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Marine Outfittings Prof. Dr. Yousri Welaya
Anchoring Systems
An anchor system provides a simple reliable means of safely
holding a ship in a
relatively fixed position in shallow water without the use of
the ships main or
secondary power source. The anchor system may also be used to
assist in
stopping or controlling the ship in an emergency to avoid
grounding or collision.
The anchor system consists of an anchor, made of cast steel with
forged steel
fittings, shackled to a chain which is engaged by a chain
sprocket driven by a
windlass; this may be powered electrically, hydraulically, or by
steam. When the
anchor is retrieved it is normally stowed in an anchor pocket or
against a bolster
surrounding the hawse pipe with the chain stowed in a chain
locker.
Two anchors are usually deployed when the swing circle of a ship
using a single
anchor is too large for the available anchorage. The radius can
be reduced by
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Marine Outfittings Prof. Dr. Yousri Welaya
using more than one anchor. The anchors are placed well apart so
that a line
drawn between the two anchors is perpendicular to the
current.
Anchor Characteristics
The capability of an anchor to dig into the bottom when dragged
depends on the
shape of its flukes, the angle between the flukes and the shank,
and the fluke
tripping arrangement. Anchor efficiency is measured as
follows:
To obtain maximum holding power the load is applied through the
chain so that
the anchor shank remains parallel to the bottom. This condition
is obtained by
using an anchor chain of sufficient length and weight for a
given depth to assure
that the chain describes a catenary parallel to the bottom at
the anchor shackle
when the pull of the chain is equal to the maximum holding power
of the anchor.
The essential characteristics of an ideal anchor can be
summarized as
follows:
Self Orientation
The anchor should engage the sea bed rapidly and orientate into
its normal burial
attitude, irrespective of drop attitude or nature of sea bed
(e.g. sand or mud).
Short Scope Capability
The anchor should be capable of engaging, orientating, and
burying at long or
short scope. If the anchor can operate with a high cable angle
at the sea bed,
less cable is needed or, alternatively, anchoring is possible at
greater water
depth for a given length of cable.
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Marine Outfittings Prof. Dr. Yousri Welaya
Deep Burial
Since holding power is a direct function of the depth of burial
of the fluke of an
anchor below the sea bed surface, the anchor should penetrate as
deeply as
possible to achieve the highest holding power. Penetration
resistance should
also be low.
Low Breakout Force
It should have a breakout force which is as low as possible
compared to its prior
holding pull. This is between 50% and 100% for conventional high
holding power
anchors.
No Moving Parts
Jamming of pivoting fluke anchors may result in failure to bury
and develop high
holding power.
No Stock
The stock is an easily damaged part of many high holding power
anchors which
may lead to fouling of anchoring and pennant lines. It offers
high resistance to
sea bed penetration and may produce handling and stowing
difficulties. If the
stock becomes damaged, it causes loss of stability at the sea
bed surface
resulting in loss of holding power.
Types of Anchor
Ships bower anchors are usually one of two general types; stock
or stockless
anchors. In each of these categories there are a large number of
variations.
1. The stockless Anchor
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Marine Outfittings Prof. Dr. Yousri Welaya
When a variety of sea bottoms are expected, to be encountered,
ships turn to the stockless anchor for reliability and versatility.
It has a relatively short shank. The flukes are allowed to swing
about 45o to either side of the shank to permit anchor to dig into
sea bottom. The stockless anchor has good holding power to weight
ratio and is the easiest type of anchors to stow with its shank in
hawse pipe. The disadvantage is its tendency to disengage flukes by
gradually turning over. The weight ranges from 100 kg to 25
tonne.
2. The stock Anchors
These are characterized by a transverse bar, or stock which
orients the flukes
in the proper position to dig in one fluke when the anchor is
dragged along the
bottom.
Danforth Anchor
The stock is incorporated in the anchor head. This allows
stowage of a stock
type anchor with its shank in a hawse pipe. The fluke motion is
limited to
about 30o each side of the shank axis. Its weight ranges from
130 kg to 14
tonne.
The Snug Stowing Anchor
It has stocklike projections made integral with the crown which
give stability
against rotation when dragged, and it develops holding power
comparable to
the stockless anchor. The small crown nests into the hawse pipe,
which has
no shell bolster, allowing the flukes to lie snugly against the
shell.
