448 Alex Goodman 
I would ask him if he has given reflexion to the following point. If the motion of the 
ship is not steady, the hydrodynamic forces and moments on the ship depend, partly, at the 
instant ¢, on the motion of the ship before this instant t. Therefore, the equations of the 
motion of the ship are not exactly the same as in the case where the forces and moments 
would at each instant equal their values in steady motion. One study made in France shows 
that the condition of stability is not modified; but the trend to the steady motion is more ac- 
centuated. Thus, the transient motions are affected in such a manner that, furthermore, 
seems to be favorable. 
Alex Goodman 
Dr. Todd’s paper raised a question pertaining to the stability and control requirements 
of a large submarine tanker. In my opinion, it would seem that such a vehicle would require 
a high degree of inherent stability as well as a properly designed autopilot. I noted a touch 
of pessimism in Mr. Newton’s comment on Dr. Todd’s paper which I cannot share. I think 
that with the techniques and means available today, solutions to the stability and control 
problems of such vehicles can be obtained without any difficulty. 
I would like to thank the discussers for their comments. I will be brief with my replies 
to the various questions that have been asked. Dr. Abramson asked whether the effects of 
hydroelasticity on the stability and control of submarines have been considered. We have 
not considered such effects. However, prediction of full-scale performance based on rigid- 
body dynamics has been very good. Answers to several of Mr. Tupper’s questions are in the 
written manuscript as well as in other Model Basin sources. Mr. Tupper questioned the need 
for the experimental determination of the acceleration derivatives by stating that theoretical 
techniques are available to estimate these derivatives. It is true that theoretical means are 
available for estimating the acceleration derivatives; however, in my opinion they are not 
accurate enough when it is required to determine the degree of stability. I can only say that 
by using the results obtained from tests using the Planar-Motion-Mechanism System we have 
been able to achieve excellent correlation with full scale performance. Correlations between 
this technique and the rotating arm technique are planned using the same model, essentially 
the same support system, and the same force-measuring-instrumentation system. I would 
like to thank Mr. Mathews for his compliments. As far as his question pertaining to the use 
of the Planar Motion Mechanism for the surface ship model testing, I can only say that some 
thought has been given to using this technique for such problems. However, I think we will 
first obtain some experience using the rotating-arm technique for the surface ship problem. 
Dr. Breslin raised a question regarding the effects of strut interference on the measurement. 
An extensive and detailed study of strut interference has been conducted at the Model Basin. 
The results of this study led to the strut design which has been incorporated in the Planar- 
Motion-Mechanism System, that is, small-chord struts in the vicinity of the model and angles 
of attack of the body being taken in the plane of the strut. As I mentioned in my presenta- 
tion, this eliminates the mutual interference effects between the strut and the body. There 
are small blockage effects. However, by choosing the proper extension of the lower half of 
the strut the resistance of bodies can be measured as accurately as is done by other stand- 
ard techniques. As to the required accuracy of static coefficients, I don’t recall to which 
paper Dr. Breslin was referring, but the one I remember which shows the comparison between 
static coefficients, obtained from straight line tests and those obtained by the extrapolation 
technique differed by as much as 10 to 20 percent. This is not a small percentage when we 
are talking about performing accurate analyses of maneuvers. With regard to Dr. Breslin’s 
comment as to the economy regarding small models compared to large models, this is an 
argumentative point. DTMB is at an advantage, since we feel that the large model is much 
cheaper to construct than a small specialized model. We use standard internal equipment 
