Marine propulsion – Screw propeller – Having means to control flow around propeller
Patent
1995-11-15
1998-03-03
Basinger, Sherman
Marine propulsion
Screw propeller
Having means to control flow around propeller
440 61, 440 75, B63H 515
Patent
active
057228663
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to a propulsion arrangement for a marine vessel, comprising a propulsion unit having a non-rotating housing in the form of a nozzle extending along a principal axis, a propeller mounted for rotation within said nozzle, a support shaft extending along the principal axis of the nozzle and to which shaft said propeller is affixed, and support shaft support means in the form of a plurality of arms extending substantially radially from the inner surface of the nozzle to a bearing hub for the support shaft.
BACKGROUND OF THE INVENTION
It is a general goal within the marine industry to provide vessels which are more efficient. The efficiency of a vessel is determined predominantly by the design of its hull and the effectiveness of its propulsion means. In terms of a vessel's hull, the greater the wetted area of the hull, the greater the frictional forces which arise. Thus phenomena has led to the development of planing-hull vessels which are particularly adapted for high speeds. As described in U.S. Pat. No. 4 597 742, to improve the efficiency of the hull and drive system at hull-planing speeds, a submerged trim hydrofoil can be affixed to the drive housing of the drive system beneath the screw propeller drive shaft.
In terms of marine propulsion means, most vessels employ one or more propellers, driven either by an inboard or an outboard motor. Including losses due to the resistance of its submerged housing, etc., a normal propeller has a total efficiency of around 55-65%, i.e. the losses can be as high as 45%. The largest undesired losses are due to the rotation of the water downstream of the propeller. This rotation is made up of a rotating "cylinder" of water as well as eddy-currents caused by water flow from the pressure side of the propeller blades to the suction side. Clearly, if the efficiency of the propeller can be increased, then the fuel consumption of the power unit as well as the required power input will be reduced. Since high propeller blade speeds induce cavitation and increase noise levels, it is advantageous if the rotational speed of the propeller can be reduced, whilst still maintaining adequate forward propulsion.
A further important consideration is that of safety. Particularly for inshore vessels, an exposed propeller can create a danger for persons who may be bathing in the vicinity.
There have been many attempts to improve conventional propellers to satisfy some or all of the above-described demands. A popular means to reduce the rotating "cylinder" of water is to employ a twin, counter-rotating propeller drive. Such a construction does not, however, offer a solution to the remaining aspects identified above.
From NO-B-143 093 it is known that the efficiency of a propeller can be increased by encapsulating the propeller in a housing having the form of a nozzle. In said document, the propeller is mounted for rotation within the nozzle and is carried on a propeller shaft whose remote end is supported in a bearing hub. The bearing hub is in turn supported by a plurality of radially extending arms or vanes within the nozzle. Vanes are also provided within the nozzle upstream of the propeller. Both sets of vanes are shaped to control the flow of water through the nozzle in an attempt to minimize rotational losses. In said document, though, these vanes present a relatively long chord length, which results in the large surface area of the vanes giving rise to frictional losses. The propulsion unit according to NO-B-143 093 may also be rotatable about a vertical axis to provide a steering function.
Further examples of encapsulated marine propulsion units are described in EP-A-0 425 723 FR-A-1 387 903, U.S. Pat. No. 4 427 393, SE-B-342 011 and U.S. Pat. No. 4 074 652. All these units are either rigidly affixed to a vessel or are capable of rotation about a vertical axis to provide a steering function.
Whilst propulsion units incorporating an encapsulated propeller have been shown to offer significant advantages over exposed propellers, none of th
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