Page:Aerial Flight - Volume 1 - Aerodynamics - Frederick Lanchester - 1906.djvu/336

§ 213 uniformly as possible the whole of the fluid, the angle allotted to successive increments will be diminished as shown in the figure; the loss of efficiency resulting from modifying the curve in this manner, or even of making serious departures from the theoretical curve, is probably microscopic. The theory as here presented will prove of more value to the designer by letting him realise exactly what he is doing, than from its too rigid and literal application.

§ 214. On the Marine Propeller.—The marine propeller, which has already been made the subject of comparison, is of especial interest as representing in a concentrated form the experience of over half a century. Beyond the advantages to be gained from an examination of established practice as a quantitative check on our investigation, there is some reasonable probability that we may arrive at facts having a bearing on the future evolution of screw propeller design.

The conclusions to which theory has led have in many cases been already tested by appeal to experience, with, on the whole, satisfactory results, but we have so far made no comparison on the very vital subject of pressure-velocity relationship.

The pressures proper to the conditions of least resistance (minimum gliding angle), given in Tables X. and XII., will be very much greater where water is concerned, owing to the greater density. On the other hand, the coefficient $$\xi$$ is less, which will have a slight effect in the opposite direction.

The form of blade employed in the marine propeller is of necessity confined to low values of $$n ;$$ long slender forms such as may be well suited to an air propeller will be too weak (unless made of disproportionate thickness) to stand the pressure required. In practice the value of $$n$$ employed is less than 3, usually very much less; we will therefore confine our attention to blades of this proportion with the knowledge that the results as to pressures and efficiency ought in general to be higher than those that obtain in practice.