Page 193 - Introduction to Naval Architecture

RESISTANCE 179

Although the different resistance components were assumed inde-
pendent of each other in the above non-dimensional analysis, in
practice each type of resistance will interact with the others. Thus the
waves created will change the wetted surface of the hull and the drag it
experiences from frictional resistance. Bearing this in mind, and having
discussed the general principles of ship resistance, each type of
resistance is now discussed separately.

Wave-making resistance

A body moving on an otherwise undisturbed water surface creates a
varying pressure field which manifests itself as waves because the
pressure at the surface must be constant and equal to atmospheric
pressure. From observation when the body moves at a steady speed, the
wave pattern seems to remain the same and move with the body. With a
ship the energy for creating and maintaining this wave system must be
provided by the ship's propulsive system. Put another way, the waves
cause a drag force on the ship which must be opposed by the propulsor if
the ship is not to slow down. This drag force is the wave-making
resistance,
A submerged body near the surface will also cause waves. It is in this
way that a submarine can betray its presence. The waves, and the
associated resistance, decrease in magnitude quite quickly with
increasing depth of the body until they become negligible at depths a
little over half the body length.

The wave pattern
The nature of the wave system created by a ship is similar to that which
Kelvin demonstrated for a moving pressure point. Kelvin showed that
the wave pattern had two main features: diverging waves on each side
of the pressure point with their crests inclined at an angle to the
direction of motion and transverse waves with curved crests intersecting
the centreline at right angles. The angle of the divergent waves to the
1
centreline is sin" !, that is just under 20°, Figure 8.2.
A similar pattern is clear if one looks down on a ship travelling in a
calm sea. The diverging waves are readily apparent to anybody on
board. The waves move with the ship so the length of the transverse
1
waves must correspond to this speed, that is their length is 2nV /'g,
The pressure field around the ship can be approximated by a moving
pressure field close to the bow and a moving suction field near the stern.
Both the forward and after pressure fields create their own wave system
as shown in Figure 8.3. The after field being a suction one creates a
trough near the stern instead of a crest as is created at the bow. The angle