Trans RINA, Vol 156, Part B2, Intl J Small Craft Tech, Jul-Dec, 2014
because the afterbody was almost completely un-wetted at the highest speeds. At the highest step height, the hull began porpoising above a speed coefficient of 3.5. This is likely the result of the very high running trim angle.
Figure 8: Photographs of Typical Run (top: above water, bottom, under water).
3.4 TEST RESULTS
The test results are tabulated in Appendix A and are reported graphically in Figures 9-11. The figures show model resistance-to-weight ratio, running trim angle and wetted surface area as a function of speed coefficient
(beam Froude number), /. The model
resistance may be expanded to any full-scale size by using the data tables in Appendix A. The Reynolds numbers in scale model testing are always lower than those at full scale, therefore the friction coefficients (and consequently the resistance-to-weight ratio) are higher for the model than for the ship. One important assumption in expanding resistance data of stepped hulls from model to full scale is that the area of the ventilated portion of the hull is not affected by scale ratio.
Figure 9 shows that at a speed coefficient between 1 and 2.5 , increasing step height increases resistance; however, the effect is much smaller than an equivalent LCG shift (as the example given in Figure 3 shows for a similar case). This is because the afterbody is wetted at hump speed, producing a bow-down pitching moment and minimizing the resistance penalty.
At the high speeds, when the flow begins to separate from the step, the resistance decreases. The smallest step height
(1.42% beam) had very small reduction in
resistance, because there was very little separation. Step heights of 2.85 beams and above reduced resistance at the highest speeds by up to 25%. Above a step height of 2.85% beam, further increases in step height changed the high speed resistance very
little. This is primarily
Figure 10: Experimental Results of Trim versus Speed Coefficient as a function of step height
Four runs in the test series exhibited porpoising
instabilities with pitch amplitude exceeding 1 degree. Porpoising is a coupled oscillation in pitch and heave, which occurs on planing hulls operating at high trim
Figure 9: Experimental Results of Model Resistance-to- Weight Ratio versus Speed Coefficient as a function of step height
Savitsky [6] demonstrated that typically 4-degree trim is the optimum angle for most planing craft, and operating below 4-degree trim results in a large increase in frictional resistance. This point was further stressed in Savitsky and Morabito [2]. The current tests agree well with that design guidance. Configurations in which the trim angle dropped below 4 degrees at the highest speeds showed increases in resistance.
B-92
©2014: The Royal Institution of Naval Architects
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