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Reducing added resistance using an anti-pitch foil Avis, John Spencer


In this study, the use of a passive bow anti-pitch foil for the purpose of reducing the vertical motions and added resistance in waves of a small marine vehicle is investigated. The difficulties, limitations and assumptions inherent in studying ships in waves are discussed. A mathematical model is developed incorporating linear strip theory, Joosen added resistance theory and oscillating hydrofoil theory in an attempt to predict the effect of a hypothetical anti-pitch foil on the heave and pitch motions and added resistance in head seas of the 12-metre yacht Canada II. The results of regular head wave towing tank experiments on a 1:8 scale model of Canada II are presented for both the bare hull and for the hull fitted with a model anti-pitch foil to demonstrate the experimental effect of the foil on vertical motions and added resistance. The results of the theoretical and experimental investigations show that the anti-pitch foil is in fact very effective in reducing the motions and added resistance of the Canada II hull. Experimental results show heave amplitude is reduced by as much as 15%, pitch amplitude by 22% and added resistance by as much as 40%. The heave and pitch motion amplitude responses predicted by the linear strip theory for the hull with no foil, do not agree well with experiment, especially in the region around resonance. It is concluded that the linear strip theory used is not completely adequate in predicting the hydrodynamic coefficients and consequently the heave and pitch motions of Canada II due to non-linearities in her hull form. These non-linearities, in particular the flared topsides and large fore and aft overhangs, violate the linear assumptions of strip theory and result in increased hydrodynamic damping and speed dependent restoring coefficients that are not accounted for by theory. The Joosen added resistance theory predictions for the bare hull do agree well with experiment despite the hull non-linearities. The change or percentage reduction in heave and pitch amplitude response due to the addition of the anti-pitch foil is predicted well by the oscillating hydrofoil theory, in spite of the inaccuracies of the strip theory in predicting the actual motions. Also, the Joosen theory predictions of the percentage reduction in added resistance due to the addition of the foil agree very well with experiment. It is concluded that the mathematical model provides a very good conservative estimate of the amount of reduction in heave and pitch motion and added resistance that can be expected from the addition of a passive bow anti-pitch foil. The model could be very useful as a preliminary design tool for naval architects, where no such tool existed before. The findings of this study indicate that the concept of using a passive bow anti-pitch foil to reduce the vertical motions and added resistance in waves is well worth pursuing.

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