Canadian Hydrofoil Program. Hydrodynamics and Simulation 



Agreement between simulated and measured response trans- 

 forms is very good. These predictions were obtained by analog simu- 

 lation of pitch and heave motions in regular waves of small amplitude 

 - a procedure equivalent to linearization. The predictions of Fig. 21, 

 on the other hand, were obtained by simulating ship motions in a ran- 

 dom State 5 sea generated as described in Section 3. The regular 

 wave technique was adopted for response transform prediction because 

 it enabled more accurate modelling of the effects of circulation delay 

 and wave orbital velocity on main foil angle -of -attack. Bow foil flow 

 re -attachment and emergence of the main foil anhedral -dihedral in- 

 tersections, both of which occurred occasionally during trials but 

 never in the simulation, are probably the cause of the only notable 

 discrepancy, under-prediction of pitch response below . 3 Hz at 35 

 knots. 



Roll and sway characteristics are presented in Figure 25 in 

 terms of rms values for lateral acceleration and roll angle, for the 

 three seaways of Figure 18. Lateral accelerations are given at two 

 ship stations, the CG and the Control Information Centre, located 

 comparatively high in the ship in the upper deck superstructure. As 

 with the pitch-heave characteristics given earlier, there seems to be 

 little effective difference between the state 4 and 5 seas. Roll angle 

 is very dependent on direction to the seaway, increasing greatly for 

 seas on and abaft the beam. The lateral accelerations exhibit a much 

 smaller dependence because the roll frequency decreases for beam 

 and stern seas. 



The effect of heading on the frequency distribution is shown by 

 the power spectral density plots of Figure 26. These show lateral ac- 

 celeration at the CIC for head and following sea runs at 3 9 knots in 

 Sea State 5. The first peak is at the main rolling frequency and is due 

 to accelerometer tilt. For the head sea, in addition to the increase in 

 frequency of the main acceleration component, there is an increase 

 in level in the 1 to 2 Hz range. This is significant because lateral 

 "jerkiness" was considered the most uncomfortable feature of the ride, 

 especially at higher speeds and for higher stations in the ship. 



The effect of increased speed is shown in Figure 27 by compar- 

 ing lateral acceleration at the CIC for head sea runs of 34 and 42 

 knots in the same Sea State 5. The effect of increased height within 

 the ship is particularly marked and is illustrated by Figure 28 which 

 compares lateral accelerations at the CG and CIC with the corres- 

 ponding roll angle plot for 3 9 knots in head Sea State 5. A very small 

 amount of roll angle energy above 0. 5 Hz seems responsible for real- 

 ly significant lateral acceleration at the CIC. The problem of lateral 

 acceleration amplification with height is clearly deserving of attention 



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