Page 274 - Practical Ship Design

232 Chupter 8

(v) Features conducive to good seakeeping and good manoeuvrability.
(vi) An LCB position at the load draft which in association with the weights
and centres of gravity of the ship and its deadweight items enables the ship
to be loaded in a way that will result in satisfactory trim.
(v) KM values at operating drafts which will ensure satisfactory stability
when the ship is loaded as intended.
(vi) The avoidance of discontinuities that may have adverse structural conse-
quences.
(vii) If possible the lines should be production kindly, with as much flat plating
as can be arranged and with the minimum amount of double curvature in
the shell plating.
(viii) Whenever possible, and particularly on passenger ships, the lines should
have an aesthetic appeal.

8.2 THE BOW AND STERN

8.2.1 Normal or bulbous bow

The first decision to be taken in relation to the bow is whether to fit a “normal” or a
“bulbous” bow. A normal bow is cheaper to manufacture and a bulbous bow
should only be fitted if doing so will reduce the resistance and thereby either
increase the speed or reduce the power required and with it the fuel consumption.
Figure 8.1 shows the range of Froude numbers and block coefficients at which
such an improvement is likely to be obtained when operating at the load draft.
The superimposition of the WatsodGilfillan Cb line on this diagram indicates
the area which is of practical concern and it can be seen that bulbous bows:
(i) are advantageous for fast ships with C, values less than 0.625 and F,
greater than about 0.26;
(ii) present no advantage for ships with cb values between 0.625 and 0.725 -
unless these are “over driven” according to the WatsodGilfillan criterion;
(iii) are again advantageous for cb values between 0.725 and 0.825, but prob-
ably not for Cb values over 0.825.
It is worth noting that at all block coefficients, bulbous bows show to best
advantage on over driven ships and are often disadvantageous on ships which are
relatively fine for their speeds.
It must be emphasised that this analysis refers only to the load draft condition. It is
generally accepted, however, that bulbous bows can offer their greatest advantage in
the ballast condition, particularly on full lined ships with block coefficients in excess
of 0.75. This being so it would have been nice to make a similar plot for the ballast
condition, but unfortunately suitable data to do this does not seem to be available.