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Marine Outfittings Prof. Dr. Yousri Welaya
Flipper Delta Anchor
Major advantages of this anchor:
Proven superior high holding power capacity - Excellent
efficiency (weight / holding power) - Open construction for smooth
and good penetration in different kinds of soil - No rotation,
which means no decrease of holding capacity and no dragging of the
anchor - Easy dismantling for transport purposes
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Marine Outfittings Prof. Dr. Yousri Welaya
JIS Type Stock Anchor
This anchor is used for relatively small ships
3. The Bruce Anchor
This anchor is of one-piece construction and is self-orientating
at the sea bed
surface irrespective of drop attitude. It penetrates the sea bed
while
orientating and has complete roll stability. The Bruce anchor
has higher
holding power and higher tolerable cable angles than other
anchors. Also the
breakout force is as low as 20 - 25% of the prior holding
pull.
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Marine Outfittings Prof. Dr. Yousri Welaya
4. The Mushroom Anchor
It is named for its shape and is used on light-ships, canal
barges, and for
anchoring permanent mooring buoys.
5. The Stern Anchor
A stern anchor is used to keep a ship from swinging with the
current in
restricted areas. It may be housed in a hawse pipe so located
that the anchor
will clear the rudder and propeller when dropped.
Anchor Chains
The bower anchor chains required by classification society rules
may be stud-link
chains of normal, high or extra high-strength steel. Anchor
chains are
manufactured in 15-fathom shots (27.4 m in length). The chain
has an enlarged
link at each end of the shot to accommodate the joining shackles
used to link up
the chain. The first shot is sometimes made 30 fathoms long to
allow anchoring
in shallow water without passing a joining shackle over the
wildcat.
The wildcat is a special type of cog-like windlass drum whose
faces are formed
to fit the links of the anchor chain. The rotating wildcat
causes the chain to be
slacked off when lowering the anchor, or hauled in when raising
it.
Chain connecting links are designed for easy assembly and
disassembly. The
one shown below is made by Baldt and consists of a c-link and
two closing caps.
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Marine Outfittings Prof. Dr. Yousri Welaya
The caps are mated to the c-link and after assembly are secured
by a stainless
steel tapered pin and backed by a lead plug.
Some of the anchor chain fittings are shown below:
Anchor 'D' endshackle
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Marine Outfittings Prof. Dr. Yousri Welaya
Pear shackle type Baldt
Shortlink chaincables
Studlink chaincables & fittings
Swivel
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Marine Outfittings Prof. Dr. Yousri Welaya
Determination of Anchor and Chain Sizes
Ships are normally equipped with a minimum of two bower anchors
and anchor
chains. The chain is usually much heavier than would be required
to withstand
the tensile load alone. The chain hangs in a catenary between
the hawse pipe
and the bottom, and must be selected so that the lower end of
the catenary will
be horizontal when the tension at the anchor shackle is equal to
the maximum
holding power of the anchor.
Chain and anchor sizes may be determined by calculation or
classification
society rules. The general design condition is:
Chain Proof Load > Anchor Holding Power > Applied Maximum
Load
To calculate the size of the anchors and chains required, the
wind load and
current load on the ship must first be determined for the most
severe conditions
under which the ship will be expected to remain at anchor. The
anchor system is
also subjected to dynamic loads as the ship surges, sways and
yaws and is
forced by wave action.
H = Z . FR
where H = total resistance corrected for dynamic forces FR =
total wind and current load Z = factor to approximate the dynamic
effect = 1.25 - 1.50 for small ships with fine lines = 1.50 1.70
for large more fully shaped ships = 1.75 2.00 for ships with more
blunt shapes
Based on the selected type of anchor H/W (Holding Power / Weight
ratio) could
be determined from the following table:
Type of Anchor H/W Mushroom 2.5 Stockless 5.0 Danforth 16
Required Anchor Weight (in air) W = H H/W
Chain size and scope are determined to suit the anchor size and
depth in which
the ship will anchor using the catenary equation:
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Marine Outfittings Prof. Dr. Yousri Welaya
where s = scope of chain, fathoms y = water depth + height of
hawse pipe above water w = chain weight (in air) per unit
length
The sizes of anchors and chains also may be determined by
reference to tables
provided by the classification societies. The sizes are
tabulated according to an
equipment number determined from characteristic dimensions of
the ship by
use of formulae given in the rules.
