Page 216 - Introduction to Naval Architecture

202 .RESISTANCE

will be a position where the overall resistance will be minimized. This
generally varies from just forward of amidships for slow ships to about
10 per cent of the length aft of amidships for fast ships. In considering
the distribution of displacement along the length the curve of areas
should be smooth. Sudden changes of curvature could denote regions
where waves or eddies will be created.

Length of parallel middle body
In high speed ships with low block coefficient there is usually no
parallel middle body. In ships of moderate and high block coefficient,
parallel middle body is needed to avoid the ends becoming too full. For
a given block coefficient, as the length of parallel middle body
increases the ends become finer and vice versa. Thus there will be an
optimum value of parallel middle body for a given block coefficient,

Section shape
It is not possible to generalize on the shape of section to adopt but slow
to moderate speed ships tend to have U-shaped sections in the fore
body and V-shaped sections aft. It can be argued that the U-sections
forward keep more of the ship's volume away from the waterline and so
reduce wave-making.

Bulbous bow
The principle of the bulbous bow is that it is sized, shaped and
positioned so as to create a wave system at the bow which partially
cancels out the ship's own bow wave system, so reducing wave-making
resistance. This can only be done over a limited speed range and at the
expense of resistance at other speeds. Many merchant ships operate at
a steady speed for much of their lives so the bulb can be designed for
that speed. It was originally applied to moderate to high speed ships but
has also been found to be beneficial in relatively slow ships such as
tankers and bulk carriers and these ships now often have bulbous bows.
The effectiveness of the bulb in the slower ships, where wave-making
resistance is only a small percentage of the total, suggests the bulb
reduces frictional resistance as well. This is thought to be due to the
change in flow velocities which it creates over the hull. Sometimes the
bulb is sited well forward and it can extend beyond the fore
perpendicular,

Triplets
The designer cannot be sure of the change in resistance of a form, as
a result of small changes, unless data is available for a similar form as
part of a methodical series. However, changes are often necessary in the