Canadian Hydrofoil Program. Hydrodynamics and Simulation 



bow foil by 



Ar=_^_L (22: 



where t is the foil base length. Over the frequency range of interest, 

 A f can be very well approximated by a time lag and constant phase 

 lead (Figure 16) : 



AT = T w" - A<f> (23) 



A block diagram of the head sea simulation system is shown in 

 Figure 17. Results derived from simulating BRAS D'OR foilborne 

 motions in rough water are presented in the next section ; surge was 

 neglected in this simulation in order to simplify the problem and also 

 because it was felt that neglecting surge should give conservative 

 results. (Consider the case where the bow foil encounters a steep 

 high wave, leading to a rapid increase in bow foil immersion depth. 

 In practice a speed reduction of several knots occurs, and the attendant 

 dynamic pressure decrease results in lower lift and accelerations than 

 on the simulated, constant speed ship. ) 



IV. BRAS D'OR FOILBORNE SEA -KEEPING TRIALS 



BRAS D'OR rough water trials were less comprehensive 

 than desired, but enough data were obtained to enable comparison of 

 measured characteristics with predictions and to reach general con- 

 clusions about sea-keeping ability. Data will be presented for three 

 key trials, two in Sea State 4 and one in Sea State 5. The wave eleva- 

 tion power spectral densities measured during these trials are shown 

 in Figure 18 and are compared with the Pierson-Moskowitz theoretical 

 spectra for Sea States 4 and 5 (significant wave heights of 7' and 

 10'). Wave measurements were made by a buoy equipped to measure 

 vertical accelerations and are admittedly inaccurate, due partly to 

 limitations of the buoy itself and partly to the practical difficulty of 

 making a single measurement representative of rapidly changing con- 

 ditions in a trials area close to the coast. 



The variation of root mean square values of longitudinal ship 

 motion parameters with heading to the sea is shown in Figure 19 for 

 an average speed of 3 9 knots. These values do not exhibit the system- 

 atic increase with sea height that one would expect, probably because 



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