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56 THE SUBMARINE TORPEDO BOAT
dom from danger by “rooting” and the consequent loss of
control would be found to be of great military advantage.
The theory of contending forces was reasoned as follows:
The forward position of the propellers being the center
of the applied force causes the direction of the force or the
movement of the vessel to be always along the line of its
Figure 4. Forward Propulsion
axis, and is the common pivotal point about which the
moments for all the forces are at work. The long lever
arm between this position of the propellers and the posi-
tion of the rudders affords the maximum turning moment
which can be obtained and insures positive control at all
times. This is evidenced by the fact that the thrust of
the water upon the upper portion of the hull, no matter
at what angle of inclination, always acts when diving to
depress the stern, in opposition to the upward thrust of
the rudders, and is never threatening to upset the whole
balance, but on the contrary it tends to right the boat and
therefore make for increased controllability.
By referring to Figure 4 it will be seen that the upward
force of the reserve buoyancy B is in this case a moment
upward about P; the thrust S is a moment downward