Propeller Excitation and Response of 230000 TDW Tankers 



different points in the deck house and in wing tank No 5 SB are shown 

 in Figures 3 and 4. The levels are given as amplitudes. In Figure 3 

 some levels, indicating the degree of unpleasantness, are marked for 

 comparison. They were calculated from the ISSC criteria, based on 

 acceleration, by using the formula 



y = a/(2 t f) 



where y = vibration amplitude 



a = acceleration, vertical 



f = frequency of vibration 



The results from the measurements in the wing tank indicate 

 weak resonance at certain shaft speeds. The results of Figure 4 are 

 the largest values measured over the speed range. 



The diagrams of Figures 3 and 4 show that the vibration am- 

 plitudes in the accomodation spaces were of reasonable magnitude. On 

 the first ship without fins, the amplitudes measured in the wing tank 

 were, however, considerable due to resonance at certain shaft speeds 

 (in general not the normal service speed). Unfortunately no measure- 

 ments were carried out in the wing tank on this ship, when fitted with 

 fins. From the results of the measurements in the deckhouse at the 

 different occasions it can, however, be concluded that the introduction 

 of fins reduced the vibration level considerably, but that the replace- 

 ment of the five-blades propeller with a six-bladed one caused no fur- 

 ther reduction of the vibration level. (In this connection it should be 

 remarked that the structures of the hull and deckhouse are identical 

 for the two ships apart from the fact that the second ship was fitted 

 with bilge keels). 



Pressure Fluctuations on the Hull 



The results of the measurements of the pressure fluctuations 

 at different points in the stern are given in Figures 5-6. In Figure 5 

 the two versions of the first ship are compared at fully loaded and 

 ballast conditions. The results shown were obtained with transducers 

 Nos 3 and 5 respectively, approximately corresponding to measuring 

 point A in model tests. The diagram shows faired mean values of the 

 peak to peak values 2p , obtained in the way shown in Figure 7, nor- 

 malised as 



K 



P 



2p 



^ 2 2 

 p D n 



589 



