Vortex Theory for Psedies Moving in Vater 
Some cases of course unstability had been stated, but regarded as 
rather exceptional and the behavior of the ship in transient motions 
was not a prior matter of concern. The situation became quite a dif- 
ferent one with submarines achieving very high speeds. A dynamic 
stability in the vertical and horizontal planes of motion are required, 
and an automatic pilot system is used for performing combined ma- 
neuvers in both planes. 
Surface vessels such as big tankers also need remarkable 
maneuvering qualities ant to that end, they are also equipped with 
automatic steering systems. 
Nevertheless, the automatic control of the ship does not 
solve all the difficulties involved in maneuvering. One can even say 
that, from a certain point of view, it gives rise to new problems. 
The writer has described some years ago some among the 
methods used at the Bassin d'Essais des Carénes for studying ex- 
perimentally the case of submerged bodies, determining the coef- 
ficients of the equations of the motion and predicting the real motion 
of the full scale vehicle [10] . Ina recent paper [1 1] , M. Gertler de- 
veloped analogous views on the purpose of this type of matched ex- 
perimental and mathematical researches. 
However powerful this way may be, it leads to the introduc- 
tion in the equations of motion of much too many coefficients and per- 
haps in an unappropriate manner. This situation does not favour the 
prcegress of the knowledge of the fundamentals in maneuverability. In 
ee) , the writer drew the attention to the time response to a maneu- 
ver and the risk of erroneous interpretation of experimental results. 
The writer is of the opinion that new purely theoretical researches 
are needed. Casal's thesis has been given as an example to base this 
opinion. The present paper has been inspired by the same line of 
thought. 
CONCLUSION 
Although the Vortex Theory plays an important role in many 
Chapters of Ship Hydrodynamics, it does not seem to be used in all 
the cases where it could really be fruitful. It is so when one deals 
with the Ship herself. 
Several explanations of the rather reluctant attitude of the 
Naval Architects with respect to the application of vortex theory to 
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