Forward Facing Props

When push comes to shove, pull is better

  In the public relations world, a slight modification is often termed 'new' and 'revolutionary.' But Volvo Penta's new IPS (inboard performance system) isn't just a slight design chance to make something old seem new. With forward facing props ... yes ... forward facing props, this diesel propulsion system can transform a sedan bridge yacht into a performance cruiser.
  With precise handling, tops speeds above 30 knots, and fuel efficiency improved by as much as 30 percent, the IPS is a genuinely new inboard motor design. "You get higher RPM, better fuel economy, and better control of the boat," said Capt. Jeff Goubeaud, of Long Island, N.Y., at Jackson Marine's Boat Show Sunday, August 21. Goubeaud skippered a Regal 3860 Commodore Express equipped with the new IPS to Hances Point so boat show-goers could get a look at the new system. "There's very, very, minimal cavitation because of the way the props are facing forward, and smaller blade diameters, tip diameters, that don't break up the water as much," added Goubeaud, a consultant for East End Marine Services. "As far as the boat construction goes, it gives you a smaller engine compartment, which means greater interior space for the size boat." The IPS features propellers that are turned in the direction in which the boat is traveling. Volvo Penta has placed pulling and counter-rotating propellers through the bottom of the hull, which provides the boat with significantly improved performance compared with traditional shaft installations. 
  According to Volvo Penta's calculations, the efficiency of the new system provides the following improvements compared with boats equipped with mechanical diesel engines and traditional shaft installations: 
* Up to 20% faster 
* Up to 15% greater acceleration
* Up to 50% lower noise and vibration levels
* Precise handling and control at all speeds.

Efficiency and performance
  Around 30 knots, the conventional propeller shaft system starts to lose efficiency (see the lower curve in the diagram). Low efficiency means that more installed power is needed, which, in turn, results in increased fuel consumption, reduced cruising range and more exhaust emissions. The IPS from Volvo Penta offers a solution to this, with outstanding efficiency over the whole speed range, 25-45 knots, (see the upper curve in the diagram). At 38 knots, efficiency with the IPS system is as much as 35% improved. The result is increased speed, reduced fuel consumption, faster acceleration, and less emissions per nautical mile. In a comparison between two identical boats with the same installed diesel power the IPS achieved a top speed of 6 knots faster, a reduced fuel consumption at 30 knots of 30 percent, and an improved acceleration by 15 percent out of the hole.

Why is IPS superior?
  Looking beneath the surface, you can see what happens: IPS dual propellers offer increased blade area with half the load on each propeller, and a smaller propeller diameter with minimized tip losses and cavitation. Furthermore, the propeller system prevents rotational losses and does not create any side forces. The thrust the propellers produce is horizontal with all the force driving the boat forward. The propellers are at the front of the IPS unit, working in undisturbed water with a minimum of pressure pulses affecting the hull. A conventional shaft system loses efficiency with the thrust angled downward and the propellers working in water disturbed by the propeller bracket and shaft.

Maneuvering and handling
  Maneuvering, handling and driving pleasure is where IPS truly sets a completely new standard. Forget everything you know about complicated docking maneuvers-with IPS it is easy and safe even with one engine. At higher speeds you will enjoy immediate response to driver commands with safe and predictable handling. The reason for this amazing maneuverability is:
* The IPS units are steerable, turning and pointing the entire thrust in the desired direction. This results in much higher efficiency and far greater response to driver commands.
* Two counter-rotating propellers on each propulsion unit negate lateral forces so tracking is completely straight. * Electronic controls give a distinct and precise feeling, and shifting is immediate. Thanks to progressive electronic steering, the wheel spins easier at low speed, further reducing driver effort.

Comfort
  Onboard comfort is one of the main factors in boating enjoyment. Minimal amounts of sound, vibration and exhaust fumes make life aboard much more pleasant. IPS technology offers major improvements to many comfort-enhancing factors.
* The propulsion forces and vibrations are absorbed by the combined rubber suspension and sealing. 
* A U-joint in the drive shaft makes it possible to have the engine soft suspended, which efficiently reduces engine vibrations.
 * The propellers are working in undisturbed water with no cavitation, and have good clearance from the hull. 
* There are an increased number of propeller blades to distribute the forces. This means the pressure pulses created by the propellers have little effect on the hull. 
* Exhaust fumes are minimized. First of all, the new engine has very low exhaust emissions, and secondly, the exhausts are emitted through the propulsion unit into the prop wash and carried well behind the boat. A sound test carried out with two identical boats, one with IPS and one with conventional shaft system, revealed sound aboard the IPS boat was much lower, reduced by up to 50 percent. 
  While demand is high for the new Volvo Penta IPS drive, the company will still manufacture conventional stern-drive diesel engines. The larger draft needed for the IPS system may make it inappropriate for certain applications. "IPS drives are always down, so you'll always have a deeper draft," said Goubeaud. "You have to watch out for shallow waters. Stern drives still give you the opportunity to raise the drive up in shallow water so you don't hit bottom." 

The End