42 
ON THE POSITION OF THE SCREW. 
of Mr. Smith. The screw would then act upon water to which the vessel has 
communicated no motion, and which must obviously be more favourable to 
obtaining the maximum resistance than when the fluid has been disturbed by the 
motion of the vessel. The dead water or following current to which we have referred 
at page 34 must deduct in some degree from the full effect of the propeller, because, 
if we assume an extreme case, and suppose the action to take place in a recess in 
the stern, as shown at fig. 30, then, as the screw cannot be fed with water in a 
Fig. 30. 
direct line, or, to speak more correctly, as the greater portion of the water in the 
recess would be carried along with the vessel, the action would approach that of a 
screw acting in a close cistern of water; inasmuch as a considerable portion of 
the power of the engine would be employed in overcoming a resistance which 
communicates no motion to the vessel. Hence it is obvious that in proportion as 
there is a disposition in the water to follow the vessel, so will power be consumed 
which does not contribute to propel her. On the other hand, if the screw be 
placed in the bow, though the defect just named would be obviated, another evil 
is generated, namely, that of throwing the water put in motion by the action of 
the screw against the bow, which, of course, causes a consumption of power, by 
its being necessary to overcome the resistance of the opposing current thus created, 
in addition to the usual and unavoidable resistance encountered by her passing 
through the water. The screw in the bow also would be more liable to injury 
than in the dead wood. 
It has also been proposed to use two screws, one on each side of the dead wood, 
as in fig. 31. 14 
14 Captain George Smith, R.N., took a patent in 1838 for applying two propellers, consisting of plane 
blades, one on each side of the dead wood.