House D.J. Ship Handling Theory and Practice

Подождите немного. Документ загружается.

ship manoeuvrability. What is being highlighted is that stopping, or increasing

speed, are alternative actions to decreasing speed and should not be dismissed out

of hand. They are and remain, options, and the circumstances of each scenario will

dictate what is considered prudent at the time.

Wheel over points

The dangers of interaction are prevalent in many different situations, but none more

so than when the vessel enters shallows and is in close proximity to the land. Tight

manoeuvres must be anticipated through rivers, canals and when making land falls.

The large vessel must anticipate that the position of the way point is rarely coinci-

dent with the time at which the helm will be applied. Masters would be expected to

ensure that passage plans include ‘wheel over points’ when vessels are approaching

positions of course alteration.

Comment: Watch Officers are reminded, however, that Regulation Six is not a stand

alone regulation, and the ColRegs also stipulate that: ‘Assumptions should not be made

on the basis of scanty information, especially scanty radar information’.

56 SHIP HANDLING

Transfer at 60°

from ships data

Advance at 60°

from ships data

Initial

Course

Wheel Over

Point

Way

Point

Beam Bearing

New

Course

The danger of collision

when wheel over point is

not applied to the way

point

Outbound

Traffic

Wheel over point. This example shown for a 60° alteration of course. Advance and transfer

details can be referenced from the ships sea trials and performance documentation.

Ch02-H8530 4/9/07 9:47 AM Page 56

Canal and river movements

Rivers and canals by their very nature have restricted water compared to open sea

conditions. When the ship is in transit through a canal, the vessel occupies a volume

of the canal space causing effectively a blocking restriction to water movement.

Squat and blockage

Blockage factor illustrated

MANOEUVRING CHARACTERISTICS AND INTERACTION 57

W/L

reduced buoyancy

increased

buoyancy forces

with greater UKC

The illustration above shows that because the underkeel clearance is small,

the volume of water under the keel is small and would not have the same buoy-

ancy affect on the hull as noted in deeper water; bearing in mind that the position

of buoyancy is defined as the geometric centre of the underwater volume. If the

vessel is heeled by external forces the water plane will increase, the position of

‘B’ would move upwards but, at the same time, also outwards towards the angle

of heel. This leaves the low side, at the turn of the bilge, liable to contact with the

ground.

When two ships are passing in the channel the blockage factor is increased and the

value of squat experienced can expect to nearly double.

Practically the vessel may expect to encounter steerage problems caused by squat

and the proximity of the canal bottom in relation to the position of the keel. Speed of

movement would be critical and the vessel must expect to move at a greatly reduced

speed. The use of tugs, fore and aft to effect steerage control must also be anticipated

as being an absolute necessity.

Blockage Factor

that proportion of a midshi



pp's section

cross sectional area of the channnel,riverorcanal

Ch02-H8530 4/9/07 9:47 AM Page 57

Squat – ship’s response

The behaviour of a ship in shallow water, where the forces of buoyancy are reduced, can

expect to be totally different to the behaviour of the same ship in deeper water, where

the buoyancy forces will have a much greater affect. Factors affecting the actual value of

squat will vary considerably but could expect to include any or all of the following:

a) Draught/depth of water ratio. A high ratio equates to a greater rate of squat.

b) The position of the longitudinal centre of buoyancy (LCB) will determine the

trimming effect and have a direct relation to the squat value.

c) High engine revolutions can expect to increase stern trim.

d) The speed of the vessel is related to the value of squat in that the value is influ-

enced by speed

. The faster the ship moves the greater the squat value.

e) The type of bow fitted effects the wave making and pressure distribution on the

under water volume.

f) The length/breadth ratio can cause an increase or decrease of the squat value, i.e.

short-tubby ships tend to squat more, than the longer narrow beam vessel.

g) The breadth/channel width ratio affects the squat value. A high ratio causing an

increased value of squat.

h) Vessels with a large block coefficient C

will experience greater effects from squat.

i) Greater effects of squat are experienced when a vessel is trimmed by the bow

than by the stern.

Evidence of squat

The indication that a vessel is experiencing squat will show from the steering being

affected. Waves from the ship’s movement will probably increase in amplitude and

the wake left by the vessel will probably be mud stained. Some vibration may also

occur with a decrease in speed and a reduced rpm.

58 SHIP HANDLING

The ‘Stena Leader’ seen departing the Roll on–Roll off river berth in Fleetwood, Lancashire.

The dangers of interaction in many forms in such confined waters is ever present for the

larger vessels. Reduced speed of operation is considered essential along with reliable

machinery and steering gear.

Ch02-H8530 4/9/07 9:47 AM Page 58

Interaction – situations within the marine environment

MANOEUVRING CHARACTERISTICS AND INTERACTION 59

A large tanker manoeuvres in close proximity to an FPSO in offshore regions. Such close

manoeuvres between vessels and fixed or floating structures are known to generate inter-

active forces which tend to hamper ship handling operations.

A small tug, seen engaged with the large carrier ‘HUAL Trotter’. The danger of interaction is a

one when small craft like tugs are engaged in close proximity to larger, parent vessels.

Ch02-H8530 4/9/07 9:47 AM Page 59

Where a vessel is brought into close proximity of a bank, as in a canal or river, it may

experience a pressure build up between the hull and the obstructing bank, known as

‘bank cushion effect’. This pressure build up would effectively turn the bows of the

vessel away from the bank and force the ship’s heading into the middle channel

area, away from the restrictions of the channel sides.

