Page 283 - Practical Ship Design

Design cf Lines 24 I

twin-screw ship can be optimised almost entirely on resistance considerations with
little need to consider the flow to the propellers which, as has been seen, plays a
major part in the design of the stern of a single-screw ship.
In thinking about the LCB position and indeed about the value of the block
coefficient, it is important not to be mesmerised by the figures themselves but to
keep clearly in mind what they mean in terms of the ships lines.
Figure 8.7 tries to illustrate this point and shows how and why the LCB moves
as the c, changes from unity to a very fine form.
(i) At C, = 1 .OO the LCB must of course be at amidships.
(ii) For a barge the first essential is a swim bow, so at about C,, = 0.95 the LCB
moves aft to say 1.5% A.
(iii) The next improvement to be made to ease movement of the vessel is a
swim stern, so at about c,, = 0.90 the LCB moves back to amidships.
(iv) For the slowest self propelled shipshape vessel the bow is now generally
very full-spoon shaped and this coupled with the need for good flow to the
propeller(s), requiring fining aft means that for a c,, of between 0.90 and
0.75 the LCB is well forward, say about 2.5-3.0% or even 3.5% The use of
big “outboard” type propellers reduces the problem of propeller support on
a full ship and to some extent the problems of flow to the propellers and
enables very full block coefficients to be used.
(v) Once the run has been made such that it provides a satisfactory flow to the
propeller, it is only necessary to fine it very gradually as the block coeffi-
cient is further reduced for ships with higher speeds and powers. The
forebody, on the other hand, is where reductions in wavemaking resistance
can best be effected and from being markedly fuller than the aft body, the
forebody changes to being much finer, with the result that the LCB prog-
ressively shifts to a position well aft of amidships.
(vi) Finally for very fine ships there is a tendency for the LCB to return towards
amidships.
Figure 8.8 gives a plot of the optimum range of LCB position for both normal
and bulbous bow forms against C,, on the assumption that the block coefficient is
related to the Froude number generally in accordance with the WatsodGilfillan
line. It will be seen that there is an appreciable range of choice without incurring an
excessive penalty - but it is worth remembering that the penalty for a “too far
forward” position is usually much worse than that for a position “too far aft”.

8.3.2 The sectional urea curve

The sectional area curve is one of the principal factors which determines the
resistance of a ship and careful attention should be paid to its form. The first step