Page 264 - Introduction to Naval Architecture

PROPULSION 249

and:

These results confirm that as expected a higher revving propeller is
smaller in diameter and is less efficient.
Figure 9,15 did not allow for cavitation and should cavitation be a
problem curves from cavitation tunnel tests should be used.

SUMMARY

As was stated at the beginning of the last chapter, resistance and
propulsion are interdependent and the separation of the two is
artificial although convenient. It is appropriate therefore in this
summary to cover the work of both chapters.
There is resistance to the passage of a ship through the water. The
resistance of the naked hull measured in model tests can be considered
as comprising two components, the frictional and the residuary
resistance. These components scale differently in moving from the
model to full-scale. The residuary resistance, for geometrically similar
hulls at corresponding speeds, scales as the ratio of the displacements.
The frictional resistance component is estimated from experimental
data and scaled in relation to Reynolds' number. The naked hull
resistance must take account of surface roughness and be increased to
allow for appendages. Where necessary an allowance can be made for
the resistance of the above water form due to its passage through the air
although in the absence of a natural wind this is likely to be small.
Fitting a propulsor modifies the flow around the hull causing an
augment in resistance the hull experiences and modifying the wake in
which the propulsor must generate its thrust The flow through the
propulsor is not uniform so the efficiency will vary from that found in
open water tests. Taking all these factors into account the power to be
delivered by the propulsor for a given ship speed can be calculated.
The power required of the main propulsion machinery follows after
making allowance for transmission losses.