Marine propulsion – Screw propeller – Propeller arrangement
Patent
1997-03-25
1998-08-18
Swinehart, Ed L.
Marine propulsion
Screw propeller
Propeller arrangement
440 89, 416129A, B63H 510
Patent
active
057952000
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
THIS INVENTION relates primarily to a type of unit having an assembly providing contra-rotating , coaxial rotors. It is more specifically concerned with the construction of a contra-rotating propeller assembly for a marine propulsion unit. The term "propeller", as used in this specification is to be understood as covering a propeller having blades, such as a marine propeller, and also an impeller having radially-projecting vanes rather than blades, which is also encountered in different types of marine propulsion systems, for example jet boat propulsion systems.
STATE OF THE ART
Proposals have been made for driving two contra-rotating propellers of a marine installation from a single drive shaft. For example, U.S. Pat. No. 4,540,396 (Caires) describes one such proposal for use with an outboard and Canadian Patent No. 2,001,357 (Couture) describes another proposal. Although both proposals enable two propellers to be driven respectively in opposite directions from a single shaft, they have draw-backs which militate against their use commercially.
The Caires proposal involves locating a set of static planetary gears between two opposed annular gears one of which is driven directly by a drive shaft in one direction, and the other of which is driven in the opposite direction indirectly from the drive shaft by way of the planetary gears. Exhaust gasses produced by the outboard motor are fed through an annular tunnel which contains the static planetary gears and extends through the hubs of the propellers. The gases then discharge into the wake of the vessel astern of the propellers.
Caires partially overcomes the loss of efficiency of the assembly caused by discharging exhaust gases into the path of the propeller blades. However this is achieved at the price of allowing water and gas to flow through the meshing teeth of the planetary gears. The risk of water-borne, solid particles washing into the meshing zones of the planetary gears is considerable, as is the likelihood of heavy wear of the gears as there is no way of lubricating them. Thus Caires does not provide a commercially viable arrangement for driving two propellers from a common shaft in an efficient manner.
Couture seeks to overcome the lubrication problem by creating an annular closed chamber between contra-rotating hub-portions of two sets of propeller blades and stuffing the chamber with grease to keep the water out. Opposed, spaced bevel gears are formed respectively on the two propeller hubs and mesh with a ring of crown gears in the chamber. The crown gears rotate about axes which are radial with respect to the drive shaft, so that the rotation of one propeller is transmitted through the crown gears to rotate the other propeller in the opposite direction at the same speed.
The Couture proposal has a number of disadvantages which adversely affect the efficiency of the assembly. For example, the spacing between two contra-rotating propellers is critical and Couture does not allow any arrangement for changing this spacing in a given installation to achieve optimum efficiency. This can only be found empirically. Indeed it seems implicit in the Couture arrangement, for the two propellers to be sited very close to one another which is a very inefficient arrangement. Secondly, Couture does not provide any arrangement for maintaining the grease pressure above that of the outside water so that there is always a risk, which grows with time, of sea water entering the chamber and damaging the gears.
For a contra-rotating propeller assembly to be commercially acceptable, due regard must be paid to a number of different features which affect its efficiency. Firstly, the axial spacing of the two sets of propeller blades can be a critical feature which can only be determined by model testing. Secondly, the thrust of the two propellers, particularly when the vessel is going astern, must be acceptable and this can be varied by designing the propellers to operate at different relative rotational speeds. Thirdly, the gearing is preferably
REFERENCES:
patent: 2282266 (1942-05-01), Swaskey
patent: 2394299 (1946-02-01), Friedrich
patent: 2765040 (1956-10-01), Darrah
patent: 4964844 (1990-10-01), Bagnall
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