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With the exception of outboard motors, all other types of power transmission from the engine room have a component that is more or less hidden. Here are the four main types to choose from, along with the strengths and weaknesses of each. We have deliberately avoided mentioning water jets and surface propellers due to their limited use.
It is the most common type of transmission; many consider it the most reliable, and it is certainly the one that requires the least maintenance. But even in-line drives have their weaknesses, such as possible misalignment between the propeller shaft and the engine-gear unit, or wear on the bushings and stuffing box. From a practical standpoint, they are less maneuverable in tight spaces (which is why bow thrusters and joysticks have become so widespread, even on medium-sized boats), and the engine’s location on board—shifted more toward the center of the boat—reduces the living space inside.
Volvo Penta’s IPS propulsion systems feature forward-facing propellers, which are more efficient because they are not affected by turbulence at the stern or along the axis; this allows for the installation of lower-horsepower engines while maintaining the same performance, resulting in some fuel savings. Optimal performance is achieved when used with a hull specifically designed for this type of drive system. In any case, Volvo Penta also deserves credit for introducing the joystick for mooring maneuvers—without touching the throttle levers—which has been a key factor in their success.
Other manufacturers’ response to IPS was swift. In particular, Cummins Zeus, ZF Marine POD 4000, and CAT Three60 POD 650 presented three types of propulsion PODs, i.e., those with propellers that push. The main advantage, compared to IPS, lies primarily in the protection the POD itself provides for the propeller. In both cases, the two PODs (port and starboard) move at different angles to optimize the turning radius and facilitate maneuvering. They often also have integrated trim tabs.
To further meet the demand for easy maneuvering, classic stern thrusters (also known as inboard-outboard units) have evolved. There are joysticks that allow them to work in conjunction with the bow thruster, while Cummins MerCruiser offers the Axius electronic control system, which enables the stern thrusters to be moved independently of one another—a significant advantage during maneuvering. All of this, of course, is controlled with the usual joystick.
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