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


FORCES AND PRESSURE INVESTIGATION OF MODERN ASYMMETRIC SPINNAKERS


I M Viola and R G J Flay, Yacht Research Unit, University of Auckland, New Zealand (Vol 151 Part B2 2009)


COMMENT


Grant Spanhake, North Sails Performance Research Group, New Zealand


I found the paper very informative and helpful. I do have some comments and questions on the paper.


In Table 1 I would suggest adding a Mid girth # or mid girth % to the table. From Section 3.3 it would be good to see a close up photo of the pressure taps and at a later date it would be good to see the results of (Twisted vanes verses non-twisted).


In Figure 4 I found it interesting that the A-3 Cx SA forces do not increase at 10 degrees of heel like the other sails. I can only think the reason for this is that the wider mid-girth (reason for adding it to Table 1) may interfere with the mainsail/Slot/up-ward flow?


In section 4.4 one good addition would be add the timing of the flaps e.g. below. This will give a sail trimmer some practical guidelines and consistency.


Kite out (1)–in(7)


# of flaps per 10sec’s


1 10


Cx m/s 0.95


2 5 1.0 3 1 (Curling)


1.05


4 0 0.9 5 0 0.8


6 0 0.6 7 0 0.3


3.5 3.5 3.5


3.5 3.5


3.5 3.5


Comments


Sail very un-stable


Sail just flicking


Sail curling Sail stable


Start to be over- trimmed


Over- trimmed


Over- trimmed


In Section 4.5, second paragraph you state “In the


present investigation it was found that with the A1 and the A2 the same maximum drive force could be achieved both by flapping and non-flapping luffs. For instance, in Figure 9 and Figure 10, trims #1 and #3 show a similar drive force but the luff was flapping in trim #1 and not in trim #3.” I suspect that this could be explained with the


© 2010: The Royal Institution of Naval Architects


bold items in the above table. I had originally thought that the reason for this may have been that the wind speed was set too high. But at 3.5 m/s (6.80 TWS) I don’t think this is the case.


I would believe your findings and conclusions based on real world on the water and wind tunnel experience. It would be interesting to further investigate the following.


1. Twisted vanes verses non-twisted. 2. Flicking luff verses curling luff 3. Keep the same sail area and increase/reduce the mitre depth at 400 to see if there is a sweet spot.


Dr W Lasher, Penn State Behrend,


Engineering, USA The authors


present a very School nice investigation of of


asymmetric spinnakers by providing detailed surface pressure measurements and analyzing them from both a flow and overall force perspective. Their discussion regarding the effect of luff flapping on the pressures and forces was particularly interesting, as this has an important impact on sail trim. I am curious as to why the flatter spinnakers produced a higher drive force when over trimmed (trim #2), whereas the fuller spinnaker did not. Is it due to a higher peak suction pressure for sail A3, or a larger


delay in trailing edge separation


compared to sail A1? Perhaps an overlay of the Cp difference from sail A1 onto Figure 11 would help to answer this question.


Also, was there anything in the pressure measurements that explains the results shown in Figure 4? Specifically, why does the drive force increase for sails A1 and A2 at 10 degrees heel, and decrease for sail A3? I recognize that this may be difficult to determine from pressure measurements alone, but any comments from the authors would be appreciated.


Dr R Ranzenbach, Donald L Blount & Associates Inc, USA


The authors are to be congratulated on their meticulous effort to obtain pressure measurement data on a flexible membrane like an asymmetric spinnaker. As noted in the paper, much of the experimental evidence collected to date has been focused on global aerodynamic forces and this additional data will prove invaluable to our understanding of the underlying physics of asymmetric spinnakers and to anyone interested in validating their CFD predictions of this complex, three-dimensional, separated flow.


Not only does it appear that great care was taken to collect data, it is clear that the authors took great pains to


B-51


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