3 October 2005
FIXED BITTS. Bitts provide a termination for tension members. Fixed
bitts, Figure 7-1, are typically placed in pairs within a short distance forward or aft of a
chock location. They are often placed symmetrically on both the port and starboard
sides, so that the ship can moor to port or starboard. Capacities of the bitts are based
on their nominal diameter. Table 7-2 provides fixed bitt sizes with their associated
capacities. NFESC TR-6047-OCN "The Capacity and Use of Surface Ships' Double
Bitts" (in preparation) provides additional information on ship's bitts. The basic
philosophy for bitts use is that mooring lines should part well below the structural yield
of the double bits in Mooring Service Types I and II to minimize the chance that ship's
mooring fittings need to be repaired. In Mooring Service Type III `Heavy Weather
Mooring' is to keep the ship moored as safely as possible, so the working capacities of
the mooring lines can approximately equal the working capacities of the ship's double
RECESSED SHELL BITTS. Recessed shell bitts, Figure 7-2, are inset
into ships' hulls well above the waterline. These bitts are used to moor lighterage or
harbor craft alongside. They also assist in mooring at facilities. The NAVSEA shell bitt
has a total working capacity of 92 kips (4.27 E5 newtons) with two lines of 46 kips
maximum tension each.
EXTERIOR SHELL BITTS. Aircraft carriers have exterior shell bitts,
Drawing No. 600-6601101, that are statically proof loaded to 184 kips (8.2 E5 newtons).
This proof load is applied 11 inches (280 mm) above the base. This testing is described
in the Newport News Shipbuilding testing report for USS HARRY S TRUMAN Bitts,
Chocks and Mooring Rings.
CHOCKS. There are many types of chocks, such as closed chocks,
Panama chocks, roller chocks, and mooring rings. Closed clocks are often used and
characteristics of these fittings are shown in Table 7-3.
ALLOWABLE HULL PRESSURES. As a ship berths or when it is
moored, forces may be exerted by structures, such as fenders, camels, and dolphins,
on the ship hull. NFESC TR-6015-OCN, Foam-Filled Fender Design to Prevent Hull
Damage provides a rational design criteria to prevent yielding of vessel hull plating.
SOURCES OF INFORMATION FOR SHIPS' MOORING EQUIPMENT.
Additional information is available from the Naval Sea Systems Command (NAVSEA
03P), NSWCCD-SSES, Military Sealift Command (MSC), and the U.S. Coast Guard
(USCG). Table 7-4 provides a list of selected referenced materials.