Strip Theory 



2.5 Influence of Ship's Length and Sea Spectrum 



After having systematically investigated the influence of parameters l/b , 

 L/i , c , and Fr on the moment coefficient c^, for a ship of 200 m length cross- 

 ing a confused sea generated by a 60-knot wind, it was decided to study the in- 

 fluence of ship's length and sea state. That was achieved without using the cal- 

 culation program shown in the last section of the present paper, because the 

 calculation had already furnished the response operators in regular waves for 

 certain \ L values. Considering that these response operators, when in non- 

 dimensional form, are not functions of L , but only of the ratio k/l , it was 

 possible to use such response operators in order to carry out the investigation, 

 simply by applying the superposition principle, on different ship's lengths and 

 different conditions of sea state (expressed by means of wind speed). 



In so doing the following very important fact was detected; the variation law 

 of bending moment coefficient c^ as a function of ship's length and of sea state 

 is in practice not affected by selected weights distribution, hull type, and ship's 

 speed and can be represented as a function of the only parameter L./k* , where 

 \* is the maximum energy wavelength for each given sea state (therefore \* is 

 a function of only the parameter w ). 



This fact is emphasized in Fig. 12, which is self-explanatory, where also 

 a curve is drawn, having the simple algebraic expression written at the end of 

 Section 1.4. 



The discovered characteristic allowed one to easily extend the investigation 

 to other w and L values; the results given above were found to be fully accept- 

 able, for w not much lower than 45 kn. 



3. CALCULATION PROGRAM 



3.1 Summary of the KK Theory 



The KK theory for the investigation of ship's motions and of loads acting on 

 a ship in a regular sea formulates and solves the differential equations of mo- 

 tion taking into account the phase relationship between ship's and wave motions 

 and the coupling effect of heaving and pitching. As this theory is well-known, it 

 seems here advisable to recall only formulas that are used in the calculation 

 program; reference is made to symbols collected in Table 15, where the num- 

 bers are shown of the formulas in Table 16 defining the relevant quantities. 

 The coordinate- system choice is shown in Fig. 13, which is self-explanatory. 



3.2 Description of the Calculation Program 



The program "Ship motions, shear, and bending moment in regular waves 

 and a confused sea" is written in FORTRAN 2 and processed on an IBM 7090 

 computer. It evaluates, for each hull and for each loading condition, shear and 

 still- water bending moment along the hull and besides heaving and pitching 

 motions in regular waves in head sea, for eight wave lengths and eight ship's 



843 



