phase lag angles of all the experimental data. The circular cylinder has 

 a mean phase lag of 75° indicating a dominative inertia force. For the 

 semi-circular model, most of the phase lags fall between 90° and 180°. 

 Therefore, the wave force on the semi-circular model is affected by both 

 drag and inertia forces. This is also true for the crescent shaped model. 

 The difference between these two is that all small wave heights the semi- 

 circular model has a higher percentage of inertia force than that for 

 the crescent model. At the larger wave heights, the trends are reversed 

 but the data are too scattered to give conclusive results. The inertia 

 force is proportional to the virtual mass which increases with the amount 

 of water displaced by the submerged object. Consequently, the trend from 

 inertia domination for the circular model to near drag domination for the 

 crescent model is probably due to the difference in the amount of water 

 displaced by the models. 



To convert the model data to the MOBS prototype, simply multiply 

 the model wave height and length by the length factor of 60 and multiply 

 the wave force by the force factor of 216,000. Notice that the net 

 peak force is neither a constant force nor a sinusoidal force. It is 

 recommended, therefore, that the test data be used with caution. 



FINDINGS 



1. For wave frequencies near the natural pitching frequency, the 

 model platform with circular legs had the fastest drift velocity. High 

 drift velocity over a wide frequency range was observed during tests of the 

 platform having crescent shaped legs with waves coming at the concave 

 side. The slowest platform was the same platform model with the convex 

 side facing the waves. For very low frequency waves, this model was 

 observed to move against the direction of wave propagation. 



2. The frequency response of the three platforms does not follow 

 the motion of a one degree of freedom mechanical system. The dynamically 

 measured natural frequencies are higher than the statically measured 

 natural pitching frequency. 



3. Net negative peak wave forces were measured only on the crescent 

 shaped platform leg model with the convex side facing the waves. These 

 forces were relatively small and were proportional to the wave height. 



4. Large net positive peak forces were measured on both crescent 

 and semi-circular leg models with waves approaching the concave and flat 

 sides. 



5. For the test conditions, inertia force dominated the wave force 

 on circular cylinders and drag force essentially dominated the wave force 

 on crescent cylinders. 



17 



