264

NICHOLLS'S SEAMANSHIP AND NAUTICAL KNOWLFDOE

per Lour, the effective speed being the velocities of the ship and current
combined. Triangle ABC is a triangle of velocities.

Moorings.—The illustration shows a ship
alongside a quay. The pull on the headrope
AB is resolved into a fore-and-aft component
AC and a transverse component AD, the force
pulling ahead being considerably greater than
the side pull, as indicated by the relative lengths
of AC and AD.

The pull on the sternrope EF is resolved
into two components EG and EH, the triangle
of forces in this case being EOF. The transverse
component is greater than the fore-and-aft
component so that the rope EF will be a good
breast but not much of a spring, whilst AB.
is a better spring than a breastrope.

Action of the Rudder.—Figures 4 and 5 show re-
spectively the turning and the retarding effect of a
rudder angled at 20 degrees and angled at 40
degrees. If XY represents the plane surface of
the rudder and PA the fore-and-aft stream
current acting on it at A, then the water will
be deflected in the direction AQ, because the angle of impact XAP and
the angle of deflection QAY are equal,'and EA is the resultant force
acting at right angles to XY and bisecting angle PAQ.

Produce RA to B so that the length of AB is made equal, in conven-
ient units, to the force acting on the rudder. This force will depend
upon the area of the rudder and the velocity of the water impinging
upon it. From A draw AD athwartship and AC fore and aft. Com-
plete the parallelogram ACBD The length of AD measured from the
same scale represents the turning force acting on the rudder, and AC the
retarding force which tends to reduce the speed of the ship.

It will be noted for the smaller angle of rudder, Figure 4, that its
turning component is considerably greater than its retarding component,
but that for the larger angle they are almost equal, that is AD—AG
nearly (Fig. 5). The maximum angle of turning efficiency of a ship's
rudder is between 35 and 40 degrees; if the rudder is set at a larger angle
than this the turning power is reduced and the retarding effect on the
snip's speed is increased.

The lines on which a ship is designed is an important factor in