating the nozzle up to 30°, port or starboard, which produces directional thrust. For single and multiple jet installations, an independent shift/throttle assembly controls each jet. Throttles rotate on swivel bases to indicate the thrust angle of the jet they direct. A console switch transfers linked steering power to the ship's wheel, which connects to the autopilot. A separate set of "emergency" backup joysticks are linked directly to the jets.

Raising or lowering the hydraulic nozzle cover - called a "bucket" - controls the boat's speed and direction fore or aft. Completely retracting the bucket under the nozzle permits the full stream to pass unhindered; raising the bucket deflects the stream toward the bow, creating reverse thrust. With the bucket in a half-mast position, the jet stream is split, causing the boat to hover in position, regardless of throttle setting.

KaMeWa manufactures its own custom electronic control systems to be used with all types of vessels. The system is flexible enough to accommodate an infinite number of steering stations, although two or three is the norm for a megayacht.

Developed in 1980 for commercial catamaran ferries, KaMeWa's high volume jets first appeared on high-speed patrol boats. A year later KaMeWa received a fateful water-jet retrofit order from the Palmer Johnson yard in Sturgeon Bay, Wisconsin. Once installed in the 98' Fortuna, a triple-jet high-performance yacht built by Palmer Johnson for King Juan Carlos of Spain, JPS became a key propulsion component of the world's fastest (51-knot) yacht and the power of choice for high-performance megayachts to follow.

PERFORMANCE PLUS 

Anders Pettersson, KaMeWa marketing manager, says the company has a hefty share of "nearly 100%" of megayacht jet installations in excess of 1,500 hp per shaft. KaMeWa's absolute minimum system is 500 hp per jet, but Pettersson considers 1,000 hp per jet to be the practical base. The boat must be a minimum of about 60'.

Rolf Svensson, JPS (jet propulsion system) development and project engineering manager, and his crew determine the most efficient rpm range and develop custom intake geometry for each installation. KaMeWa works with the yard to fabricate the intake out of hull material. A critical piece of engineering, the intake has to maximize flow without creating unnecessary drag on the hull. Its distance from the transom is determined by the hull's resistance curve.

Besides customized mega-thrust, KaMeWa JPS units feature welded stainless steel construction and a reduction gear mounted between the JPS pump shaft and the engine. KaMeWa jets operate across a 608 to 1,600 range, with 1,100 rpm as the average.

Jet power has distinct advantages for high-speed megayachts: excellent maneuverability, good shallow-water capability, no hull appendages, no resistance to debris in the water, and a low level of noise and vibration.

The most widely acclaimed virtue is shallow-water operation. Many prime cruising grounds like the Bahamas have an average depth of 6' to 8'. At ports of call, large deep-draft yachts are often forced to anchor out - sometimes outside the harbor itself (particularly inconvenient for offloading Mopeds and a Rolls-Royce).

Jets keep the big boats off shallow bottoms because there's no underwater gear adding draft to the megayacht's relatively flat hull profile. At speed, the boat's transom is dry and the jet nozzle is completely above the water's surface.

LOW MAINTENANCE 

KaMeWa's eight year track record proves that jets aren't highly susceptible to damage from ingesting sand and debris. The company's field representatives report that impeller blades with 20,000 hours of commercial service are still not showing wear. When necessary, impellers can be removed without docking the boat.

KaMeWa jets provide other mechanical benefits over propeller systems. There's no power loss in a turn because there's no change of engine rpm. With a standard prop, resistance causes shaft speed to drop when the boat slows down. Even at full stop, the shaft speed of a jet unit drops only 1% or 2%.

According to Svensson, jets reduce maintenance because they don't stress the engine, clutch, or gearbox at low speeds, and reversing is done by the jet nozzles, instead of the engine. He says it's simply not possible to overload or overspeed a diesel engine with JPS because " ... the power absorbed by the pump is only dependent on rotation speed of the shaft, not vessel speed."

