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enforce a specified blade angle as a function of blade orbit position. Intermediate supports distributed over the span are applied where necessary to ensure sufficient stiffness of the blades (see Fig.2).

Fig. 2 Stern view of an aftbody with Whale Tail Wheel propulsion.

The allowable large area of the propeller and the almost two-dimensional flow about the blades give it an unequalled high performance potential for the requirements of many ship types. The large area of the propeller is expressed in a low thrust loading per unit envelope area. This allows the required change in momentum of the flow to be obtained by a relatively small acceleration of fluid. A corresponding high ideal efficiency is obtained. The trochoidal propeller with a horizontal axis, ‘Whale Tail Wheel' in the following, is a combination of the tail propulsion of a living creature and the wheel, invented by mankind.

The choice of the dimensions of the Whale Tail Wheel can lead to a minimum thrust loading per unit envelope area and consequently to the optimum of the maximal efficiency curve of the trochoidal propeller with a horizontal axis. A shape of afterbody adapted to the two-dimensional character of the lightly loaded Whale Tail Wheel will realise promising conditions for the control of cavitation and vibration risks.

2.
HISTORY

A summary of the historic review by Brockett (1) is presented here:

‘Different configurations of the cycloidal propulsor were independently developed by Kirsten (2), (3) and Schneider (4) during or prior to the 1920's. The blade angle control of Kirsten is fixed by gears and has experienced only limited development. The more complex but controllable blade angle orientation using linkages proposed by Schneider has received considerable development.

There are two traditional regions of pitch control. One for low-pitch propellers (associated with orbit circumferential velocities ωD/2 greater than the advance speed V0), and one for high-pitch propellers (associated with circumferential velocities less than the advance speed). Low speed manoeuvring requirements have led to extensive development of the low pitch configuration (Schneider (4)). Such propellers have top speeds usually less than 20 knots. High pitch propellers have hydrodynamic efficiency values greater than for low pitch propellers and permit high speed operation. These latter propellers have to date however, received most attention at only research level (e.g. van Manen (5)).

A survey of cycloidal propulsion has been given by Henry (6). He presents a history and a description of some kinematics of the motion of a cycloidal propeller, as well as a critical review of early attempts to model the flow field. Some of his suggestions for improved modelling have yet to be implemented. Improvements in performance were addressed by Sparenberg (7), (8) and James (9), who attempt to define blade angle control specifications for improved efficiency based on two-dimensional flow models.

Systematic series data are presented by Nakonechny (10), (11), Van Manen (5), Ficken (12) (who includes Nakonechny's data in his faired curves), Bjarne (13) (see also Kallstrom and Loid (14)), and Bose and Lai (15). The most extensive of these series are those of Van Manen and Ficken. The main variable in these investigations is pitch control, but several blade shapes are also evaluated. Van Manen includes evaluation of the blade angle motions derived by Sparenberg (7), which did produce improvements in efficiency over his conventional cycloidal blade angle variations.

Van Manen's experiments (with low pitch propellers) produced generally lower efficiency values than expected, which has been partially explained by Ruys (16) as a viscous effect. Van Manen's data for high-pitch propellers had quite respectable efficiency values (η0~0.7).' (Brockett (1)).

3.
WORKING PRINCIPLE
Propulsion fundamentals

The general principle underlying the thrust production of any propeller is strongly summarized in the conservation law of momentum. A conse



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