This chapter will provide you with a
general overview of the forces involved
during maneuvering and sailing of ships
increased velocity gives decreased
pressure water is a liquid it flows and
it is nearly incompressible when water
has to pass through a smaller space its
velocity will increase the increased
velocity will decrease the pressure an
example of this is squat when the water
has to pass between the hull and the
seabed its velocity will increase this
increase will result in a decrease
pressure which will make the vessel sink
deeper in the water accepting that ship
handling can be defined as using forces
under control to manage forces not under
control these forces can be divided into
the following three categories forces
under direct control forces under
indirect control forces not under
control click the pictures to learn more
about these forces these forces should
be under direct control during ship
handling ship's main engine ship's
propeller ships thruster ships rather
ship's anchor ships moorings
professional ship handlers should
familiarize themselves with the
performance and limitations of various
ships systems used during maneuvering
the most important forces said to be
under indirect control are the ship's
inertia other hydrodynamic factors such
as bank effects squat interaction etc
the ship inertia varies with different
loading conditions but the loading
conditions can normally not be changed
during a maneuver to a certain extent
changing the speed
either the vessel can control the
hydrodynamic moment of inertia other
hydrodynamic factors such as squat bank
effects etc can to a certain extent to
be avoided or minimized if the unknown
and understood in practice it is not
possible to calculate the influence from
all these forces continuously in real
life
these forces must be estimated based on
experience and up-to-date information
there is no substitute for experience in
ship handling the most important forces
not under control are tied wind waves
current fouling although we cannot
control these forces we can through
close observation and by using available
information sources obtain relevant
information which can be used to
minimize the effect of these forces the
pivot point is the point of the ship
turns around
click this text to continue when the
ship is dead in the water
the pivot point is generally in the
center of the ship when initially
ordering engines ahead the pivot point
shifts forward as the speed increases
once the ship is steady state steaming
the pivot point settles back at about
one third from the bow when ordering
engines are stern the opposite takes
place the pivot point shifts aft and
settles at about one third from the
stern
the distance from the pivot points to
the applied force gives you the turning
livre a long lever gives you more
turning moment than a short click this
text to continue when making headway you
get a long turning liver and
consequently you get a greater turning
moment when making stern way you get a
short turning liver and consequently you
have less moment to turn the ship with
as you can see from the animations the
rudder is much more effective when
making headway the same principles apply
when you use bow thrusters or tugs
the water flow from the propeller around
the rather creates forces that help you
turn the vessel if you do not have a
water flow around the rudder you will
have no turning effect when turning the
rudder the water flowing around it will
create a pressure zone on one side and a
suction zone on the other these two
zones together will create a force
called lift if you study the animation
to the right you will see that the water
flow is faster on one side of the rudder
by employing a Bernoulli's law you will
understand how the lift force is created
ship handling in practice can be said to
be a circular movement between observing
estimating assessing deciding and
executing a good advice to all ship
handless is to remember the old saying
full speed is full-speed the first step
on the way to becoming a professional
ship handler is observation of the whole
situation estimating and assessing
possible actions deciding what to do
executing the decisions
relevant and up-to-date information is
essential for the ship handler such
information can be obtained by visual
observation instrument observation
studying the ship's maneuvering
characteristics estimating the external
forces correctly obtaining detailed
local knowledge communicating with
relevant information sources ie other
ships vessel traffic services etc it's
not possible and relevant to calculate
all the forces and their influence on a
particular maneuver
however in order to undertake a correct
estimate of what to do in a particular
maneuver situation as many of these
forces as possible should be known and
taken into account a ship's ability to
maneuver depends on the ship's steering
ability
the ship's turning ability
the ship stopping ability
it's important that the ship handler is
aware that the majority of the ships in
service today are operated without any
proven documentation of the ships
maneuvering performance standards the
professional ship handler should
familiarize themselves with the
advantages and limitations of available
equipment in order to optimize its use
whenever needed another important
qualification necessary for the
professional ship handler is an ability
to think and plan ahead of the ship
thus being at all times prepared for the
next move