Unlike conventional prop performance, jets deliver full engine power and thrust throughout the entire speed range. This makes it possible to utilize full engine power during single shaft operation and helps explain why jets are the only system for ultrafast megayachts. A jet is the only propulsion system that can combine diesel and turbine power in a single installation - with all power plants operating efficiently at full throttle.

Jets produce a stepless transition - from full ahead, through idle to full astern -continuously and in a matter of minutes. While all KaMeWa systems are designed to the "crash stop" thrust-bearing standards of the world's navies, the yacht's guests, crew, and contents may be somewhat less durable.

GAS TURBINES AND JETS  

Current top-end KaMeWa jet capacity is a turbine-driven 15,000 hp per shaft. Experience with the Shergar, the Aga Khan's 152' superyacht, and Fortuna paved the way for KaMeWa's unique diesel/gas turbine design program. (Shergar is often cited as an example of how long-lasting raw jet power can be. All three jets have been functioning successfully for more than four years since the Aga Kahn took possession of the extraordinary megayacht.)

Shergar's port and starboard MTU "wing" diesels each power a size-63 KaMeWa jet. This combination alone provides excellent low-speed cruising at 15 to 16 knots, as well as turning power for the vessel. Centered in the hull, two gas turbines pour 14,000 hp into a single 112 cm "booster" jet that isn't equipped with a steering bucket. In consort with the wing jets, this 112 cm fixed outlet unit delivers the overdrive to propel the 240-ton Shergar at speeds in excess of 45 knots.

Jets greatly reduce cavitation, the main source of erosion to the vessel's wetted propulsion system, caused by collapsing "cavitation bubbles" around the metal. The minimal cavitation produced by jets also decreases hull vibration. Pressure waves striking the hull not only produce an uncomfortable ride, but they're capable of cracking the hull plating over time.

KaMeWa originally reported that jet efficiency doesn't greatly exceed prop efficiency below about 30 knots. But recent comparative field tests "showed a remarkable performance improvement starting at 20 knots. At 30 knots the JPS reduced fuel consumption by 15%, compared to a propeller with 66% efficiency in this condition." As an added plus, internal noise levels were reduced by 7 to 10 dB-A.

On Kalamoun, a glamorous 120' patrol boat in the Aga Kahn's fleet, actual performance at speeds above 13 knots was proven to be 7% to 8% more efficient with jets compared to the earlier prop version. At the other extreme, KaMeWa reports that their jets achieve 70% total ship propulsive energy at 45 knots, resulting in an excellent 50% to 60% vessel efficiency at that speed.

ARNESON'S UPWARD MOBILITY  

Arneson, a relatively new presence at the dock, is the only megayacht speed supplier to work up the LOA ladder. The original Arneson surface-piercing drives with high-performance cleaver props were designed by racers for racers, according to Arneson chairman Craig Dorsey.

Since taking the helm in '85, Dorsey has broadened the company base by marketing drives with low-rake props to a handful of passenger ferries, police and patrol boats, as well as to builders of megayachts up to 80'. In the meantime, the last two Miami-Nassau-Miami offshore races were won on stock, fully warrantied Arneson surface drives.

The present Arneson Marine, Inc., is a "consolidation" of the company which had fragmented under Howard Arneson. Through mergers and complex negotiations, Dorsey finally secured the rights in '85 to Arneson's patterns, patents, and licenses held variously by Castle & Cook, Borg Warner and Second Effort Performance Products.

Dorsey took over amid a lot of confusion. By his own assessment there was little continuity of production or service. When he analyzed the product, he didn't like what he found. He weeded out unsuccessful applications. Race models that were designed to go straight ahead at 130 mph - but didn't reverse well - were segregated from the production program. Dorsey also insisted on stricter torque limits for each drive model. Dorsey says the beefed up drives are targeted to diesel installations of up to 5,000 hp, with a rating of 23,000 to 25,000 ft/lbs of torque. Arneson engineers also say the drives have been completely re-engineered from within, despite the fact that they don't look different from prior models.

