The free-stream characteristics of ship skeg-rudders

The results of an investigation into the free-stream characteristics of semi-balanced skeg rudders are presented. The investigation simulated the condition of a rudder operating in a uniform stream and without the influence of hull and propeller, extensive force and moment measurements were carried outin a wind tunnel on three model rudders having taper ratios of 0.59, 0.80 and 1.00 and skeg and overall characteristics which are typical for the rudders fitted to many modern ship types. Detailed pressure measurements were also carried out for one of the rudders in order to determine the distributions of load. A full description is given of the five-component dynamometer developed and built for the purpose of these wind tunnel tests. A modified lifting line theory is developed which supports the form of the skeg-rudder experimental results and provides satisfactory predictions of the spanwise loadings. The theory is used to provide a limited extension to the experimental data, showing the effects of varying the aspect ratio and skeg proportions. 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; the flow breakdown accounting for the discontinuities is discussed. Sealing the gap between rudder and skeg was seen to lead to a significant improvement in the lift developed at higher angles of attack. Predictions using the modified theory show that, for fixed aspect ratio and taper ratio, changes in the skeg depth have the largest influence on the production of lift whereas the principal effects of changes in the skeg chord and sweep are on the stock position and balance area. Compared with an equivalent all-movable rudder it was found that the skeg rudder is generally inferior for most aspects of performance, having a smaller lift curve slope, larger drag and larger movement of its chordwise centre of pressure with change in incidence.