LOADS ACTING ON A SHIP AND THE ELASTIC RESPONSE OF A SHIP 



-Tow Bars 



267 



Accelerometer 



Accelerome+er 



Fig. 1 1 Diagram of model setup for determining bending moments in waves (from E. V. Lewis, 1954) 



stem accelera- 



The dcHections 



1 fso that it was 



rigid bodv with 



111 Fig. 11. A wooden model, 4.79 ft long, had lieeii 

 divided at the midsection and was jointed liy a hinge 

 at a neutral axis of the ship's midsection. The ahgu- 

 ment of two halves was maintained by metal flexiu-e 

 bars. A deflection transducer had be(>n located at the 

 stern and was operated Ity a cantile\'er truss from the 

 forward part of the model. The relationship between 

 deflections and bending moments was established l>y 

 calibration. Hull deflections, bnw and 

 tions, and wave profiles were recorded, 

 of the model had been sufficiently smal 

 permissible to consider the model as a 

 regard to slowly applied loads. The rigidity of the 

 flexure bars was such that the natural vibration period 

 of the model corresponded to a two-node frer|uency of the 

 ship. This correspondence between the model and tlie 

 ship was expected to give a faithful representation of 

 slamming phenomena. All data were transmitted from 

 a model to a shore station and were recorded by a gal- 

 vanometer on the same photographic tape. In the 

 early tests the pitching and heaving motions were ob- 

 tained from the photographs of the targets installed at 

 the bow and stern of the model. In later tests the motions 

 transtluccrs were used and the motions were recorded 

 by the galvancimcter together with other data. 



4.11 Experimental data. Fig. 12 .shows the bending 

 moments for the model at zero speed as well as the bend- 

 ing moments computed by the standard static procedure, 

 with and witlKjut Smith effect. The (lualitative agree- 

 ment with Schnadel's sea measurements is evident: at 

 L/20 wave height, the measured hogging bending 

 moment is equal to half of the one computed by the con- 

 ventional procedure. It is about 74 per cent of the static 

 moment when the latter is computed with Smith ett'ect. 



In order to gain a better understanding of the phe- 

 nomena involved, a test was made with the rear part of 

 the model rigidly attached to the te.st carriage. In this 

 case the model inertia forces w'ere eliminated and only 

 hydrodynamic forces resulting from wave motions were 

 present. A much larger moment was recorded in this 

 condition. This test clearly demonstrated the load 

 relief which had resulted from the model freedom to 



250 



200 



S 150 



SO 



c 50 



•S<t 



i 30 



t 10 



Wove Lcngth=Ship Leng+h 

 i"n all Cases 



Legend; E/perlmen+ol Poinds, 

 + Hoc 

 o Sag 



Model 

 Retrained^ 



Wave Height, in. 



Fig. 12 Comparison of calculated bending moments with ex- 

 perimental values, model of T2-SE-A1 tanker at zero speed, both 

 restrained and free (from E. V. Lewis, 1954) 



yield to the waves. E. V. Lewis' explanation of this 

 phenomenon is shown in Fig. 13. At zero model veloc- 

 ity the phase relationships are favorable in that the 

 maxima of moments occur when the model and water 

 move in the same tlirection. This reduces the forces 

 exerted by waves. 



Fig. 14 shows the effect of the forward speed of the 

 model on bending moments and motions in regular waves. 

 It appears that ship speed has no significant effect on 

 bending moments as long as it remains sufficiently 

 below the synchronous one. This speed range probably 

 corresponds to a ship's operation in rough weather. 

 In following seas the sagging and hogging moments are 

 approximately equal, but in head seas the sagging mo- 

 ment is increased liy about oO per cent. 



Model tests also included higher than practical speeds 

 in order to investigate conditions which may occur in 

 faster ships. The hogging moment was found to in- 

 crease gently and the sagging moment more rapidly, 

 with the further increase of spe(>d beyond the synchronous 

 one. Fig. 14 (also Fig. Hi) shows a pronounced reduction 

 of the bending moment at synchronous speed, but this 



