Practical Ship Hydrodynamics

152 Practical Ship Hydrodynamics
Understanding ship manoeuvring and the related numerical and experimental
tools is important for the designer for the choice of manoeuvring equipment
of a ship. Items of the manoeuvring equipment may be:
ž rudders
ž fixed fins (e.g. above the rudder; skeg)
ž jet thrusters
ž propellers (including fixed pitch, controllable pitch, slewable, and cycloidal
(e.g. Voith–Schneider propellers)
ž adjustable ducts for propellers, steering nozzles
ž waterjets
Both manoeuvring and seakeeping of ships concern time-dependent ship
motions, albeit with some differences:
ž The main difficulty in both fields is to determine the fluid forces on the hull
(including propeller and rudder) due to ship motions (and possibly waves).
ž At least a primitive model of the manoeuvring forces and motions should
be part of any seakeeping simulation in oblique waves.
ž Contrary to seakeeping, manoeuvring is often investigated in shallow (and
usually calm) water and sometimes in channels.
ž Linear relations between velocities and forces are reasonable approximations
for many applications in seakeeping; in manoeuvring they are applicable
only for rudder angles of a few degrees. This is one reason for the following
differences.
ž Seakeeping is mostly investigated in the frequency domain; manoeuvring
investigations usually employ time-domain simulations.
ž In seakeeping, motion equations are written in an inertial coordinate system;
in manoeuvring simulations a ship-fixed system is applied. (This system,
however, typically does not follow heel motions.)
ž For fluid forces, viscosity is usually assumed to be of minor importance
in seakeeping computations. In manoeuvring simulations, the free surface
is often neglected. Ideally, both free surface and viscous effects should be
considered for both seakeeping and manoeuvring.
Here we will focus on the most common computational methods for manoeuvring
flows. Far more details, especially on manoeuvring devices, can be found
in Brix (1993).
5.2 Simulation of manoeuvring with known coefficients
5.2.1 Introduction and definitions
The hydrodynamic forces of main interest in manoeuvring are:
ž the longitudinal force (resistance) X
ž the transverse force Y
ž the yaw moment N
depending primarily on:
ž the longitudinal speed u and acceleration Pu
ž transverse speed v at midship section and acceleration Pv