Mooring Line purchase requirements

Hi all,

I’m analyzing mooring operations as part of operational excellence at our company.

Mooring manufacturers are using much more improved materials then 10-20 years ago. This results in significantly stronger lines for same diameter. To recommend the correct MBL and die. of the line we should refer to the drawings and manuals. The OCIMF Mooring Equipment Guidelines suggest a terminology ship’s design MBL or alternatively if not able to find, the original lines provided by the yard.

Unfortunately many vessels doesn’t specify the MBL or material of the recommended lines, only the diameter. The alternative could be to use known winch capacity and apply correction factor of 22-33% as per OCIMF MEG4 ‘Winch motor - pull- between 22-33% at nominal heaving speed (ISO)’ of MBL. However, that seems to be too low, in example bulk carrier of 180m and recommended 63mm lines would have a MBL of around 600Kn with winch capacity of 100Kn. Following the guideline 100Kn/0.22 = 454Kn.

I’ve checked with global supplier and they simply follow Master’s advise and/or use factor of 2.5 of winch brake capacity, which is very different form OCIMF.

Any feedback on above would be welcomed.

Our winches (200 m PCTC) are 300 Kn and the lines are 1000 kn MBL, that’s about 33%, the upper end of the 22%-33%.

That’s not enough to keep us alongside in 50 kts of wind but better lines go up in cost quickly so the thinking evidently is call a tug if needed rather than buy more expensive lines.

As far as the winch brake, I don’t know the specs off the top of my head but I’d expect if the brake was in good shape and tightened properly the line will part before the brake slips.

Beyond that there are cheaper lines out there that have the requisite MBL but are not suitable. For example the same diameter line of different (cheaper) material will be too slippery and will bury itself three layers deep on the reel under a big wind load.

Chaffing would be another important factor, we don’t have time generally to put out chaffing gear although we have in some cases.

If I was responsible for buying lines I’d check with the masters if you haven’t already.

I’d be curious to see what others have to say about this.

Thanks mate,

Could you clarify please if 300Kn you mentioned is winch capacity (pulling force) or brake capacity?

You are correct that winch brake must be set-up to render before line parts. The recommended ratio is 60% of line’s MBL. So in your case the brake should be set-up to render at 600Kn.

Unfortunately in Australia, there are many open sea ports with occasional strong winds and due to remoteness, the tug only available for arrival/departure. I remember coming off the wharf around 20-30m resulting in couple more grey hairs.

I did speak with the Masters but appropriate characteristic of the lines are grey area and most seems like order based on history and intuition rather than scientific approach. I’m trying to establish a base line.

I thought the ideal was winch brake renders at less than line strength. And line parts before bollard fails.

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Yes, looks like you are correct, that is the recommendation.

As far as the actual situation we used to part lines in high winds from time to time before the brake slipped. However some years ago the company upgraded to better lines and we haven’t parted one since.

I don’t know off the top of my head what the brake holding force is. I was assuming it was more than MBL of the lines but I may be wrong about that.

Yes, pulling force of the winch.

This topic was apparently addressed here at

Basically, calculate the IMO equipment number (, look up the breaking load on MSC circular 1175, then double it to get mooring line specs.

I feel a little more educated now.


One of our naval archs led me in the right direction. As Lemurian2 mentioned, class calculated equipment number depending on the windage and current forces. This number is then used to extract from the class table the mooring line and cable characteristics.

All very well explained is below article.

I’ve already located the drawings titled “Equipment Number Calculation” which provided me with mooring line MBL and diameter.

Learned something new and useful today.


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Jones Act Tanker.

We have to follow the new Mooring Equipment Guidelines Edition 4. So our brakes are set to 60% of the ships design MBL.

When a lot of these ships were launched, they had wires of X size. Over the years, the wires were changed out for soft lines, also ordered in X size. Problem is, when you compare mooring wires and HMPE lines of the same size, the HMPE line is going to be significantly stronger than the wires ever were. So on vetting inspections we get nailed for having oversized and overstrenghtened lines.

With the wires, the brakes on the winches were set to 80% of ships design, which was 80% of the breaking strain of the wire. With the lines, 80% of the line was too high for the brake, so we still had them set to 80% of the ships design MBL.

We’ve overpaid for the lines, but that’s what we have until they reach the end of their life (10 years) and we can get the ‘proper’ sized lines onboard. The office could have solved this a long time ago if somebody had done a little research, but here we are.

Also…63mm lines? Is that circumference or diameter? If diameter, those are enormous. Ours are 32mm diameter and have something like a 85 ton MBL on them. Couldn’t you take the original size wire and do the math and see what the strength of them was and then convert to whatever line manufacturer you’re going to use? Also, if you see what the chocks and bits onboard are rated for, you can do the math and see how strong a line you need.

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In the mentioned video I’m confused about 'rule of thumb is to take recommended MLB from equipment number tabel and multiply by two. Why would you multiply by two? Isn’t the whole purpose of recommended MBL is to be ‘recommended’ and not changing it by 200%?

If I multiply by two the recommended MBL to identify required mooring line strength then the line would be more stronger than ship’s bits and chokes. So you possibly rip the bollard from deck before the line would break.

63mm is a diameter and some vessels are actually using 72mm. Are yours 32mm are Dyneema? Who is manufacturer?

Yes. Samson.

Maybe you need wayyyyy stronger lines than we do.

I believe the mentioned “rule of thumb” is accounting for the working load limit of the mooring line.

Unless I heard a very compelling argument to the contrary I’d want my lines to be ~25% stronger than the original wires were as a buffer against chafe and degradation.

SIRE VIQ & MEG4 says they can’t be. At least for tankers.

As for chafing and degradation, we’re 6 years into our set of lines with very little signs of wear, and that’s running coastwise, being at a dock at least once a week if not more.

Would you be able to get away with 10% or 5% extra?

Depends on the inspector. But by the letter of the law, no.

You could always put a pendent on the end of your “oversized” lines with the “correct” line size. The strength of your mooring lines would be that of the line size used on the pendants.

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That’s a rather brilliant workaround.

Yes, except the pendants are supposed to be 125% of the mooring line…

The workaround was to just buy the right size lines in the first place lol. Or now just accept the vetting observations as they come. The office has learned their lesson by now and won’t get caught with this again, that’s for sure!