MEG4
Ship Design MBL vs LDBF
5 min read
These two numbers get confused constantly in purchase orders, and the confusion is expensive. One belongs to the ship. The other belongs to the rope you are about to buy.
Ship design MBL belongs to the vessel
The ship design MBL is set when the mooring system is designed. Bitts, fairleads, winch brakes, foundations and the surrounding structure are all engineered around it. It does not change because you buy a different rope, and it does not change because the vessel changed trade.
You should be able to find it in the vessel's mooring system documentation. If nobody on board can produce it, that is a finding in itself — line selection cannot be done properly without it.
LDBF belongs to the rope
LDBF is what the rope manufacturer designs and certifies the new spliced line to achieve. It varies with fibre, construction, diameter and manufacturer, and it is the number you are actually buying.
How they should relate
The selection principle is that the line should be the intentional weak link. Under MEG4 guidance, a new line's LDBF is normally chosen in a band around the ship design MBL — commonly cited as 100 to 105 percent — so the rope fails before the ship's structure does.
Two failure modes follow from getting this wrong. Choose an LDBF too far below the design MBL and lines part more often than they should, which is a safety problem in itself. Choose an LDBF too far above it and, in an overload, the bitt or fairlead becomes the failing component — a far worse outcome than a parted rope.
Worked example of the reasoning
Suppose a vessel's mooring documentation gives a ship design MBL and the current lines are being replaced. The correct sequence is: read the design MBL, apply the percentage band from the vessel's mooring management plan, arrive at a target LDBF, and only then ask which fibre, construction and diameter delivers that LDBF within the winch drum capacity and the fairlead geometry.
The wrong sequence — which is how most urgent purchases actually happen — is to start from the diameter of the old rope and buy the same again. That works only if the old rope was correct in the first place, which is exactly what nobody checked.
What to include in an RFQ
- Ship design MBL from the vessel's mooring documentation
- The LDBF band your mooring management plan requires
- Winch drum capacity and fairlead type
- Current line specification and its age, for context
- Required length, eye configuration and whether tails are included
Frequently Asked Questions
- In the vessel's mooring system documentation — typically the mooring system management plan or the original mooring arrangement design data.
Need this applied to your vessel?
Send us the ship design MBL, the required diameter, construction and length, the delivery port and the date. We will come back with a line configuration and the documentation it will carry.
Request a quotation