is to reduce the model drift velocity in the direction of wave travel 

 and, conversely, the effect is to increase the drift velocity when the 

 forms are in the opposite direction. 



Mechanics of Drift 



To aid in the interpretation of the test results the mechanics of 

 the wave forces which cause the model to drift will be discussed. First, 

 it is important to note that an object floating on the surface of the 

 water in the wave tank did not drift for the wave conditions used in 

 the test. The drift of the MOBS model is caused by unequal wave forces 

 on the buoyant columns. This inequality in forces is due to the wave 

 acting over a longer portion of the buoyant column when a wave crest 

 passes than when a wave trough passes. There is therefore a net force 

 impulse in the direction of wave travel which causes the model to drift. 

 Pitching and heaving motions of the model will considerably effect the 

 length of the bouyant column over which the wave acts. The drift velocity 

 is therefore influenced by the motion of the model. This fact makes the 

 establishment of a meaningful baseline using a model without hydrodynamic 

 forms very difficult. For this reason the effect of the forms was 

 established by comparing the drift velocity with the forms in the normal 

 orientation to the drift velocity with the forms in the opposite 

 orientation. 



Test Results 



The results of the test are presented in Figure 4. The model drift 

 velocity is shown in the lower curve for the normal hydrodynamic form 

 orientation and in the upper curve for the opposite orientation. The 

 dotted line represents the mean between the upper and lower curves. The 

 effect of the hydrodynamic form on the drift velocity is estimated as 

 the difference between the lower curve and the mean curve. This compari- 

 son is considered the most accurate when the model was stable which was 

 outside of the region represented by values of H/T^ from 0.6 to 4.8. For 

 the very long wave lengths the model was stable and drifted opposite to 

 the direction of wave travel with very little relative motion between the 

 model and the surface of the water. This case corresponds to the extreme 

 left end of the lower curve. For the short wave lengths corresponding 

 to values of H/T^ greater than 4.8 the model was very stable with very 

 little pitch or heave. There is no apparent explanation for the peaking 

 of the upper curve in the range of H/T^ of 2.0 except possibly the fact 

 that the model was pitching considerably. The region does not, however, 

 correspond to the natural pitch or heave periods of the model as one 

 might expect. 



49 



