Anchoring equipment

The anchoring equipment fitted to the majority of vessels consists of two matched units, offering a degree of redundancy.These units consists of an anchor, chain (or for smaller vessels wire), a gypsum or chain lifter wheel, brake, lift motor and various chain stopper arrangements.When not in he use the chain is stowed in a chain locker, systems fitted with wire are stowed on a drum in the same way as winches.

Chain locker

A false bottom is fitted to the chain locker consisting of a perforated plate. This allows water and mud to be removed from the space. The end of the chain is attached to the hull by a quick release mechanism known as the 'bitter end'.The strength of the 'Bitter End' fixing arrangement for a moderately large vessel is in the region of 6 Н tons, this will not be sufficient to prevent a run away unbraked chain. The arrangement must be easily accessible.

The proof load for the windlass (the load the windlass must withstand without being pulled from the deck) is given by; 6.18 dc² (44 + 0.08dc) [kN] Where dc is the diameter of the chain metal.

This will prove that it is strong enough. It must also be as strong as the braking load on the cable.The windlass must be capable of pulling the anchor from a depth of 25% of the total cable carried, i.e. 50% of the length of chain on one side.It should be capable of lifting the anchor from 82.5m to 27.5m at 9m/min.

Motive force

May be steam, electrical, hydraulic or even engine driven although the latter is rare. A gear box arrangement, heavy tooth pitch on final drive, are fitted

Brake

Generally consists of ferodo lined half cylinder steel bands which are joined by a pivot point at the rear. The bands are closed and opened by the action of a threaded brake wheel spindle acting on the fixings at the front of the band ends

Hawser

The chain is led overboard by a strengthened and reinforced pipe called a Hawser. One of the reasons for bow flare is to allow the anchor and chain to lie well clear of the hull when in use, preventing damage.

Chain stopper

For anchoring operations the stopper bar is locked upright. When it is required to fix the position of the chain the stopper is lowered into the position shown. This allows the brake to be released and is typically used for stowing the anchor. chain stopper arrangements are not design to stop a runaway chain. Alternately an arrangement known as the 'devil's claw' may be used which has a forked locking piece. For smaller vessels, and where extra security is required bottle jacks with wire strops passed though the chain may be used.

Chain

End pull will cause the link to collapse in. This repeated many times will lead to fatigue failure. Hence, stud linked chain is insisted upon. Here a stud is welded on one side in the link to brace it against deformation. An alternative to this albeit in limited use is shown below.

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Chain sizing

Each vessel is given an equipment number which is calculated with use of a formula and tkaens into account the vessels size, underwater area and sail area. From this a 'look-up' table may be used to give an appropriate size of cable. The diameter of the chain may be read from this table and differs depending on the grade of steel. This grade of steel varies from U1 ( mild steel), U2 (Special Steel) to U3 (extra special steel).

The size of cable that is to be used is found by the use of a formula which is

Equipment number = D2/3 + 2Bh +A

where
D = Displacement
B = beam
h = Freeboard + height of deckhouses over B/4 wide
A = Transverse area including deckhouses over B/4 wide

Connecting chain and components

To join two sections of chain a 'kenter' (don't quote me on the name) shackle is used. This consists of two half sections and removable bridge all held together by a tapered pin. This arrangement works remarkably well and can be found on all sizes of chain.

Ranging Anchor Chain

During docking the anchor chain is lowered from the chain locker to the dock bottom and laid out for inspection.

This allows the inspecion of the chain for broken or lost chain studs. A random set of links are measured from each shackle length ( Shackle refers to a standard length- nominally 27.5m, of chain joined to other shackle lengths by a splitable link). There is an allowable wear limit allowed nominally 12%. The link showed below has wear in excess of this and the shackle length required replacement.

Anchor designs

All of the anchors shown below are of the 'flipper' type. Regulations allows these to be smaller than standard types .Below is an anchor of typical design seen in many small to medium sized tankers.

Below is a high grip anchor typical of that used for four mounting mooring of vessels required to fix their positions accurately

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This anchor weighs about 20 tonnes without attachments. The chain link in comparison the anchor size probably indicates this came from a jack up barge. For vessels the holding comes not only from the anchor but the weight and lay of the anchor chain . Again the chain link in comparison the anchor size probably indicates this came from a jack up barge.