Swaan and Wahab 



GENERAL CONSIDERATIONS 



The model of the G.E.M. was tested in the Seakeeping Laboratory and in the 

 Shallow Water Laboratory of the N.S.M.B. 



It would have been desirable to use a model with six degrees of freedom but 

 that requires an autopilot in order to keep the model in its track. Because of 

 weight considerations it was decided to be content with only three degrees of 

 freedom for the model. In terms normally used in naval architecture they are: 

 heave, pitch and roll. This restriction is only of importance for the behaviour 

 of the model in oblique seas. 



In order to compensate for the lack of information caused by restricting 

 some of the motions the exciting forces were measured for surge and sway to- 

 gether with the yawing moment. 



The vehicle considered here proceeds over a free water surface, therefore 

 the Froude number (v/v^) must be the same for model and prototype in order 

 to equalize the scale factors for inertia forces and gravity forces. The same 

 rule must be applied in order to simulate the dynamic properties of the air 

 cushion as shown by Tulin [4]. 



The Reynolds number is of importance in order to take into account the 

 effect of viscosity. This has some effect on the flow around the vehicle in for- 

 ward flight and for the behaviour of the jets. 



The fact that this Reynolds number is different for model and prototype will 

 not be of importance for the frictional resistance because this will be small in 

 comparison with the total drag. 



The jet flow will be highly turbulent in the actual vehicle. Because the 

 Reynolds number of the model jets exceeds the theoretical critical value of 5500 

 it is justified to assume the model jets to be turbulent as well. It is therefore 

 expected that the flow around the model will be to a large extent similar to that 

 around the prototype. 



Another aspect which had to be considered is the effect of surface tension 

 which was clearly visible in the amount of spray generated by the jets. The 

 surface tension will be primarily of importance for the energy needed for main- 

 taining height. This aspect was not included in the program and therefore it is 

 considered to be of minor importance that the Weber number necessary to en- 

 sure similarity with respect to surface tension, is not the same for model and 

 prototype. 



DESCRIPTION OF THE MODEL 



The 1/14 scale model of the SKMR-I "Hydroskimmer" was made of plywood, 

 aluminum and plastic foam in order to provide for sufficient stiffness and 

 strength combined with light weight. A general arrangement plan of the model 

 is given in Fig. 1. 



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