Page 275 - Practical Ship Design

Design of Lines 233

0.32
0.30

0.28
0.26
0.24

Froude 0.22
Number
0.20
0.18
0.16

0.14
0.12
0.55 0.60 0.65 0.70 0.75 0.80 0.85
Block Coefficient

Fig. 8.1. The combination of Froude number and block coefficient at which a bulbous bow is
likely to be advantageous.

Some deductions can, however, be made from Fig. 8.1 and in general it appears
that if a bulbous bow is not advantageous at the load draft, it will only become
advantageous in ballast if the ship is operated at or near its full power giving a speed
in ballast at least 10% or say 2 knots or so, more than the loaded service speed.
In the past, when tankers and bulk carriers making lengthy ballast voyages used
their full power on this leg, the gain in ballast speed was a clinching argument for
fitting a bulbous bow. Today fuel economy often keeps ballast speeds down to, or
lower than, the loaded service speed, and the argument for a bulbous bow is reduced.
It is worth emphasising that overall economy may require a balance between
designing for optimum performance fully loaded and in the ballast condition.
A bulbous bow will generally help to reducing pitching, but on the other hand it
is more likely to cause slamming.

8.2.2 Bulbous bow shapes

Bulbous bows come in a variety of shapes and sizes, as shown in Fig. 8.2. One
main division is that between a fully faired bulb and one in which there is a sharp
knuckle line between the bulb and a normal bow configuration. Claims have been
made for the advantages of each type, but the “added” bulb is generally simpler to