- Fairleads should be spaced 2.0 metres centre to centre apart, if practicable. and iD no case
be more than 3.0 metres apart. Ships of 150,000 tonnes deadweight or less at maximum
summer draft need only provide one fairlead which shouJd be on the centre line. - The chock opening should be oval or have roWlded corners, as in Figure AS. The curved
shape tends to centre (he chain in the chock and thus minimize chain lateral movement and
abrasion. A chock "";th square corners would allow excessive chain lateral movement and
aHow the chain to jam in the corners. - The outboard lip of the chock should be flush with the tanker hull or smoothly faired into
the bulwark. Experience has shown that hawser assembly components often hang-up 00 bow
chocks whicb have protruding lower lips or which are improperly [aired into adjacent
structure. - Chock strength must be adequate to withstand the high loads imposed by towing. The
highest loading will be lateral when the tanker yaws. How much of the towing assembly
tension will be lransmitted to the chock wiU depend on tbe aogle of the towing assembly.
It is recommended that the chock be designed to withstand a load of 3900 kN (397 tons
metric) applied lateraHy, with the towing assembly at right angles to t.he tanker ceDlreline.
This should ensure adequate chock strength. The tanker srructure to which the chock is
secured may require reinforcement to withstand such loading. - The chain bearing surface diameter of the chock must be sufficiently large so that it does not
significantly decrease the chain strength. Bending chain over a curved surface decreases its
axial strength; the sharper the chain bend the greater the strength decrease. Until recently a
precise relationship between the chain-bearing surface diameter and the decrease in chain
slrength could not be given. As part of tbis study the stresse-s in chain bent over a curved
surface was analytically examined and a lest program was conducted on actual chain.
Appendix F gives the results of this work. Based OD the results, a minimum chain bearing
surface diameter [0 chain diameter ratio of 7: I is recommended. This should not result in a
significant reduction in the breaking strength of lhe chain. The chock design of Figure AS
meets this ralio for the recommended 76 mm (3 in.) diamet er chafing chain.
A.4 POSITION OF PEDESTAL FAIRLEADS,
WINCH DRUMS OR CAPSTANS
To enable the mooring operation to be carried out safely, it is imperative that the forward pedestal
roller fairleads are positioned correctly. It is essential that roller fairleads/winches/capstans are
positioned to enable a direct pull to be achieved on tbe continuation of the direct lead line between
the bow fairleads and bow stoppers. The 'distance betwee·n [he bow stoppers and pedestal leads
should be consi.dered. so that an unrestricted line pull can be achieved from the bow fairlead and
through the bow stopper. (See Figure A4).
Winches or capstans used to handle moorings should be capable of lifting at least 15 tonnes (147 kN).
I f a winch storage drum is used to stow the pick-up rope, it should be of sufficient size to
accommodate 150 metres of 80 mm diameter rope.