Schmitke and Jones 



of different sea energy distributions. For all three seaways there are 

 similar trends with change of heading : vertical accelerations are high- 

 est in head and bow seas and lowest in following seas, while pitch 

 angle shows the opposite trend. 



The longitudinal distribution of vertical accelerations is in- 

 dicated in Figure 20, the points representing average root mean square 

 values for the three sea trials. The most interesting feature of these 

 plots is the comparatively small change in vertical acceleration along 

 the length of the ship, illustrating the well-controlled pitch response 

 of this canard configuration, with its special bow foil. 



Figure 21 compares predicted and measured root mean square 

 vertical accelerations at the bow. The predictions were derived from 

 analog simulation of pitch and heave motions in a theoretical head Sea 

 State 5 with significant wave height of 10. 7 feet, while the measure- 

 ments were obtained during head and bow sea runs in Sea States 4 

 and 5. Predicted acceleration levels are higher than measured, re- 

 flecting the higher theoretical sea state and perhaps to some degree, 

 supporting the intuitive argument that neglect of surge is a conserva- 

 tive simplification of the simulation problem. 



Figure 22 presents typical power spectral density plots of 

 vertical acceleration at the bow and pitch angle for head sea runs at 

 speeds of 34 and 42 knots. Also given are the corresponding spectra 

 for the encountered seaway, derived from the nominal Sea State 5 plot 

 of Figure 18. There are two dominant seaway frequencies, and al- 

 though the effects of both are clearly apparent in the pitch angle plots, 

 pitching is associated mainly with the lower frequency. Pitch response 

 falls off with increasing speed, particularly at the lower frequencies. 

 Bow vertical acceleration peaks at the higher seaway frequency ; the 

 magnitude of this peak increases with speed and there is also a shift 

 in the energy distribution toward higher frequencies. 



The response transforms of Figures 23 and 24 quantitatively 

 characterize BRAS D'OR's pitch and heave response to random head 

 seas at speeds of 3 5 and 40 knots. Although the experimental results 

 are scattered, reflecting inaccuracy of sea state measurement, sys- 

 tem non-linearity and limited statistical confidence, they nevertheless 

 furnish a reasonable indication of mean ship response, as given by 

 the dashed lines in the Figures. Pitch response peaks at approxima- 

 tely .2 Hz, in agreement with the prediction of pitch natural frequency 

 obtained from linear stability analysis. Bow vertical acceleration res- 

 ponse is fairly flat above . 2 Hz, but falls off rapidly below this fre- 

 quency. 



306 



