On the Results of Experiments on Rudders placed behind the Ship

Abstract

The author discussed the performance of open rudder, particularly the effect of angular velocity of steering in Reports Nos. 1 & 2 of this series of experiments, and described, in Reports Nos. 3 -& 4, the effect of propeller race on the rudder performance. Therefore, in Report No. 5, the present report, the author principally discusses the experimental results pertaining to the effects of wake and propeller as the remaining problem, and lastly he mentions the practical formulae applicable to the normal force of rudder and the position of the centre of pressure, integrally considering Reports Nos. 1-5.The following points were made clear as the results of model experiments of the rudder placed behind the ship and ship's propeller.(1) When the propeller is located in.froht of the rudder, the effect of wake can be presumed by considering the mean effective wake fractions within the propeller actuator disc, and when the propeller is not in front of the rudder, by considering the nominal wake fractions around the rudder.(2) As to the effect of propeller race on the normal force of rudder, it is the function of the slip-ratio of the propeller just the same as in the case of open propeller, and its increment ratio is in proportion to 1.5 power of the slip-ratio, but the coefficient is greater than in the case of open propeller.(3) In the case of the rudder behind the right-handed turning propeller situated at the stern, when normal force is zero, the helm angle shifts to the port in many cases, though slightly, but in the case of a great rudder angle, normal force indicates an almost equal value for both port and starboard. The position of the center of pressure alone is affected by unsymmetry of the upper and lower portions in the distribution of wake.(4) Regarding the composite effects on the normal force of rudder located at the stern, the effect of wake and that of race mutually offset and in an ordinary propelling condition, there is shown an increase of an equal degree, or by 10-20%, as compared with normal force working on open rudder at the time of an equal velocity to the ship's speed.Lastly, the practical formulae for the normal force and the center of pressure of ruddes in the single screw vessel are given, and the calculated values of the pressent formula were compared to the measured results of the actual vessel.