Lewis 



- % = I7.5 



.05 



"-/h =110- 



S = RELATIVE BOW MOTION 

 5/ ] = AVERAGE OF '/|o HIGHEST 

 "-'i/lQ VALUES OF 5/L 



% = 24.0 



10 



15 



25 



SHIP SPEED (KNOTS) 



lOOr 



75- 



10 15 



SHIP SPEED (KNOTS) 



20 



25 



Fig. 9 - Relative bow motion for series 60, 

 Cg = 0.70 ship in Pierson 62-knot spectrum, 

 trends with speed 



Of more direct interest in evaluating a ship's seagoing performance than 

 relative bow motion are two derived quantities: 



(a) Foredeck immersion, as an index of shippir^ water. 



(b) Forefoot emergence, as a rough indicator of possibility of slamming. 



For a particular forward freeboard or draft these quantities can easily be 

 worked out statistically from the response spectra. A convenient form of pres- 

 entation is in terms of percentages of cycles of motion in which the foredeck is 

 immersed or the forefoot emerges, as shown in the lower part of Fig. 9. 



It is interesting to see from this figure that increasing speed is even more 

 unfavorable to wet decks than was suggested in the upper part of Fig. 9. For 

 increasing the speed from 7-1/2 to 20 knots almost doubles the frequency of 



204 



