The Marine Steam Engine ... by Richard Sennett

Author:Richard Sennett
Language: eng
Format: epub
Publisher: Longmans
Published: 1898-03-25T05:00:00+00:00

^ Negative apparent slip. —From the nature of the medium in whicb

r the screw-propeller works, it is clear that in every case there must

^ necessarily be positive real slip, as it is the change in the backward

\r momentum of the water which causes the propelling reaction or thrust;

^ hut it is not difficult to imagine cases in which the water following

the ship has such an initial velocity that the apparent slip might

, be negative -that is, the speed of the ship might be greater than that

^ /W^s^ of the screw-propeller. From the formula (3) above, it will be seen

iX ^ \^ r^ .y^j. *bat if yx be considerable, V may be greater than v quite consistently

with a positive value for the real slip, in which case the apparent slip ^ /V N - ^ooAr^^^^^^ ^ negative. This has sometimes occurred, and before the .M ' 'matter was sufficiently understood gave rise* to a vast number of

nX ti^\^><^^i^ theories to account for its existence.

Q » d^ Possibly in some cases the apparent negative slip may be attributed

' "^"^ to some extent to the difficulty of correctly estimating the true mean

I .v>i 8* CK^^k P^^c^ of an ordinary screw. The method adopted practically is to

. divide each blade by a number of circular arcs at equal distances •^v%J^apart, as in Fig. 300, and to measure the pitch or pitches at each arc "^- " -' separately. It is generally found that the pitches at the various radii are somewhat different; and frequently at each radius the pitch of the leading part differs from that of the following part of the blade. The arithmetical mean of all the pitches thus measured, for all the screw-blades, is called the * mean pitch' of the screw ; but considering the different velocities with which the several sections pass through the water, and their different obliquities and areas, it is by no means certain that this method gives the true mean pitch of the propeller as regards its propulsive effect, and a comparatively small error in estimating the mean pitch might considerably affect the calculated apparent slip of the screw.

Propeller race. —The action of the screw-propeller is to drive stern ward a cylindrical column of water, usually called the * propeller race,* and the thrust of the screw is measured by the stem ward momentum generated in this race in a unit of time. The area of this race is approximately equal to that of the screw disc, less that of the boss of the screw, the race being in fact approximately a revolving annular column. In consequence of the obliquity of the propelling surfaces, the race receives a rotatory as well as a sternward motion, and this centrifugal action causes a certain loss of thrust. The race of a screw-propeller may be conceived to be a series of concentric cylinders of water moving sternward, and rotating at different velocities. It is evident that the thrust must be diminished both by the centrifugal motion and by the frictional action of the particles of water.

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