Page 232 - Introduction to Naval Architecture
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PROPULSION 217
Handing of propellers
If, when viewed from aft, a propeller turns clockwise to produce ahead
thrust it is said to be right handed. If it turns anti-clockwise for ahead
thrust it is said to be left handed. In twin screw ships the starboard
propeller is usually right handed and the port propeller left handed. In
that case the propellers are said to be outward turning. Should the
reverse apply they are said to be inward turning. With normal ship
forms inward turning propellers sometimes introduce manoeuvring
problems which can be solved by fitting outward turning screws.
Tunnel stern designs can benefit from inward turning screws.
Forces on a blade section
From dimensional analysis it can be shown that the force experienced
by an aerofoil can be expressed in terms of its area, A; chord, c, and its
velocity, V, as:
Another factor affecting the force is the attitude of the aerofoil to the
velocity of flow past it. This is the angle of incidence or angle of attack.
Denoting this angle by a, the expression for the force becomes:
This resultant force F, Figure 9.6, can be resolved into two components.
That normal to the direction of flow is termed the lift, L, and the other
Figure 9,6 Forces on blade section
in the direction of the flow is termed the drag, D, These two forces are
expressed non-dimensionally as:
