Lewis 



DISCUSSION 



G. Aertssen 



University of Gent 

 Gent, Belgium 



This paper is an excellent approach to the trend of the wave-induced mo- 

 ments in extreme seas and the investigation comes at the right moment. It is 

 known that in extreme seas some waves are exceptionally high (heights of 80 ft 

 have been recorded in the North Atlantic). But on the other hand strain gages 

 applied to the stringer plating of the main deck of usual cargo ships of 500 ft 

 showed in these extreme seas bending moments which were not greater than the 

 calculated bending moment, the ship being poised on a trochoidal wave of a 

 length Lpp and a height Lpp/12, i.e., a height of about 25 ft. It has been argued 

 that the reason for this was the ability of the ship to adapt herself to the actual 

 shape of the sea, especially when it is considered that in this extreme sea the 

 ship of 500 ft is hove to at a speed of about 5 knots. 



Prof. Lewis comes to a better explanation when applying to the bending mo- 

 ments the superposition principle and accepting a spreading function for the en- 

 ergy of the assumed short- crested sea. The surprising result is that for the 

 500 ft cargo ship having Cg = 0.8 the wave induced bending moments are then 

 quite the same as the conventional static bending moments. 



A second important result of this work is the deviation from the L/20 law 

 for long ships. It was known that for these ships a smaller wave height must be 

 taken and a wave height 0.6L°^ was proposed. This again, as Prof. Lewis 

 shows in Fig. 6, is a very good approximation for all bulk carriers and tankers 

 now under construction and ranging from 500 to 800 ft. 



There is an old rule limiting the bending stresses calculated on a basis L/20 

 to 5L + 500 Kg. per sq mm, L is ship length in m. This rule holds good for 

 L = 150m where the allowable stress is 1250 Kg. per sq cm and for the largest 

 bulk carriers up to L = 200m where the allowable stress is 1500 Kg. per sq cm, 

 which means 20 percent more for the longer ship. This allowance of 20 percent 

 is exactly — as Prof. Lewis shows in Fig. 6 — the error in excess when applying 

 for large bulk carriers the static method on a base L/20. This is a support for 

 the static calculation based on L/20, even for ships up to 200m, provided the 

 allowable stress is given by the formula 5L + 500 Kg. per sq cm. 



There are other remarkable results emerging from Prof. Lewis' paper. 

 Stresses and bow motions in the realistic short- crested sea are reduced in beam 

 and in following seas as compared with head seas but less than would be expected 

 and this is especially true for the stresses. The relative bow motions are 

 roughly the same in beam and in following seas. The writer recently, in a paper 



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