140 



THEORY OF SEAKEEPING 



-2 -0. 



Experimental motion study, 0.8 lb towing force 

 Regular seas: X = 5 ft, \lh = 16.7 Natural periods: (T„)g = 0.65 sec; (T„). = 0.70 sec 

 Ballast condition: A= 38.19 1b Gyradius: 1.394 ft 



Fig. 34 Towing tank record of slamming of a model of Liberty ship in steep regular waves (from Szebehely and Lum, 1955) 



smooth water. The difficulty shifts to the evakiation of 

 ship motions and the definition of the ship-water surface 

 rehxtionship at the instant of slam. 



Szebehely and Todd (1955) pointed out that occur- 

 rences of high local pressures may not coincide with the 

 total force felt at a slam, but they did not pursue this 

 important topic further. High local pressures are gen- 

 erated, in accordance with Wagner's theory, whenever a 

 ship section of small deadrise penetrates the water sur- 

 face with sufficient vertical velocity. If the draft of a 

 ship and the heaving and pitching motions are such that 

 iiigh pressures are generated successi\'ely at one section 

 of a ship after another in a sufficiently slow succession, 

 the total force will remain small and a slam will not be 

 felt. Such conditions are found in most cases of bow 

 emergence, since the calculations of M. A. Todd (1954) 

 and of Bledsoe (1956) show that a high sectional force 

 occurs at a very small instantaneous draft. This force 

 can occur simultaneously over a large area only when a 

 ship's bottom is almost parallel to the water surface. 

 The high pressures then occurring simultaneously 

 over a number of ship sections add up to a large force 

 which is felt as a slam. In the case of a ship in waves 

 this postulates a certain amount of up-heave as is evi- 

 dent in Figs. 33-36. This condition of parallelism also 

 is more readily fulfilled at shallow draft than at deep 

 draft. 



Slamming appears to be one of the main reasons for re- 

 ducing a ship's speed in bow seas. The work of Szebe- 



hely and his associates has thrown a considerable amount 

 of light on the nature of this phenomenon. The value of 

 their work lies not so much in the successful perform- 

 ance of a difficult analysis as in the vivid demonstration of 

 the harmful effects of nearly flat bottoms in bow areas. 

 Others also have commented on the importance of ship 

 form in slamming; e.g.. King (1934-35) and Kent (1949). 

 C)chi's (1956) experiments in a towing tank on U and 

 V-section form ships, Fig. 37, demonstrate that slam- 

 ming acceleration is much lower for the V-forms than 

 for the U-forms. Practicing naval architects appear to 

 be slow in adapting themselves to this effect of ship 

 form . 



7.4 Calculated Slamming Pressures. Bledsoe (1956) 

 calculated slamming pressures for fixe models of the 

 Series 60, ranging in block coefficient from 0.60 to 0.80. 

 These calculations were made for slamming-approach 

 conditions similar to the ones assumed in the experiments 

 of M. A. Todd (1955). A characteristic result of these 

 calculations was the extremely high value of the peak 

 pressure. 



Quoting Bledsoe: "The absolute magnitude of the 

 maximiuii pressure (for example, 1450 psi for Station 3, 

 of ....-'■' is not in itself too significant. The susceptibil- 

 ity of the ship to damage depends not only on the mag- 

 nitude of the slamming pressures but on their extent and 



2' Figure referred to is not reproduced here. It gives the pres- 

 sure distribution on the 0.75 block coefficient hull. 



