Mooring rope Strength Reduction 1

[djw2k3]

Hi all,

I am interested to know the true performance of a mooring line on large ships.

For say a 350m LOA vessel;
- What is the typical ability of a pretensioned winch - ie how much tension can they really put into a line.
- When a mooring line is placed around a fairleads what is the effective strength reduction due to compression of fibres and deformation of rope shape.

Dave

 

[pauljohn]

For strength reduction of line it depends on many factors. Diameter of capstan/winch versus diameter of line, also type of material, how it was made (three strand, five strand, platt, etc). Obviously the bigger the winch the less the deformation. I think it would be difficult to actually measure deformation.

Tying any kind of knot usually reduces the strength by around 50%. My experience has been the rope nearly always breaks at a knot or where the line chafes on the fairlead. By the way the fairlead is the hole in the hull where the line goes through. The line is tied off on capstan, winch, cleats or bitts.

Not quite sure what you mean by pretensioned winch.

 

[djw2k3]

Hi Pauljohn,

I am really wanting to find out the true effective breaking load of a mooring line. Literature indicates a minimum breaking load and states that strength is reduced when the angle is changed around a fairlead etc.

A pretension winch is used to control vessels position by paying in/out rope whilst berthed, normally it just sits there with a constant tension in the line. Was wanting to know how much tension these can actually apply.

D

 

[pauljohn]

I don't know anything about pretensioned winches the boats I work on are too small to use them.

You can find tensile strengths, which is the average breaking load, of mooring lines on New England Rope's web site but to actually measure it in situ would be difficult if not impossible. Even if you could find apparatus to measure the huge loads you would probably end up breaking cleats/winches etc. Modern 3 inch line has a breaking strain of about 250,000lbs.

Mooring lines and anchor rodes are so overbuilt they only break when badly chafed or tied in a knot.

 

[eureka]

There is a good coverage of this in:
Design of Marine Facilities for the Berthing, Mooring, and Repair of Vessels,
by John W. Gaythwaite
Van Nostrand Reinhold, New York, 1990
Section 6.2 Mooring Lines, Hardware, and Equipment, has a good discussion, graphs, and tables with the capacity of various ropes.