Page 215 - Introduction to Naval Architecture

RESISTANCE 201

reasonable as the full ship can be expected to create a greater
disturbance as it moves through the water. There is evidence of
optimum values of the coefficients on either side of which the
resistance might be expected to rise. This optimum might be in the
working range of high speed ships but is usually well below practical
values for slow ships. Generally the block coefficient should reduce as
the desired ship speed increases.
In moderate speed ships, power can always be reduced by reducing
block coefficient so that machinery and fuel weights can be reduced,
However, for given overall dimensions, a lower block coefficient means
less payload. A balance must be struck between payload and resistance
based on a study of the economics of running the ship.

Slimness
Slimness can be defined by the ratio of the length to the cube root of
the volume of displacement (this is Froude's circular M) or in terms of
a volumetric coefficient which is the volume of displacement divided by
the cube of the length. For a given length, greater volume of
displacement requires steeper angles of entrance and run for the
waterplane endings. Increase in volumetric coefficient or reduction in
circular M can be expected, therefore, to lead to increased resistance.
Generally in high speed forms with low block coefficient, the
displacement length ratio must be kept low to avoid excessive
resistance. For slow ships this is not so important. Fast ships require
larger length to beam ratios than slow ships.

Breadth to draught ratio
Generally resistance increases with increase in breadth to draught ratio
within the normal working range of this variable. This can again be
explained by the angles at the ends of the waterlines increasing and
causing a greater disturbance in the water. With very high values of
beam to draught ratio the flow around the hull would tend to be in the
vertical plane rather than the horizontal. This could lead to a reduction
in resistance.

Longitudinal distribution of displacement
Even when the main hull parameters have been fixed it is possible to
vary the distribution of displacement along the ship length. This
distribution can be characterized by the longitudinal position of the
centre of buoyancy (LCB). For a given block coefficient the LCB
position governs the fullness of the ends of the ship. As the LCB moves
towards one end that end will become fuller and the other finer. There