A Catechism of the Steam Engine

by C. E. John Bourne

A.--Beattie's screw is an arrangement of the screw propeller whereby it is projected beyond the rudder, and the main object of the arrangement is to take away the vibratory motion at the stern,--an intention which it accomplishes in practice. There is an oval eye in the rudder, to permit the screw shaft to pass through it.

600. Q.--When the diameter of the cylinder of water projected backward by a screw, and the force urging it into motion are known, may not the velocity it will acquire be approximately determined?

A.--That will not be very difficult; and I will take for illustration the case of the Minx, already referred to, which will show how such a computation is to be conducted. The speed of this vessel, in one of the experiments made with her, was 8.445 knots; the number of revolutions of the screw per minute, 231.32; and the pressure on each square foot of area of the screw's disc, 214 lbs. If a knot be taken to be 6075.6 feet, then the distance advanced by the vessel, when the speed is 8.445 knots, will be 3.7 feet per revolution, and this advance will be made in about .26 of a second of time. Now the distance which a body will fall by gravity, in .26 of a second, is 1.087 feet; and a weight of 214 lbs. put into motion by gravity, or by a pressure of 214 lbs., would, therefore, acquire a velocity of 1.087 feet during the time one revolution of the screw is being performed. The weight to be moved, however, is 3.7 cubic feet of water, that being the new water seized by the screw each revolution for every square foot of surface in the screw's disc; and 3.7 cubic feet of water weigh 231.5 lbs., so that the urging force of 214 lbs. is somewhat less than the force of gravity, and the velocity of motion communicated to the water will be somewhat under 1.087 feet per revolution, or we may say it will be in round numbers 1 foot per revolution. This, added to the progress of the vessel, will make the distance advanced by the screw through the water 4.7 feet per revolution, leaving the difference between this and the pitch, namely 1.13 feet, to be accounted for on the supposition that the screw blades had broken laterally through the water to that extent. It would be proper to apply some correction to this computation, which would represent the increased resistance due to the immersion of the screw in the water; for a column of water cannot be moved in the direction of its axis beneath the surface, without giving motion to the superincumbent water, and the inertia of this superincumbent water must, therefore, be taken into the account. In the experiment upon the Minx, the depth of this superincumbent column was but small. The total amount of the slip was 36.53 per cent.; and there will not be much error in setting down about one half of this as due to the recession of the water in the direction of the vessel's track, and the other half as due to the lateral penetration of the screw blades.

601. Q.--Is it not important to make the stern of screw vessels very fine, with the view of diminishing the slip, and increasing the speed?

A.--It is most important. The Rifleman, a vessel of 486 tons, had originally engines of 200 horses power, which propelled her at a speed of 8 knots an hour. The Teazer, a vessel of 296 tons, had originally engines of 100 horses power, which propelled her at a speed of 6-1/2 knots an hour. The engines of the Teazer were subsequently transferred to the Rifleman, and new engines of 40 horse power were put into the Teazer. Both vessels were simultaneously sharpened at the stern, and the result was, that the 100 horse engines drove the Rifleman, when sharpened, as fast as she had previously been driven by the 200 horse engines; and the 40 horse engines drove the Teazer, when sharpened, a knot an hour faster than she had previously been driven by the 100 horse engines. The immersion of both vessels was kept unchanged in each case; and the 100 horse engines of the Teazer, when transferred to the Rifleman, drove that vessel, after she had been sharpened, 2 knots an hour faster than they had previously driven a vessel not much more than half the size. These are important facts for every one to be acquainted with who is interested in the success of screw vessels, and who seeks to obtain the maximum of efficiency with the minimum of expense. [1]

[1] See Treatise on the Screw Propeller, by John Bourne, C. E.

PROPORTIONS OF SCREWS.

602. Q.--In fixing upon the proportions of a screw proper to propel any given vessel, how would you proceed?

A.--I would first compute the probable resistance of the vessel, and I would be able to find the relative resistances of the screw and hull, and in every case it is advisable to make the screw as large in diameter as possible. The larger the screw is, the greater will be the efficiency of the engine in propelling the vessel; the larger will be the ratio of the pitch to the diameter, which produces a maximum effect; and the smaller will be the length of the screw or the fraction of a convolution to produce a maximum effect.

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