When building a new superyacht many questions arise concerning the breakloads of the bollards, winches, and the mooring lines.
A good advice is to check DNV-GL Lloyds requirements by looking at the equipment number of the yacht to select the strength of the mooring line.
However this will cause many questions to arise;
The equipment number will specify a required breaking load of the mooring line and towing line which is chosen according to the breaking load of the deck equipment.
In practise this means that the mooring line will be very thin in diameter. For instance a yacht of 70 mtr with equipment number 600 would need a minimum of 122kN breaking load.
A 26mm polyester mooring line would do the job according to this. However in practise a 70mtr yacht would go for 40mm polyester mooring lines.
Some things we need to realize is that the breakload of a rope is without any splicing or knotting. A Spliced mooring line loses roughly 15% at the splice. Knots in a rope will be even worse; you would lose about 50% of the breaking load when tying a knot in a rope.
Another thing is that with polyester mooring lines both the core and cover work together on the breaking load of the line. With dyneema lines (dyneema core and polyester cover) it is purely the dyneema core which defines the breaking load.
If with a polyester line the cover starts to abrase or gets damaged, it means that the breakload of the line suddenly drops.
Specially on spring lines the rope makes a 180 degree angle on fairleads. This generates so much friction that the load on the bollard will be much lower compared to the load on the rope fro fairlead to the shore. Spring lines are often the lines which take the greatest loads and also the lines which might snap first.
When using small diameter it will mean more load per square inch on the cover of the line in fairleads. This might cause the cover to abrase or even melt.
Also on bollards small diameter lines will be very slippery and therefore could be dangerous.
For the above reasons it’s virtually impossible to quantify the required working loads on ropes. Docklines are dynamic items that are being worked with and there are to many variables to guarantee anything.