A fundamental study on the gap effect between rudder and stern-bottom

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A fundamental study on the gap effect between rudder and stern-bottom

Nakato, M. / Kudo, K. / Kato, R.

The results of model basin experiments on rectangular flat plates near temporary wall are presented. The experiments simulated the condition of a rudder operating near stern-bottom with finite gaps. Tests were carried out on two flat plates having aspect ratios of 1.0 and 1.35. The open water tests were also carried out for each and/or double aspect ratio flat plate which is equal to the limiting case of infinity and/or zero gap-to-span ratio. Results are presented for each rectangular flat plate as normal force coefficients. The leading edge was sharpened in every case. In case of aspect ratio 1.0 the results are compared with Winter's experiments. Both agreed very well, so that we are able to be assured of our measuring system. When the gap ratio becomes less than about 0.1, the normal force coefficient tends to be equal to that of double aspect ratio flat plate. However, the effect of increasing aspect ratio seems to advance the stall at the leading edge. The theoretical calculations were also made and compared with the experimental results. The linear lifting surface theory gives lower normal force coefficients but predicts higher effect of gap than the experiments. The strip method for the nonlinear lifting surface was applied to this problem. Assuming that the free vortex shedding angle Theta is equal to the angle of attack Alpha, this method compares well with the experimental results. (Wassmann)