PropulsionSystems

Propulsion Systems

Design Considerations

At its simplest, a propulsion system consists of an engine, reduction gear, propeller shaft and bearings, and a propeller. Most designs feature a long rigid drive shaft to get the correct propeller angle. AAmpere (amp), SI unit of electrical current lot of attention is paid to getting the shaft and engine aligned to prevent twisting and vibration. Periodic realignment is not uncommon. The engine can suffer wear and tear because it bears the end thrust from the propeller. The engine is hard mounted to its bed, resulting in a lot of vibration and noise transferred through the hull.

Other industries take different approaches. Racing cars have long used flexible Constant Velocity (CVConstant velocity) joints, with engines of over 1000 hphorsepower. Tractors use flexible drive shafts running from a power-take-off (PTO) on the back. These shafts are extensible on splines, and use universal joints for alignment. There is little direct loading on the PTO bearings. Some vehicles also use a fully hydraulic transmission and drive.

Boats are retro because: (1) tradition; (2) hard fixed mounts are cheaper; (3) simpler is better (more reliable); and (4) tradition (i.e., mistrust of unknown technologies). All of the alternatives are more complicated, which is not what you want with a breakdown in a crisis.

The other reality is that the marine market for small boats is too small for many manufacturers to develop specialized versions of their drives for continuous duty marine applications. This is why some areas innovate slowly, and at great expense.