142 



THEORY OF SEAKEEPING 



Lw = 600 en 

 Hw= 23cm 



Fig. 36 Towing-tank record of slamming of a flat-bottom 



shallow-draft ship model in regular waves (from Akita and 



Ochi, 1955) 



up to the time of niaximuin force was of the order of Vs 

 ft full scale. The presence of sharp-crested wavelets 

 2 ft high would have increased the length of the shock- 

 absorbing tra\'el threefold, and would considerably re- 

 duce the impact force. Theoretical and experimental re- 

 search on the impact of flat and small-deadrise plates on 

 a rippled water surface is suggested. The impact of a 

 flat surface on a single wave crest of a specified included 

 angle can be theoretically investigated. This can next 

 be generalized to apply to a complex sea surface by means 

 of the statistical methods discus.sed iu Chapters 1 and 

 3. 



7.6 Forces Generated by a Bow Flare. As has been 

 stated earlier, strong slams usually occur when the de- 

 scentling velocity of the bow is high and the ship-wave 

 phase relationship is such that the water surface is rising 

 to meet the bow. In natiu'al irregular seas this usually 

 happens at the recurring periods of vmusually high and 

 steep waves. Since slamming absorbs but little energy, 

 the downward pitching motion continues and often 

 water is shipped over the bow. Walls of green water de- 

 flected by the flared sides of a ship near the bow are fa- 

 miliar to mariners. Large water jjressures are produced 

 as a reaction to this deflection. In accordance with Wag- 

 ner's (1931) theory, equations (62), (63) and (64), water 



0.8 1.2 l.G 2.0 2.4 



Ship Speed Vm M/sec 



0.4 



0.8 



1.6 



F'M/JT 



ZA 



Fig. 37 Eflfect of bow form on slamming acceleration of a ship 

 model in regular waves (from Ochi, 1956) 



pressure increases with decrease of the angle between the 

 tangent to the body surface and the horizontal. This 

 angle, even in the largest flare, is ne\'er small, usually over 

 45 deg. Unit pressures are, therefore, not high, and the 

 rate of force de\'clopment is relatively slow. Flare pres- 

 sures, however, can cover a large area and generate a 

 large force which seriously increases the bending stress in 

 a ship. The "flare impact" will he discussed in greater 

 detail in Chapter 5 in connection with observations made 

 on destroyers at sea. 



The forces caused by slamming are usually applied at 

 such a rapid rate that their effect on a ship cannot be 

 expressed by the laws of statics. It lieeomes necessary 

 to consider the elastic response of the ship's structure. 

 I'\irther discussion of the slamming process will be de- 

 ferred therefore to Chapter ,5. 



