## Relative rotative efficiency

Theoretically, the relative rotative efficiency accounts for the differences between the open-water test and the inhomogeneous three-dimensional propeller inflow encountered in a propulsion test. In reality, the propeller efficiency behind the ship cannot be measured and all effects not included in the hull efficiency, i.e. wake and thrust deduction fraction, are included in In addition, the partial recovery of rotational energy by the rudder contributes to . This mixture of effects makes it difficult to express as a function of a few defined parameters.

Holtrop and Mennen (1978) and Holtrop (1984) give

= 0.9922 - 0.05908 • AE/A0 C 0.07424 • (CP - 0.0225 • lcb) for single-screw ships

= 0.9737 C 0.111 • (CP - 0.0225 • lcb) - 0.06325 • P/DP for twin-screw ships lcb is here the longitudinal centre of buoyancy taken from L^/2 in AE/A0 is the blade area ratio of the propeller P/DP is the pitch-to-diameter ratio of the propeller

Helm (1980) gives for small ships:

The basis is the same as for Helm's formula for

= 1 ± 0.05 for propeller propulsion systems; Alte and Baur (1986) recommend, as a simple estimate, = 1.00 for single-screw ships, = 0.98 for twin-screw ships.

Jensen (1994) gives = 1.02-1.06 for single-screw ships depending also on details of the experimental and correlation procedure.

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