already pointed out that the consequences of such an omission will not be serious.
It has been assumed that the whole resistance of the vessel is due to its skin friction. In marine propulsion “wave making” plays a prominent part; the resistance from this cause may be regarded as a force applied from without, since the waves travel away, taking their momentum with them; the consideration of wave making resistance would destroy the precise balance between and on which expression (5) is based; the matter of wave making has, however, no interest to us from an aerodynamic standpoint.
§ 201. Propulsion under Actual Conditions.—Under actual conditions neither of the hypotheses discussed in §§ 198 and 200 applies in its entirety. The requirements of the former hypothesis are most nearly met in the case of a paddle boat (with the paddles at the sides); the latter case is best exemplified in the stern-wheeler, a flat-bottomed type of craft whose propeller is particularly well placed for capturing the frictional wake. In the forms that succeed in practice the propeller is usually behind in the frictional wake, never in front; and the successful forms of propulsion are those in which a sufficient mass of water per second can be conveniently handled; thus, jet propulsion has become practically extinct. We are, therefore, led to appreciate the soundness of the Newtonian method.
There are many methods of mechanical propulsion, that is to say, there are several known mechanical devices for producing the reaction on the fluid which we have so far regarded as being accomplished by action at a distance. Firstly, we have the numerous devices employed by nature in the locomotive mechanism of birds, fishes, etc.; secondly, we have the primitive devices employed by man—the paddle, the oar, etc.; and finally, we have the two great inventions of marine engineering, the paddle wheel and the screw propeller. Of these various types of propeller, only two will be discussed as of interest in connection
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