Page 221 - Practical Ship Design

Except for shallow draft ships the variation in TIL is limited and factor 3 has
little effect as can be seen by comparing lines 20 and 21.
Factor 4 is of course dependent on the LCB position and to a lesser extent on the
C,, both of which may on occasion vary from the norms assumed in the table.
A variation in Cp from the norm, such as applies to frigates with a high rise of
floor and a low C, will also change factor 6 increasing (1 + K), an effect shown in
lines 21 and 22 of Table 6.2.
Some further thought suggested that it might be useful to have a basic value of
(1 + K) which applies to a ship which is “middle of the range” in all respects and
develop a series of factors that correct its (1 + K) value for changes in the various
factors from the assumed basic values.
It was hoped that this would give both a better feel for the value of (1 + K) likely
to apply to a particular type of ship form and of the relative effects exercised by the
various form factors.
For the middle of the range ship it seemed appropriate to use the 122 x 16.76 x
7.32 m ship with Cb = 0.70, C,,, = 0.975, C, = 0.718, normal stern sections, LCB
amidships, for which (1 + K) has already been calculated. This gives a standard
( 1 + K) value of 1.185 and the following guidelines for changes from the standard.
Stern lines: basic is “normal”
for U sections add 0.02
for V sections subtract 0.02

LIB ratio: basic is 7.3
normal range is 5.5 to 8.5
per unit of (LIB - 7.3) subtract 0.02
per unit of (7.3 -LIB) add 0.03

TIL ratio: basic is 0.06
normal range for cargo ships is 0.058 to 0.064
other ship types may go down to 0.03 (hardly worth
consideration)
LCB position: basic amidships
per 1 % aft of amidships subtract 0.02
per 1 % forward of amidships add 0.04

Block coefficient: basic 0.70
per unit of 1o(cb - 0.70) add 0.06
per unit of lO(0.70 - Cb) subtract 0.03

Further comments on (1 + K) are given in the next chapter.