"To arrive at a bulletproof product," Dorsey says, "the company created new patterns and tooling and adopted a new shaft design which contributed to increased torque rates of 20% to 25% depending on the model." He claims it has increased the life of the drives 20 times over. Manganese bronze models were added for corrosion resistance to saltwater.

THE RIGHT PROP 

Another improvement is prop selection. Arneson stocks 450 sizes of propellers that roughly fall into three basic styles. Megayachts use the low-rake, round or "elephant ear" surface-drive prop. With three, four or five blades, it resembles a normal propeller and accounts for 80% of Arneson's installations. With 10% each, the straight edged "cleaver" is designed for gas-powered boats reaching speeds over 60 mph, while the slightly de-tuned "high-rake" prop is used with gas engines at speeds of 35 to 60 mph.

Cross sections of surface-piercing propellers are different from underwater blades. Not airfoils, they have a slightly cupped, cambered shape, with the leading edge flaring less than the trailing edge. Most have 10% to 15% more diameter than underwater props.

Standard propellers develop maximum thrust in the lower half of their rotation. Surface drives use surface-piercing propellers which operate 50% submerged in relatively calm water behind the transom at planing speed.

''Raw power sets the new breed of megayachts apart.'' 

Surface drive systems reduce drag at planing speeds, since underwater appendages are reduced to a fin tip and a portion of the prop blades. Arneson claims its surface-piercing props eliminate cavitation because they ventilate, rather than cavitate, beneath the hull. Reduced cavitation also means less vibration, a quieter ride, and longer prop life.

While a gent named C. Sharp patented the first surface-piercing-type prop in 1869 (as a possible shallow water alternative to the paddlewheel), it proved too unstable. By the '40s, the emphasis on surface-drive development gradually shifted from shallow-draft uses to high-speed applications.

SloMoShun, an unlimited hydroplane equipped with surface-piercing props ran at speeds of 175 mph during the 1950s. Although squirrelly to control, the drives found wide acceptance in racing. Drawbacks to their general acceptance included torque effects from a fixed level of prop submergence - making it tough for planing boats to get the prop out of the hole - plus serious steering control problems.

THE INVENTOR 

In 1970 Howard Arneson, inventor of the Arneson Pool Sweep and an avid offshore racer, created an "articulated," two-way adjustable Arneson Surface Drive to overcome fixed drives' worst habits. Shortly after that he began serious production of surface drives.

The heart of the Arneson Drive is its trim capability. Drives can be adjusted vertically for optimum submergence, despite varying loads, speeds, and sea state conditions on a given day. To achieve positive prop thrust steering control, Arneson developed and patented a ball and socket connection which allows the prop shaft to be rotated hydraulically to either side.

From a design standpoint, Arneson surface drives have a lot of desirable features. They have less than 20 moving parts arranged in a simple system. Power loss to the prop is minimal since there's no "Z" drive changing the direction of power through gear sets.

Although jets are still the preferred drive system for the super megas, Arnesons are becoming more widely recognized for their efficiency and controllability. Some builders even prefer the Arneson over the jets. "If somebody came to me and asked, what should I use to go the fastest, get the most power out of my engine, no question I would say the Arnesons," said Joe Langlois, chief designer at Denison Marine, a high-speed yacht builder.

Most of us, of course, would never dream of putting in the sort of engine/drive combinations these megaboats like to eat up. Too big, too expensive .... Naw .... Who'd want to bother? On the other hand, maybe a couple of big 3,500 V-16s in your fishboat might get you out there a little quicker. Might win a tournament or two, also .... Wonder what an MTU would look like in a Hatteras? Awesome.

This review/article originally appeared in Boating Magazine, February 1988 and is written by Dugan Meluso. For more great powerboat reviews, visit their website and subscribe at: http://subscriptions.boatingmag.com/