Distribution of Hydrodynamic Forces on a Shipmodel 



strip theory. In view of the above mentioned results such a simplification does 

 not hold for actual shipforms. 



For symmetrical shipforms at forward speed, it was shown by Timman and 

 Newman [7] that the damping cross- coupling coefficients for heave and pitch are 

 equal in magnitude, but opposite in sign. Their conclusion is valid for thin or 

 slender submerged or surface ships and also for non- slender bodies. 



Golovato's work [8] and some of our experiments [5] on oscillating ship- 

 models confirmed this fact for actual surface ships to a certain extent. 



The effects of forward speed are indeed very important for the calculation 

 of shipmotions in waves. The two-dimensional solutions for damping and added 

 mass of oscillating cylinders on a free surface, as given by Grim [9] and Tasai 

 [10] show a very satisfactory agreement with experimental results. When the 

 effects of forward speed can be estimated with sufficient accuracy, such two- 

 dimensional values may be used to calculate the total hydrodynamic forces and 

 moments on a ship, provided that integration over the shiplength is permissible. 



In order to study the speed effect on an oscillating shipform in more detail, 

 a series of forced oscillating experiments was designed. The main object of 

 these experiments was to find the distribution of the hydrodynamic forces along 

 the length of the ship as a function of forward speed and frequency of oscillation. 



THE EXPERIMENTS 



The oscillation tests were carried out with a 2.3 meter model of the Sixty 

 Series, havii^ a block coefficient Cg = 0.70. The main dimensions are given in 

 Table 1. The model is made of polyester, reinforced with fibreglass, and con- 

 sists of seven separate sections of equal length. Each of the sections has two 

 end-bulkheads. The width of the gap between two sections is one millimeter. 

 The sections are not connected to each other, but they are kept in their position 

 by means of stiff strain-gauge dynamometers, which are connected to a longitu- 

 dinal steel box girder above the model. The dynamometers are sensitive only 

 for forces perpendicular to the baseline of the model. 



By means of a Scotch- Yoke mechanism a harmonic heaving or pitching mo- 

 tion can be given to the combination of the seven sections which form the ship- 

 model. The total forces on each section could be measured as a function of fre- 

 quency and speed. 



A non- segmented model of the same form was also tested in the same con- 

 ditions of frequency and speed to compare the forces on the whole model with the 

 sums of the section results. A possible effect of the gaps between the sections 

 could be detected in this way. The arrangement of the tests with the segmented 

 model and with the whole model is given in Fig. 2. 



The mechanical oscillator and the measuring system is shown in Fig. 3. In 

 principle the measuring system is similar to the one described by Goodman [11]: 

 the measured force signal is multiplied by cos cot and sin cot and after 



221 



