Ship handling

Overview

When handling a ship, particularly in narrow waters, a number of factors must be taken into account

These can be divided into:

ship dependent factors

- type of machinery

- number and type of propellers

- number and type of rudders

- number and type of thrusters

- draught

- trim

- shape and condition of the hull

external factors

- wind, (sea and swell)

- tide or current

- depth and extent of surrounding water

- distance to other ships

Type of machinery

Depending on the machinery there are two factors that may differ

These are:

- a difference in ahead and astern power

- a difference in time needed for changing from

ahead to astern

The majority of modern merchant ships have diesel engines with astern power being up to approx 80 per cent of ahead power (with different ratios when it comes to steam or gas turbines)

Type of propellers

Most ships still have a single right-handed fixed propeller (traditional propeller with blades in the fixed position), i.e. turning clockwise when going ahead and viewed from astern

More and more ships have a CPP, controllable pitch propeller

Now the shaft and propeller rotate in one direction only and astern power is achieved by reversing the pitch of the blades and is the same as ahead power

Some ships have more than one propeller

- twin screws or triple screws

All propellers have something called 'turning effect' or 'transverse thrust' (sideways moving effect of a turning propeller)

In other words they do not only 'push' the ship ahead or astern but also sideways

This effect for a ship with a right-handed propeller is:

1. going ahead> stern is 'pushed' to the right and consequently the bow to port

2. going astern> stern is 'pushed' to the left briskly and the bow to starboard

It is obvious that with a right-handed CPP the effect is always as mentioned under 1.

Type of rudder

Most ships are fitted with a single balanced rudder or semi-balanced rudder

A rudder angle of about 35° is considered to give maximum efficiency

A rather new type is the flap rudder

- That's a rudder with a hinging back part

- They are able to "bend" the waterflow almost to 90 degrees to the fore and aft line

For a rudder to be effective the ship needs to make sufficient steerage way (enough movement of the ship for ther rudder to be effective)

Type of thrusters One type is built-in

It consists of a straight tunnel through the bow and sometimes also one through the stern

Hence bow thrusters and stern thrusters

Another type, not built-in, is the azimuthing propeller (also called pod drive)

This is a retractable system - when needed, it is lowered from a recess in the ship's bottom

It is able to turn 360°

Some ships have pod drives as their main

propulsion system

In that case they are fitted permanently

outside the hull

A ship's ability to move perfectly sideways

without turning or forward speed is called

'crabbing' in modern jargon

Draught and trim

In general ships that are trimmed by the

stern by 2 or 3 feet have the best steering

properties

Trim – fore and aft balance of a ship

If either the bow or the stern is deeper in

the water, the vessel is said to be down by

the head or down by the stern

Wind The effect of the wind depends on the area

presented to the wind before or aft of the ship’spivoting point

This point is not a fixed point but depends on whether the ship moves ahead or astern

Ships with the main superstructure aft, such as tankers, will turn towards the wind (Also called: to bear down)

An offshore supply boat will turn away from the wind (Also called: to bear away) – effect on steering

Another effect is that the ship as a whole is moved sideways, certainly with a strong beam wind > this is called: to make leeway

Speed Full sea speed and manoeuvring speed are

different things

At full sea-speed the engine cannot suddenly be

reversed but the RPM need to be reduced

For a ship with steam turbines, needing the astern

turbine, the minimal time for slowing down is about

one minute and for a diesel engine it will be close

to two minutes

For ships with a CPP the required time will be far

less since the shaft will keep turning in the same

direction

Moving stern-first (moving stern

foremost) To move stern-first means that a ship is moving

backwards over same distance

When there is not sufficient room to turn ahead of

her, she may have to move stern-first until in an

adequate position

If a ship is making sternway (same meaning) the

rudder effect is opposite to and much smaller than

the effect when making headway

For this reason, ships that move stern-first

frequently (ferries, drifters - fishing with floating

net) may be equipped with a bow rudder

Making fast

Often the first line is taken to the quay by the

boatmen (people in a small mooring boat

who receive the first ship’s line and take it to

the bollard)

