603. Q.—­Will you illustrate this doctrine by a practical example?

A.—­The French screw steamer Pelican was fitted successively with two screws of four blades, but the diameter of the first screw was 98.42 inches, and the diameter of the second 54 inches.  If the efficiency of the first screw by represented by 1, that of the second screw will be represented by .823, or, in other words, if the first screw would give a speed of 10 knots, the second would give little more than 8.  The most advantageous ratio of pitch to diameter was found to be 2.2 in the case of the large screw, and 1.384 in the case of the small.  The fraction of a convolution which was found to be most advantageous was .281 in the case of the large screw, and .450 in the case of the small screw.

604. Q—­Were screws of four blades found to be more efficient than screws with two?

A—­They were found to have less slip, but not to be more efficient, the increased slip in those of two blades being balanced by the increased friction in those of four.  Screws of two blades, to secure a maximum efficiency, must have a finer pitch than screws of four.

605. Q.—­Are the proportions found to be most suitable in the case of the Pelican applicable to the screws of other vessels?

A.—­Only to those which have the same relative resistance of screw and hull.  Taking the relative resistance to be the area of immersed midship section, divided by the square of the screw’s diameter, it will in the case of the Rattler be 380/100 or 3.8.  From the experiments made by MM.  Bourgois and Moll on the screw steamer Pelican, they have deduced the proportions of screws proper for all other classes of vessels, whether the screws are of two, four, or six blades.

606. Q.—­Will you specify the nature of their deductions?

A.—­I will first enumerate those which bear upon screws with two blades.  When the relative resistance is 5.5 the ratio of pitch to diameter should be 1.006, and the fraction of the pitch or proportion of one entire convolution should be 0.454.  When the relative resistance is 5, the ratio of pitch to diameter should be 1.069, and fraction of pitch 0.428; relative resistance 4.5, pitch 1.135, fraction 0.402; relative resistance 4, pitch 1.205, fraction 0.378; relative resistance 3.5, pitch 1.279, fraction 0.355; relative resistance 3, pitch 1.357, fraction 0.334; relative resistance 2.5, pitch 1.450, fraction 0.313; relative resistance 2, pitch 1.560, fraction 0.294; relative resistance 1.5, pitch 1.682, fraction 0.275.  The relative resistance of 4 is that which is usual in an auxiliary line of battle ship, 3.5 in an auxiliary frigate, 3 in a high speed line of battle ship, 2.5 in a high speed frigate, 2 in a high speed corvette, and 1.5 in a high speed despatch boat.

607. Q.—­What are the corresponding proportions of screws of four blades?