Dynamics of Naval Craft- System Identtficatton 
Since the hydrodynamic contributions to Mg by the sidewalls are 
known to be small (from other analytical studies), and there is also 
a small influence of the destabilizing moment due to pressure, these 
two quantities generally negate each other leaving mainly the hydro- 
static term. This result appears to be sufficient, since the estimate 
of Mg given by Figures 4-6 is close to the value of -9.0 (for 
Mg /ly) obtained from hydrostatics, and can then be considered to 
be a good estimate of this particular parameter. 
The magnitude of Mg determined by this technique was 
somewhat different from the values estimated by theory due to the 
influence of the sidewalls. The predominant contribution to this 
stability derivative is due to the rigid stern seal hydrodynamic forces, 
which depend upon the seal wetted length. Since it is difficult to 
determine this contribution due to the seals, and the simplified 
equation system does not include any representation of seal dynamics, 
the procedure only gives a measure of the overall seal contribution 
to the resulting vehicle dynamics in this case. 
It would appear on the basis of these results that the pre- 
sence of the seals has an important influence on the values of effec- 
tive stability derivatives in the linearized equations representing 
SES craft motions. Since they are not statically fixed, and transmit 
their forces directly to the craft, some other representation would 
be necessary to adequately model an SES craft together with the 
seals and their influence on the craft dynamics. This would certain- 
ly require some representation of the seal motion as a separate 
degree of freedom, together with measurements of the seal motions 
in order to characterize the seal parameters adequately. Since this 
was not done in a way that would provide useful information in the 
full scale measurements, no mathematical modeling was carried out 
for the seals (i.e. for coupling the seal motions with the vehicle 
motions) in the case of simulation studies used for the application of 
system identification to such a system. One possible influence of the 
seals that has not been included in the determination of various sta- 
bility derivatives is the fact that the seal forces due to the varying 
seal and bubble pressure have an effect on the seal as well, which in 
turn influences the resulting dynamics of the craft. Since only limit- 
ed treatment of the hydrodynamic forces on the seal has been includ- 
ed, that may be one reason why there is poor matching between the 
theoretical prediction of the damping-type stability derivatives that 
included the seal terms, as compared to the estimates obtained from 
system identification. A much more precise match of measured data 
and quantities that must be modeled in the mathematical representa- 
tion has to be made in order to carry out successful system identi- 
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