Page 222 - Introduction to Naval Architecture

RESISTANCE 207

(2) Using Schoenherr.
Total resistance = 334 100N, allowing for roughness

Effective power = 2578 kW
(3) Using the ITTC line.
Total resistance = 324 200 N

Effective power = 2502 kW
As will be seen in Chapter 9, the effective power is not the power
required of the main machinery in driving the ship at the given speed.
This latter power will be greater because of the efficiency of the
propulsor used and its interaction with the flow around the hull.
However, it is the starting point for the necessary calculations.

SUMMARY

The different types of resistance a ship experiences in moving through
the water have been identified and the way in which they scale with size
discussed. In pracdce the total resistance is considered as made up of
frictional resistance, which scales with Reynolds' number, and residuary
resistance, which scales with the Froude number. This led to a method
for predicting the resistance of a ship from model tests. The total model
resistance is measured and an allowance for frictional resistance
deducted to give the residuary resistance. This is scaled in proportion
to the displacements of ship and model to give the ship's residuary
resistance. To this is added an allowance for frictional resistance of the
ship to give the ship's total resistance. Various ways of arriving at the
skin friction resistance have been explained together with an allowance
for hull roughness.
The use of individual model tests, and of methodical series data, in
predicting resistance have been outlined. The few full scale towing tests
carried out to validate the model predictions have been discussed.
Finally the concept of effective power was introduced and this
provides the starting point for discussing the powering of ships which
is covered in Chapter 9.

References
1. Milne-Thomson, L. M. Theoretical hydrodynamics, MacMillan.
2. Lamb, H. Hydrodynamics, Cambridge University Press.
3. Froude, W. (1877) On experiments upon the effect produced on the wave-making
resistance of ships by length of parallel middle body. TINA