The ship is then still at same distance away

from the quay

This line is slowly lowered into their boat

from the ship's forecastle while they give

directions from below

If not assisted by boatmen, there will be

linesmen waiting

A heaving line is thrown out from the ship

and caught by a linesman

This will sometimes take a few efforts which

must be the reason for having boatmen

Sometimes 'the shore' also requires the use

a messenger

In that case the messenger - a thinner Iighter

line - is attached to the heaving line first and

next the thick ship's line on to the messenger

The lines are put on bollards

The holes in the ship's side that the lines

pass through are called fairleads

The mooring lines

- ropes and wires > the number depending

upon the size of the ship, the prevailing

conditions of wind, tide and currents

Ropes and wires leading forward and aft will

stop the vessel ranging up and down the

quay, while "breastropes" leading close to

the quay will keep the ship firmly alongside

Shifting berth and warping

Shifting berth means both moving to

another anchorage berth or alongside

berth in port

Warping > originally meant to move a ship

from one place to another by means of

ropes only

Nowadays it is also used as a synonym for

shifting berth

Number of tugs

Port regulations (by-laws) will prescribe

how many tugs the ship will have to use

This will depend on her size, handling

characteristics and whether or not she is

carrying dangerous cargo

Moreover, the pilot will take the weather

conditions into account

Pilot card

With many leading shipping companies it is

customary to give such a completed card to

the pilot as soon as he enters the

wheelhouse

As a VTS operator it helps you realize what

the important manoeuvring essentials are, in

particular when communicating in English

with a ship with pilot exemption

Crash Stop When the engines are put astern to stop as quickly

as possible after going full ahead the manoeuvre is known as a crash stop

Because of the turning effect of the rudder the ship will gradually turn away from its initial course

The total distance travelled in the direction of the ship's initial course is the advance distance

The distance the ship has moved away from this same imaginary line in stopped position is called transfer distance

For a loaded 200,000 tonnes tanker it may take 25 minutes with an advance and transfer distance of 12 and 4 ship lengths respectively

Turning circle

If the rudder is put hard over when the ship

is going ahead and kept in this position, the

ship will describe a turning circle

The diameter is usually between 3 and 5

ship lengths for a merchant ship of any size

going at full ahead and using full helm

A listed vessel will normally have a smaller

turning circle towards the high side

Shallow water effects

When moving in relatively shallow water,

there may be certain "hydrodynamic"

interactions influencing ship handling

The effect depends on the depth/draught

ratio

If less than two it should not be neglected

Squat The effect will be that the ship will "sink" deeper into

the water and there will be a change in trim

The overall decrease in the UKC

If a ship moves ahead at too high a speed in

shallow water, the grounding may occur at the bow

or stern

An increase in average draught of more than 2 m

may happen to a ship moving at a fairly high speed

Squat is approximately proportional to the square of

the ship's speed

So halving the speed reduces the squat effect by a

factor of four

Bow cushion and bank suction

In a narrow fairway the bow may be pushed

away from the bank and the hull attracted to it

The first effect, called bow cushion, is caused

by the bow wave or more precisely the

pressure field of the bow

The second - bank suction - is caused by the

increased speed of the water in the restricted

space between the hull and the bank

Interaction between passing ships

A similar effect will take place when two

ships moving at high speeds pass close to

each other. The effect will be more important

with both ships moving in the same direction

because this takes much longer than on

opposite courses

When a big ship is overtaking a smaller one

at acIose distance there will be two

dangerous stages

1. The first is when the bow of the big ship

approaches the stern of the smaller one >

the latter will tend to swing across the path

of the big one

2. The second dangerous stage is when

the two ships are paralleI > The smaller

one will tend to turn into the side of the big

one