Trans RINA, Vol 152, Part B1, Intl J Small Craft Tech, 2010 Jan-Jun


present the results in a compact but meaningful fashion as evidenced by Figure 6 which is an especially good qualitative representation of


the pressure results and


provides an excellent template for any subsequent CFD validation efforts that might follow.


I do feel compelled to comment on one additional element of the paper. I understand that Figures 9 and 10 are an amalgam of results, Cx and leeward side pressure from sail A1 but windward side pressure from sail A3. The presentation may have been better served if the leeward side A1 and windward side A3 pressure results had been shown on different graphs rather than relying upon the undocumented assertion that the windward side pressures are very similar for every trim setting. In addition, greater comment might be offered by the Authors on the important differences that are evidenced on the windward side between trim condition #1 which shows the windward side Cp varying greatly from the nearly constant value of 1 that dominates the results for nearly every other trim condition. It also seems that these results are not consistent with a claim made earlier in the paper regarding the Cp maximum being equal to or less than 1.0 everywhere.


I am sure that like me, others in the sail testing/design community will look forward to the next instalment promised by the authors to discuss how twisted flow changes the pressure distribution on the spinnaker. I personally hope that


they will not limit their data


collection and analysis to only one uniform and one twist profile so that the impact of varying twist profiles may also be illuminated.


Michael Richelsen, North Sails Performance Research Group.


I would like to applaud the authors for their work and bringing it into the public.


The measured pressure sections help understand the amount of attached flow on the sails. This is valuable not just for feeding back information from a test to the sail designer, as it also allows for more detailed comparison with CFD results. Without the pressure measurements CFD results can only be compared on the basis of total forces whereas now, given measured section pressure values, one can get a better understanding of how well the simulated flow field in the CFD matches the tunnel flow. This is a valuable aid in improving CFD based predictions, which is


becoming another tool evaluating the performance of a sail design.


Obviously to do such a tunnel versus CFD comparison one will also need to obtain an accurate 3D geometry of the flying shape corresponding to a set of pressure measurements. Presumably the tunnel at the Auckland University already has capabilities in this field, either by laser scanning or photogrammetry?


for AUTHOR’S RESPONSE


Thank you for the generous comments and the very interesting points highlighted. The sail aerodynamics is still far from being fully understood and we hope that this


paper answered some questions, conscious that it also raised many other questions.


The large amount of data collected in the presented experiments couldn’t be fully discussed due to the restricted space available. Hence, a new manuscript titled Pressure Distributions on Modern Asymmetric Spinnakers has been submitted to the Journal of Small Craft Technology, where the pressure distribution on five sections of the three sails is discussed.


In particular, the effect of the twisted flow is discussed. The pressure measured on three horizontal sections of the sail A3 sailing at 55° AWA and 10° heel, both with and without the twisted flow, is presented. With regard to the question raised by Robert Ranzenbach, only


twisted flow profile was tested due to the time demand of the test.


The effect of the heel on the pressure distribution over the five sail sections is discussed, which shows the strong three-dimensionality of the phenomena. For instance, heeling the A2 by 10° causes the pressure to increase on the highest sections, and to decrease on the lowest sections.


Some interpretations are hypothesis and some possible highlighted. With regard to the


comment by Grant Spanhake, the different behaviour of the three sails can be due to the mid-girth interference with the mainsail. The mid girths of the A1, A2 and A3 are 1.21m, 1.35m, and 1.61m respectively. Hence, the A3 has the maximum absolute mid girth and also the maximum mid-girth/foot


ratio. However, the deeper


mitre and the longer sections can significantly affect the complex three-dimensional


phenomena, which can


explain the opposite trend of the A3 compared to the A1 and A2. Moreover, heeling the model, the spinnaker sheet can be eased without causing the sail to collapse. The sheet ease changes the sail geometry, which is correlated with the force increase. Hence, the way the geometry changes can also explain the trend differences.


Some clarifications are necessary about the flapping and non-flapping trims. With regard to the comment by William Lasher, the non-flapping luff leads to a significant force reduction in the A3 because it is correlated to a significant increase in separated flow. In fact, Figure 11 of the paper shows the separation point of the A3 moving from roughly the 60% of the chord to roughly the 50% of the chord, when the sheet is tightened to stabilize the luff. The anticipated


trailing edge


separation leads to a significant reduction of the suction after the turbulent reattachment. Conversely, the flapping and non-flapping trims of the A1, named trim #1 and #3 respectively in figure 10 of the paper, show similar pressure suction


and trailing edge separation. In


but we are


one


B-52


© 2010: The Royal Institution of Naval Architects


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