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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.
From an emergency perspective, the debate over one engine or two will never be settled. Each person’s comfort zone is different when it comes to managing risk. From this perspective, there are four choices:
* No backup: ** One main engine with one propeller.* Emergency power: ** One main engine with one propeller, plus and 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 unim-portantunimportant).
If you’re comfortable psychologically with the proven reliability of diesel engines, you will happily go to sea with one engine and one propeller, and save some money in the bargain. You will be in the excellent company of most small fishing trawlers.
Realistically, a small wing engine with a wimpy propeller is unlikely to have enough power in a severe-weather emergency. Two main engines is the only way to provide true redundancy. Alternatively, you might consider a single engine with an electric drive – it can be configured to use the house bank as an emergency source of power.
From a [[:Category:Arrangement|layout perspective]], twin engines require a larger engine room, and may dictate an amidships layout. Twins mean twice as much cost and maintenance, and duplicate charging and fuel systems (not necessarily bad). Sometimes this results in two systems in some degree of disrepair.
==References ==
[[Category:PropulsionSystemsGeneral]]