Anchor Handling Arrangements
The anchors are housed against the forward side shell, sometimes
in specially
recessed pockets. The anchor is shackled to the anchor chain
(cable). The chain
passes through the shell via the hawse pipe on to the forecastle
deck. It travels
over the chain stopper and on to the windlass wildcat (cable
lifter) drum. From
the wildcat it drops vertically down through the "Spurling pipe"
to the Chain
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Marine Outfittings Prof. Dr. Yousri Welaya
Locker under the deck. The bitter end of the chain is connected
to bitts fixed in
the chain locker bulkhead using the bitter pin, which should be
able to be
released from outside the locker to "slip" the Anchor. This
would occur if the
Windlass brake has slipped in a storm for example and you have
reached "the
bitter end".
The most common arrangement utilizes a stockless anchor and a
horizontal
windlass wherein the wildcats are mounted on horizontal shaft as
shown in
figure.
http://en.wikipedia.org/wiki/File:Ship_anchor_windlass_diagram.gif
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Marine Outfittings Prof. Dr. Yousri Welaya
The normal windlass arrangement utilizes one prime mover to
drive two
declutchable wildcats and also two warping ends (gypsy heads).
The warping
ends are not declutchable and rotate continuously when the
windlass is in use.
When mooring light line speeds of 0.75 1.0 m/sec are
required.
The windlass should be able to heave a certain weight of chain
(cable). This full
load is generally between 4 6 times the weight of one anchor and
the speed of
haul at full load is usually between 0.12 0.2 m/sec (LR
requirement is 0.15
m/sec).
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Marine Outfittings Prof. Dr. Yousri Welaya
For very wide ships such as large tankers, and for ships with
large bulbs, it is
necessary to split the windlass and set each wildcat at an angle
to the centerline
in order to obtain proper leads to the hawse pipe.
Large passenger ships and naval ships generally are equipped
with a vertical
windlass where each wildcat is mounted on a vertical shaft.
This is driven through spur and worm gears by electric motors
located on the
deck below, thus lowering the centre of gravity. Capstan heads,
driven by the
windlass machinery, usually are fitted on each side outboard of
the wildcats for
handling the forward mooring lines. Vertical windlasses have the
advantages of
greater wrap of the chains when dropping the anchor, and the
machinery is more
easily maintained at sea. It also allows a flexible angle of
pull (which means rope
or chain can be run out to different fairleads).
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Marine Outfittings Prof. Dr. Yousri Welaya
Chain Stopper
A chain stopper made of cast steel is usually placed between the
wildcat and the
hawse pipe in line with the run of the anchor chain. It is used
to hold the anchor
chain in place while the ship is riding at anchor or the anchor
is fully housed. The
stopper is not designed to stop the moving chain, but only hold
it in place. In the
one shown in the following figure a hinged bar is designed to
fall between two
vertical links and hold the chain in place.
Devils Claw
The devil's claw is a device that is used as a chain stopper to
grab and hold an anchor chain. It consists of a turnbuckle, usually
attached at the base of the anchor windlass, and a metal hook with
two curved fingers that grab one link of a chain.
After hoisting the anchor and setting the windlass brake, the
claw is placed on a chain link and the turnbuckle is tightened to
take up the tension on the chain. If more than one stopper is used,
the turnbuckles can be adjusted to evenly distribute the load.
A devil's claw cannot be released while it is under tension. To
release it, the tension must first be taken up by the windlass
brake. Then the turnbuckle can be loosened and removed.
http://en.wikipedia.org/wiki/Turnbuckle
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Marine Outfittings Prof. Dr. Yousri Welaya
Types of Anchor Handling Machinery
1. Horizontal Shaft Windlass
The steam driven spur-geared windlass with a two cylinder engine
is the type
used on many tankers. The speed is controlled by the engine
throttle. The engine
may be installed on the deck below. This protects engine
location and provide
access at all times for overhaul.
A similar windlass powered by an electric motor is the most
common type fitted
on merchant ships. Speed variation is achieved by an electric
controller which
gives multiple speeds with DC and full and quarter speeds with
AC. The
exception of using this type is where there is danger of spark-
initiated explosions
or fires because of the nature of the cargo carried.
2. Vertical Shaft Windlass
This is driven through spur and worm gears by electric motors
located on the
deck below. Vessels equipped with AC may use electro-hydraulic
machinery to
drive the windlass at varying speeds. The power is supplied by a
constant speed
electric motor driving a variable stroke hydraulic pump which is
piped to a
hydraulic motor which drives the windlass through shafting and
gearing. The
speed and direction of rotation are regulated by varying the
stroke and reversing
the discharge of the pump end.
http://en.wikipedia.org/wiki/File:Securing_devils_claw.gif