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→Design Considerations
== Design Considerations ==
At its simplest, a propulsion system consists of an engine, reduction gear, propeller shaft and bearings, and a [[Propeller|propeller]]. Most designs feature a long rigid drive shaft to get the correct propeller angle. A 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 (CV) joints, with engines of over 1000 hp. 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.
These advantages can be obtained in a boat by using a hard-mounted thrust bearing to take the shaft thrust from the propeller, and flexibly coupling and mounting the engine.
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.
** One main engine with one propeller
* Emergency power:
** One main engine with one propeller, and** A a small wing engine with a small propeller
* Redundant power:
** One main engine with dual electric drive, two propellers and battery backup, OR
** Two main engines with two propellers (drive systems are unimportant)