60 SHIP HANDLING

Stage 1

Stage 2

Stage 3

Suction

Area

Interaction forces – vessels meeting ‘End On’ passing too close.

Ch02-H8530 4/9/07 9:47 AM Page 60

In itself, this can be countered by applying helm, if it is expected and catered for.

However, the movement from a bank cushion effect could have serious consequences

if, say, the vessel is being overtaken or meeting an oncoming vessel moving in the

opposite direction, the vessel close to the bank taking a sheer towards the oncoming

traffic.

The pressure cushion generated cannot be avoided, but the violent reactive move-

ment can be curtailed by reducing the speed of approach towards the bank. The

speed of the vessel being reduced to steerage way only, will expect to minimize the

outward turning effect of the vessel.

MANOEUVRING CHARACTERISTICS AND INTERACTION 61

Stage 1

Stage 2

Stage 3

Turning

Force

Bows deflect outward,

Stern quarters attracted.

Ship ‘B’ will experience an

increase in speed.

Interaction forces – one vessel overtaking another, too close.

Ch02-H8530 4/9/07 9:47 AM Page 61

Shallow water effect

When ships make a landfall from a deep sea position they may experience a form of

interaction with the sea bed, known as ‘Shallow Water Effect’. It is especially notice-

able where the shoals and the change in depth becomes abrupt and may cause the

ship’s steering to be affected, the bows being pushed off course to either port or star-

board as the vessel experiences a sharp change in underkeel clearance.

62 SHIP HANDLING

Bank cushion effect shown on a

vessel where the rudder is retained

in the midships position and the

vessel sheers away from the bank

with the pressure build up without

any helm movement.

Interaction forces – bank cushion effect.

Shoals

Deep

Water area

Area of

expected sheer

Ch02-H8530 4/9/07 9:47 AM Page 62

MANOEUVRING CHARACTERISTICS AND INTERACTION 63

Following Tide

Tide Ahead

Negotiating bends in tidal riders. ‘P’ represents the position of the ship’s pivot point when

going ahead.

As the vessel approaches the shoal area, the interaction between the hull and the

closeness of the sea bed may cause the vessel to sheer away. A reaction that can be

quickly corrected by alert watchkeepers but could generate a close quarters situ-

ation if other traffic is in the near vicinity.

Ch02-H8530 4/9/07 9:47 AM Page 63

Interactive forces between tug and parent vessel

With the following example a tug is to engage with a parent vessel on the starboard

bow. Interaction between the smaller and larger vessels could generate a collision

scenario. Prudent use of the helm and speed by the tugmaster will be crucial in

collision avoidance.

64 SHIP HANDLING

The smaller tug experiences

an outward turning effect

along the parallel hull lines of

the parent vessel

The outward turning effect can be

countered by applying Port Helm in

this example

The tug experiences the maximum

outward turning force at the shoulder

position of the larger vessel

Because the tug is carrying Port Helm

and still moving ahead, a loss of

turning force is experienced under the

flare of the ships bow. The tug could

sheer across the bows of the larger

vessel with inevitable collision

1 2

3 4

Ch02-H8530 4/9/07 9:47 AM Page 64

Introduction; Anchor terminology; Anchor types – securing and stowage arrangements;

Anchor planning – single anchor procedures, safety precautions when employing anchors;

Anchor watch – dragging anchors; Anchor work operations; Baltic and Mediterranean

moorings, running and standing moors; Dredging an anchor; Large vessels; Kedge

anchor; Emergency use of anchors; Mooring operations; Hanging off an anchor; Clearing

foul hawse; Clearing fouled anchor; Mooring by cable to buoys; Use of slip wires.

Introduction

It has been suggested that the first anchors might have appeared as a basket of

stones used by vessels engaged on the River Nile around 6,000 BC. In the twenty-

first century, the deployment and holding power of anchors has improved some-

what from the basket of stones of the ancient Egyptians.

The high holding power anchors, the use of multiple anchor moorings and the

sheer size of anchors for the larger vessel, have all brought with them associated oper-

ations and relevant complications. Anchor cables are brought into use, sometimes

without the anchor as in mooring to buoys, while the problems of fouled anchors, foul

hawse and lost anchors present concerns for seafarers as well as insurers.

Developments in the offshore industry have spilled over into aspects of commer-

cial shipping, while the early experiences of shipping fuelled advances in the roots

of offshore. Historically, terminology from the sailing ship days has lingered on, but

technological advances in windlasses, braking systems, anchor design and the need

for greater holding power have all changed the face of modern day, anchor oper-

ations (see Appendix C).

A ship’s use of anchors when berthing has become common practice. More ships

will expect to moor with two anchors as opposed to coming to a single anchor which

requires a larger swinging room. Vessels will still experience the dragging anchor,

when the elements affect the exposed vessel; we cannot control the weather but we

can be prepared for the worst and the anchored ship only remains safe when the

personnel within continue their effective duties as watchkeepers.

Even though anchor plans have become more formalized by ships’ Masters, the

weather and tidal affects cannot in any way be guaranteed. Watchkeepers must be

effective in their duties, especially in the task of monitoring of the ship’s position. Far

better to be party to good seamanship than have to rectify mistakes of inefficiency in

Anchor operations and

deployment

Ch03-H8530.qxd 4/9/07 9:49 AM Page 65