Further free-stream characteristics of semi-balanced ship skeg-rudders

Abstract

The results of free-stream wind tunnel experiments on semi-balanced skeg rudders are presented.

The tests on these rudders are an extension to earlier tests on a parent skeg rudder, which were the subject of a separate report, and complete a series of tests on three skeg rudders having taper ratios of 0.59, 0.80 and 1.00.

The experiments establish a better understanding of the working and performance of semi-balanced skeg rudders, and provide design data. Results are presented as lift and drag coefficients and centre of pressure for the rudder plus skeg combinations, for the rudders alone and skegs alone; complete ranges of rudder angles of attack were tested for selected positive and negative angles of attack on the skegs.

Supplementary tests were carried out to determine the influences of Reynolds No., the size of the gap between rudder and tunnel floor, the transition strip and the effect of sealing the gap between rudder and skeg.

In order to provide bases for comparison, the all-movable cases were simulated for each rudder by sealing the gap between rudder and skeg and varying the angles of attack on rudder and skeg simultaneously. The results for the all-movable cases compare very satisfactorily with existing published data.

The results for the skeg rudders show that with increasing angle of attack, discontinuities occur in the growth of lift together with a large movement of centre of pressure. Sealing the gap between rudder and skeg was seen to lead to a significant improvement in the lift developed at higher rudder angles of attack. The results for the three skeg rudders are compared, and the flow breakdown accounting for the discontinuities in the skeg rudder characteristics is discussed.

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