Page 214 - Practical Ship Design

Powering I I79

Correction for any variation in BIT from the standard value of 2.5 is made as
follows:

103[C,(B/T> - C,(2.5)] = 0.16 [BIT- 2.51 (6.4 1 )

Corrections for the form of the sections are suggested if these are either
extremely U or extremely V-shaped:
- U-shaped bow sections and V-shaped stem sections reduce C,;
- V-shaped bow sections and U shaped stem sections increase C,.

The standard form has an orthodox non-bulbous bow and corrections are given
for bulbous bows of different sizes at a range of Froude numbers. In general a
bulbous bow is shown to be advantageous at high Froude numbers relative to block
coefficient. The corrections are given for the loaded condition but there is a
statement that bulbous bows can give a remarkable decrease in resistance for full
forms in the ballast condition.
A ship-model correlation factor for roughness and scale effect based on ship
length is applied as a correction factor C,. The value of this ranges from
C, = +0.4 x for a ship L= 100 m

through a zero value for a length of 200 m to
C, = -0.3 x lo-’ for L > 300 m

The similarities which this method has with some aspects of Taylor’s method
and some aspects of Ayre’s method may be noted.

6.9 HOLTROP AND MENNEN’S METHOD

The fact that the C, method just discussed is already out of date to modern tank test
procedures following the introduction of the form factor has been noted. Addition-
ally it is not computer friendly as C, must be read from a graph.
Holtrop and Mennen’s method, which was originally presented in the Journal of
International Shipbuilding Progress, Vol. 25 (Oct. 1978), revised in Vol. 29 (July
1982) and again in N.S.M.B. Publication 769 (1984) and in a paper presented to
SMSSH’88 (October 1988), meets all these criteria with formulae derived by
regression analysis from the considerable data bank of the Netherlands Ship Model
Basin being provided for every variable. Many naval architects use the method,
generally in the form presented in 1984 and find it gives acceptable results
although it has to said that a number of the formula seem very complicated and the
physics behind them are not at all clear, (a not infrequent corollary of regression
analysis).