Page 187 - Introduction to Naval Architecture

8 Resistance

Although resistance and propulsion are dealt with separately in this
book this is merely a convention. In reality the two are closely inter-
dependent although in practice the split is a convenient one. The
resistance determines the thrust required of the propulsion device.
Then propulsion deals with providing that thrust and the interaction
between the propulsor and the flow around the hull.
When a body moves through a fluid it experiences forces opposing the
motion. As a ship moves through water and air it experiences both water
and air forces. The water and air masses may themselves be moving, the
water due to currents and the air as a result of winds. These will, in
general, be of different magnitudes and directions. The resistance is
studied initially in still water with no wind. Separate allowances are made
for wind and the resulting distance travelled corrected for water
movements. Unless the winds are strong the water resistance will be the
dominant factor in determining the speed achieved.

FLUID FLOW

1 2
Classical hydrodynamics ' leads to a flow pattern past a body of the
type shown in Figure 8.1.
As the fluid moves past the body the spacing of the streamlines
changes, and the velocity of flow changes, because the mass flow within
streamlines is constant. Bernouilli's theorem applies and there are
corresponding changes in pressure. For a given streamline, ifp,p, vand

Figure 8.1 Streamlines round elliptic body
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