OCEAN Magazine's Anthony Twibill reminds us that the ultimate test of high speed marine propulsion is not as simple as just the engines on board but what delivers that power.
' Step aboard the stunning silver Pershing 115 superyacht, or for that matter the equally alluring Pershing 90 you are entering a very special realm of style, luxury, and exotic design afloat. You are also about to experience the fastest leather lounges and luxurious Italian furnishings ever to blast their way across the Mediterranean at speeds of up to 55 knots.'
High technology marine propulsion systems such as waterjets and surface piercing drives have applications across many military and commercial marine operations with high speed requirements. But nowhere is the dedication to speed on the water more beautifully executed than in high performance, luxury leisure boats such as the Pershing.
OCEAN was invited on board Pershing’s top-of-the-line luxury vessels, the 115 and 90, for high speed sea trials to compare these thoroughbreds and their high tech yet totally different drives. Both (magnificent) vessels took to sea off Cannes and played a high speed game of cat and mouse, throwing their massive silver hulls through the tightest turns, carving massive wakes of white water and sky high rooster tails, then racing each other in exhilarating straight line dashes side by side at close to 50 knots. Unlimited excitement delivered by remarkably different propulsion systems.
The Pershing 90 can propel its 66 tonnes (unladen) to 44 knots, driven by twin MTU 2435 HP turbocharged common rail diesels coupled to Twin Disc Arneson surface piercing drives. Arneson drives feature large diameter, fully trimmable props which are designed to achieve maximum speeds by piercing the water’s surface behind the transom of the vessel. The Arneson drives are perhaps as well known for their characteristic rooster tail as they are for their high speed performance characteristics and the extraordinary directional stability, or traction, they command in a turn as the trimmable drives angle the propellors in the precise direction of motion.
A conventional underwater propeller system on standard shafts can allow speeds of about 35 knots or, in a few cases, up to 40 knots. But to achieve speeds in the realm of 50, 60 knots or greater requires propulsion alternatives such as surface piercing propellers on drives that can be trimmed both horizontally for steering and vertically to allow the ideal thrust angle for varying load and sea conditions.
This system allows for the propeller submergence to better match the engine power output, delivering more thrust. Together with larger propellers designed for surface operations, Arneson drives provide speed increase and fuel efficiency savings of up to 30 percent over conventional set-ups. The Arneson drives behave like a cross between an outboard and a conventional shaft. But they eliminate some of the issues of typical shaft installations such as blade tip clearance from the hull (reducing the optimal propeller size), maximum vessel draft, shaft angle and engine location. These issues can cause propeller cavitation and excessive vibration due to low tip clearances, or a steep shaft angle inducing loss of efficiency and consequent parasitic drag. All become much greater as speed increases.
Hence, Arneson drives are regarded as one of the most efficient marine propulsion systems available, not just a high speed propulsion technology. Twin Disc Inc. is the international manufacturer and distributor of the drives and offers them in nine different model sizes for engines up to 10,000 HP.
The waterjet is another proven, high speed propulsion technology and is the system chosen for the largest Pershing yet designed. The Pershing 115 is an even more extraordinary package and almost defies belief. Its revolutionary and seductive design, ultimate performance and luxury, places it at the very top of its class in the elite world of superyachts.
The 115 may only be 25 linear feet longer than its stablemate, but the volume and performance of the boat is in an entirely different class. The big Pershing displaces 114 tonnes unladen (almost twice the 90) and up to 133 tonnes when loaded. It is no lightweight and you might imagine with those extra staterooms, luxury appointments and consequent kilos, that the legendary Pershing performance could be compromised? Never! After all, this is the flagship.
The 115 has a top speed of 55 knots on its high lift Pershing hull, designed to run Kamewa Rolls Royce hydrojets as propulsion units, powered by twin MTU diesels totalling 7,400 HP. Plus it features a centreline mounted 5,100 HP Textron Lycoming turbine (jet engine) that boosts speed to 55 knots from its ‘nominal’ 42 knot top speed if only the diesels are engaged.
To achieve such speeds a 115 foot vessel has high power requirements and 12,500 HP total installed power with the gas turbine engaged is quite sufficient. Converting all that power to speed on the 115 is theKamewa waterjet system from Rolls Royce. Used for many years on larger, high speed superyachts and military vessels, the system principally consists of a water inlet duct, an engine driven pump with an outlet nozzle shaping the jet and steering and reversing gear. Steering is accomplished by a nozzle which deflects the waterjet thrust in the required direction. A reversing bucket in the steering nozzle deflects thrust forward when reverse is required.
Waterjets are relatively smooth and vibration-free when compared to conventional shaft and propeller arrangements. Waterjets expel tonnes of water rapidly aft (or in the direction of the nozzle) providing forward velocity. As there are no propellors or rudders, or even shafts, there is very little appendage drag from exposed rudders, shafts, struts or even propellers themselves which would otherwise contribute to parasitic drag at high speed.
Waterjets provide smooth acceleration and cruise performance and the ducted steering nozzles provide good directional control at speed. As there are no propellers in the water, there is less grip on the water and during high speed turns the stern of a waterjetpropelled vessel tends to slip more than a conventionally propelled vessel. But for smooth, low-vibration, high-speed excitement, the waterjet propulsion is in a class of its own.
The Pershing 90, on the other hand, adopts an entirely different propulsion model to achieve its equally exciting performance. Which is better? Neither really; they are simply different and suit the individual requirements of each vessel.
The Pershing 90 with its Arneson drives is best compared to a high performance Italian sports car. The pure exhilaration of storming across the sea at 44 knots, throwing a 50 foot rooster tail into the sky from the trimmed up props, then tossing the 66 tonne hull through hairpin turns casting aside tonnes of white water from the Arnesons’ relentless grip on the water is unequalled for pure excitement.
The Pershing 115 Turbine is comparable to the ultimate Grand Tourer. It is awe-inspiring, luxuriously appointed, supremely powerful and effortlessly smooth and fast.
With the gas turbine booster engaged and the twin MTU diesels at maximum rpm, there is nothing of its displacement class that can match its speed and sea-keeping. Ultimately, the waterjets are a more subtle form of high speed propulsion than the extreme performance of the Arneson drives which are unmatched in turns.
But if a decision had to be made based on th