F.B RECOMMENDATIONS FOR CHOCK
SURFACE DIAMETER
Practicality must be considered in specifying a chock diameter for SPM and (owing chains. Rct ro-
fill.ing OT modification might be necessary on many tankers. [f an impractical or too large diameter
is required, many operators and owners may cboose to resist or defer making modifications.
Many recommendations have been made in the pasr regarding t.he safe diameter for surfaces on
which cbains bear. Rev iew of these past recommendations indicates that only some of (hem apply
to chain tensioned over an ungrooved surface, the case of interest in SPM and towing chain chocks.
The LJoyds investigation reported by Buckle, the only other known analysis, applies onJy for a
grooved surface. No other experimental work of chain tensioned over ungrooved surfaces is known.
Thus most past recommendations are apparently only conservative assumptions based on experience
in various djssimila r ervices.
In the present inve.stigation. both analy!ical and experimental findings indicate that stud link chain
will not be significantly weakened provided the diameter of the surface over which it is l.cnsioned
is not I~s than 7 times the diameter of the chain.
Chocks witb a 600 by 450 mm (24 by )8 in.) opening and a sUIface diameter of 560 mm (22 in.)
are available. Such chocks are known as Mississippi fairleads. and are manufactured and marketed
by several companies. When used with tbe 76 mm (3 in.) chain, recommeDded for SPM and towing
cha fing chain, the 560 mm surface curvature of such chocks provides a surface d.iamcter rario of
7.4. Thus, the recom mended surface diameter to chain diameter ratio of 7 is practi cal.
Many tankers are already filted with bow chocks which comply with rhis recommendation. Somc
tan kers probably have chocks with smaller surface diameters. The most common commercially
available 600 by 450 mm (24 by 18 in.) opening chock has a surface diameter of 360 mm (14.1 in.)
as specified by Japanese Indus! rial Standards (JTS). Such chocks provide a surface diameter 10
chain diameter ralio of only 4 .7 with 76 mm chah:l. However, such chocks can be readily replaced
with larger surface diameter cbocks.