Lewis 



The results of the calculations for tanker type vessels with Cg = 0.80 in 

 the average severe spectrum (Fig. 3) are shown in Fig. 6, which gives effective 

 wave height as a function of length [11]. A low ship speed of Froude number = 

 0.10 (8.25 knots for a 600-foot ship) was considered to be a reasonable maximum 

 speed in an extremely rough sea. The curve crosses the L/20 line at L = 500 

 feet, and coincides with the 0.6L°^ wave that has been proposed from about 500 

 feet to 650 feet. The matching of the calculated trend with these other criteria 

 thus provides a sound basis for the comparison of the larger ships with those of 

 500 to 650 feet, even if the absolute significance of the statistical parameter is 

 doubtful. The calculated trend indicates that at lengths greater than 600 feet the 

 increase in effective wave height with length is less rapid than is shown by the 

 other criteria. 



The results for the finer ships are also shown in Fig. 6. A somewhat higher 

 speed (Froude number = 0.15; 12.4 knots for a 600 foot ship) was used since the 

 finer ships could be expected to make better speed in rough seas. Possible in- 

 creased stresses caused by slamming were not included. The trend with length 

 is similar to that for the fuller ships, and from 15 to 20% lower. Thus the bend- 

 ing moment coefficient is not quite proportional to block coefficient, since in 

 that case the reduction would have been 25%. However, it should be noted that 

 fullness is already taken into account in the bending moment coefficient h^/L 

 which includes the waterplane coefficient. 



RESULTS -BOW MOTIONS 



The trends of ship motions in irregular seas have also been investigated, 

 with particular reference to relative bow motion. This work has been carried 

 out under the sponsorship of the Society of Naval Architects and Marine Engi- 

 neers, Panel H-7 of the Hydrodynamics Committee. Calculations are based on 

 Vosser's Series 60 model tests in regular waves [4], showing the effect of both 

 speed and proportion. 



Figure 7 shows the results for a ship of Cg = 0.70 and L/H = 17.5 at vari- 

 ous speeds in short- crested head seas, using one of the severe sea spectra used 

 in the bending moment study [12]. It may be seen that the response amplitude 

 operator peaks increase steadily with speed. They also move to the right with 

 increasing speed, which has a favorable effect — because of the downward slope 

 of the wave spectrum. However, the overall effect of speed is unfavorable, as 

 shown by the response spectra at the bottom of the figure. 



Figure 8 shows in a similar way the effect of varying the L/H ratio when 

 heading into the same sea at constant speed. It may be seen that the reduction 

 in height of the response amplitude operator peaks with increasing L/H results 

 in a corresponding reduction in response spectra. 



The trend with ship speed is shown more clearly in the upper part of Fig. 9. 

 Also shown in the figure are two points from Fig. 8 for ships of different length/ 

 draft ratio at the same speed. 



200 



