surface drive underwater propeller pulse drive

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The implication is that we've been wrong - or at least quite a distance drive away from optimum - for an awful long time. So it is with understandable skepticism that the idea of using -piercing propellers on more-or-less conventional small craft is greeted by the boatbuilding community. What is a -piercing propeller, anyway? Simply stated, a drive -piercing propeller drive (or propeller) is a propeller hat is positioned so that when the vessel is underway the waterline passes right through the propeller's hub. This is usually accomplished by extending the propeller shaft out through the transom of the vessel, and locating the propeller some distance aft of the transom in the relatively flat water that flows out from the transom's bottom edge. (The exception being single-shaft catamarans, where the propeller hub intersects the undisturbed waterline.) In the case of articulated drive systems, the propeller shaft is driven through a double universal joint inside an oil-tight ball joint, allowing the shaft to rotate athwartships for steering and to trim up and down for control of propeller submergence. Fixed-shaft drives can use conventional shafts and stern tube bearings, but require rudders. In many racing applications, outboards and outdrives can be positioned sufficiently high on the vessel for the propellers to operate in a -piercing mode. The important operating feature is that each surface drive propeller blade is out of the water for half of each revolution. And here is another reason for skepticism. Surely a propeller blade is more efficient surface drive if it operates continuously in the smoothest possible flow, rather than splashing through the water twice with each revolution. But nature surface drive can play tricks on our intuition. Sometimes an unsteady process is actually more efficient than its continuous surface drive counterpart. Why use a propeller? A summary surface drive of the principal reasons for the high performance of propeller systems relative to conventional installations follows. Propeller Efficiency: surface drive Traditional propeller design and selection is almost always an exercise in trading off diameter against several other performance-limiting parameters. Basic momentum theory tells us that for a given speed and thrust, the larger the propeller, the higher the efficiency. While there are exceptions, most notably the effects of frictional resistance on large, slow-turning propellers, surface drive it is generally borne out in practice that a larger propeller with a sufficiently deep gear ratio will be more efficient than a small one. A number of design considerations conspire to limit the maximum surface drive feasible propeller diameter to something considerably surface drive smaller than the optimal size. These include blade tip clearance from the hull, maximum vessel draft, shaft angle, and engine location. While this may surface drive at times make life easy for the designer - the propeller diameter specified is simply the maximum that fits - it can also result in a considerable sacrifice of propulsive surface drive efficiency.

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