“The aim of the new drivelines is to handle the new engines’ higher output and, notably, their even more powerful torque,” Benny Hedlund, Technology Project Manager at Volvo Penta, and adds:
“In particular, the torque produced by these engines imposes tremendous stress on transmission and propellers. A normal drive can’t handle the stresses, so this led us to develop drivelines specifically for these diesel engines. The driveline is designed to power a 9-ton boat at a speed of 20 meters per second!”
Exhaust pipe through the gearbox
Volvo Penta uses the drive as an exhaust and cooling-water channel so that everything is integrated in a single unit, removing the need for additional inlets through the hull. The new marine diesels feature a large exhaust and require considerable amounts of cooling water.
“Put simply, you could say that we build a gearbox and run an exhaust pipe straight through it. The new drive design copes comfortably with extremely high exhaust flows and we’ve reduced back pressure by 30%, thereby boosting engine output. Also, cooling water capacity is twice that of previous models, as required by the new engine series,” notes Benny.
Completely new steering
The control and feel gained by the driver are some of the key aspects of safety and boating pleasure. DP and DPR feature servo-assisted hydraulic steering as standard. This completely new system offers peak precision and high reliability. The powerful servo has a special feedback valve that gives the driver a feel of control resistance and response. That adds zest to driving and provides superb sensitivity. The control cylinders are mounted externally and act directly on the drive. In the case of a twin installation, the drives are linked up with a tie-rod. The tilt angle in the drive is a full 50 degrees, but can easily be limited to avoid any damage to a bathing platform.
Advanced development tools
Volvo Penta’s driveline development department comprises two units – one in Göteborg Sweden, and the other in Chesapeake, in the U.S.
The work involved in developing the new drives proceeded via intensive trans-Atlantic cooperation, using advanced computer-based development tools.
“Using advanced technology we can go very far and finalize everything at the blueprint stage before we begin testing. In our efforts we’ve used all the software programs that offered any potential and have even borrowed from the automotive industry. We’ve had no trouble in coordinating activities in Sweden and the US, and this, of course, also applies to the development of the actual engines,” explains Mikael Åsberg, Technical Project Manager at Volvo Penta in the U.S.
The various sub-solutions were continually tested at Volvo Penta’s test centers at Krossholmen, in Göteborg and in Suffolk, in Virginia. Hydrodynamics were tested using a full-scale model in a cavitation tunnel.
“The tank tests are important in verifying our computations. The tank tests also allow us to utilize our experience and make minute adjustments to ensure we get the maximum possible. Only when we are absolutely confident of the design do we build prototypes for testing in boats, with which we conduct a wide-ranging test program. Among other trials, we run a large number of 300-hour nonstop tests – the equivalent of a Gothenburg-New York round trip. Overall, we’ve clocked up almost 15,000 hours in various types of boats, occasionally in extremely demanding weather conditions,” says Benny Hedlund.
New propeller series
The new drives feature a completely new propeller series. Volvo Penta develops its own propellers and has unique expertise in terms of propellers for drives in general and for Duoprop in particular. Volvo Penta conducts its own R&D, using advanced computer simulation, plus laboratory tests in cavitation tunnels and boats. Kåre Jansson is responsible for propeller development.
“Only by developing propellers specifically for each drive and engine can we achieve maximum efficiency. The new propellers are customized to match the character of the new D4 and D6 engines with their powerful torque and relatively low engine rpms.
Developing propellers for Duoprop differs considerably from conventional drives with single propellers. The front and rear propeller must interact and perform an equal share of the work, otherwise transmission wear will increase. In this case, Volvo Penta has gained a great deal from its unique experience, permitting it to build ever more sophisticated computation systems.
“We’re continually shifting the boundary of the possible. We can now make far greater progress in computations than in the past, which means that when we begin testing in tanks we’ve only got a few minor details to adjust. The new propellers work completely without cavitation, engine power is used entirely for driving the boat forward. The front unit has three blades and is slightly larger than the rear four-bladed prop, which functions inside the propeller stream created by the front unit. This balances the forces and provides extremely high efficiency throughout the power range,” explains Kåre.
The propellers are available in nine versions, with each set designed for a specific speed.
“We’ve developed propellers for the DP drive to match speeds ranging from 25 to 45 knots. Each propeller pair is tailored for its particular speed and thus provides high efficiency with a good margin against cavitation. Boat builders or boat owners don’t need to consider size and pitch, the series is completely systematic, so you simply select on the basis of the boat’s top speed,” adds Kåre.
The propellers for the DP drive are made of nickel, aluminum and bronze alloy.
“This alloy is a very common material in propellers for inboard installations. We are the first to use in a drive. The primary advantages are that it permits the manufacture of high-precision propellers and copes with high engine output. Corrosion resistance is also very good. We’ve developed a stainless steel series for DPRs designed for speeds above 45 knots.”
The best to date
According to Volvo Penta’s tests, the new drives – matched with the new engines and propellers – provide a highly impressive performance
“We were really competing against ourselves in the development period. The existing drive was a good candidate in terms of efficiency. But because we ourselves control the entire chain and can adjust all components to match particular conditions, we’ve been able to boost efficiency to a new level. In addition, the drive is larger, stronger and handles higher output and torque than previous models. Moreover, I must say that the design is really a success, this drive is simply beautiful,” concludes Benny Hedlund.