Page 302 - Practical Ship Design

260 Chapter 8

A rudder is more effective if its top is arranged with only a small clearance from
the shell as this effectively increases the aspect ratio and reduces eddies.

8.7.10 Ship manoeuvring calculations

Like ship motions, ship manoeuvring has only become the subject of calculations
in recent years. Considering how important a role a ship’s manoeuvring capability
can have in the avoidance of collisions or of grounding in coastal waters, it is
surprising how long it has taken for statutory and classification societies to become
interested in this capability as an essential safety feature in a ship’s design.
In the past the interest of these authorities was limited to the time taken by the
steering gear to put the rudder hard over and even owners seemed content to record
the turning circle and rarely specified a maximum diameter except for ships where
manoeuvring was of particular importance. This aspect of design was left almost
entirely to the shipbuilder and they, in turn, contented themselves with providing a
rudder area based on a coefficient related to the product of the ship length and draft
that had given satisfactory performance for a previous ship of similar character-
istics for which they had records.
It is not surprising that quite a number of ships, particularly those with high
block coefficients, have had poor manoeuvring and/or poor course stability.
Modern computer-based calculation methods take account of a rudder force
coefficient based on the aspect ratio of the rudder, the rudder position relative to
midships, the flow velocity to the rudder based on the propeller race in which it
operates and the rudderhull interaction based on the turning velocity of the ship. If
necessary shallow water effects can also be evaluated.
Results can be presented for the turning circle, for a zig-zag manoeuvre and for a
reverse spiral curve, the last of these being used to detect potential directional
instability.

8.8 THE LINES ABOVE THE WATER LINE

8.8.1 Section shape
Above the waterline the shape of the sections can be determined by a number of
factors. The freeboard at bow, stern and amidships helps to determine the flare. If
the freeboard is relatively small, the angle of flare must be large partly to achieve
the required deck area and partly to help limit the water shipped on deck. If the
freeboard is large both of these objectives can be obtained by a gentle flare.
Flare at the bow and outward sloping sections aft have been practised for many
years; more recently, however, the advantage of flaring the midship section has
been pointed out. This advantage lies in the gain in large angle stability that flared