International Journal of Small Craft Technology DISCUSSION AN EXPERIMENTAL
STUDY OF THE
HYDRODYNAMICS OF A YACHT WITH A CANTING KEEL AND FORWARD RUDDER
F S Maes and M J Downie, University of Newcastle upon Tyne, UK
(Vol 149 Part B1 2007)
COMMENT P van Oossanen, Van Oossanen & Associates, The Netherlands
The results shown in Fig. 11 have led the authors to conclude that "the worse way of producing the side force is with a fixed keel or monoplane". This figure cannot be used for such a wide-sweeping conclusion. The results only show that the hull (without appendages), when fitted with a forward and an aft rudder, has less resistance than the hull itself, at equal side force. Since the hull (or canoe body) of a sailing yacht can only develop side force at the expense of considerable resistance, it follows that this result is rather trivial in nature. Figure 12 shows a more relevant result in that the biplane configuration (with the forward and aft rudders aligned to the centre plane) possesses more resistance than the other cases in which forward and aft rudders are deflected to produce more lift. But this result also does not provide a sound basis for the conclusion drawn.
Perhaps the authors would like to comment on the reason for not
also having included a conventional keel and
rudder configuration in their experiments. It is to be noted that a fixed keel, when of sufficient aspect ratio and fitted with a high-lift device (a trim tab) and winglets, time and time again, is found to possess very good overall performance (as in America's Cup yachts). The addition of a keel-canting arrangement no-doubt constitutes an improvement, but our research shows that a forward rudder does not.
Perhaps the authors would also like to comment on the reason for the lack of a linear relationship between resistance and side-force squared in figures 11 and 12. The results shown in Fig.11 seem to indicate that the forward and aft rudders suffer from flow separation at the higher lift values. This might indicate that the flow over these appendages was laminar during the tests in which case more doubt is cast over the conclusions drawn.
AUTHOR’S RESPONSE
Having reviewed Mr Van Oossanen’s comments, we would to offer the following remarks:
Since it was impossible to simulate a real fixed keel situation (model was modified for canting keel setup), the fixed keel situation where all the lift is generated by having a leeway angle was simulated by leaving both rudders on the centreline and adopting several leeway angles. In the latter setup the lift was not only generated by
the hull but also by the two less induced resistance is appendages.
It is appreciated that this setup has to be interpreted with care as an actual fixed keel would produce the lift more efficiently. Hence the setup is only used in figure 11 to prove that
providing lift by turning the rudders while the hull remains without or with a limited leeway angle.
For biplanes and triplanes, the relationship is not linear due to interference effects (Ref equation [3] in the paper)
Low Reynolds number separation (which is what we most probably had) is, or can be, a complex phenomenon and model experiments in this regime are always faced with this difficulty. Modification of characteristics of the foil
the surface turbulence stimulators, is a demanding
using roughness, or other procedure
needing extensive investigation to ensure that it hasn't initiated some unforseen and undesirable consequence, such as unrepresentative thickening of the boundary layer. Within the scope of the resources available we were unable to pursue this rigorously and the results should be viewed with this limitation in mind. Anyhow, the main effect of laminar rather than turbulent separation is that our CLmax (before stalling) would be smaller than for turbulent flow , but in any case the foil would stall at a high enough angle of attack.
generated by
©2007: Royal Institution of Naval Architects
B-61
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