Strength of Deck Fittings and Tug Bollard Pull

Interestingly, a lot of pilots I meet in various ports tell me that a majority of these newer and larger ships have shockingly low tonnage safe working limits on their bits. Some as low as 30 tons maximum.

I have no way to verify this, but it is a regular topic of conversation when I tell them the safe working load of the bits on my ship.

Good point.Tugs can pull as hard as they want until the other gear can’t handle the forces present or needed. That full bollard pull by those modern tugs were not exhibited here, nor that pressure stressed or used on deck installations (bits) on the ship.

It’s a real hodge podge of what you get with deck equipment on ships. Some of it will make you scratch your head. Some old, smaller container ships with 110ton bits and some new, huge ones with 35 ton bits, and everything in between, and sometimes less!

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Oh I’m on one of those older ships with 110 ton bits, but the question has to naturally be; if they’re increasing the size and displacement of these ships 4 fold, why in the hell are they quartering the capacity of the bits? The ships are getting bigger and the tugs are getting bigger but the linkages between them are getting smaller? It just doesn’t make sense.


Oh, I’m sure it makes sense to some accountant somewhere.

Evidently the SWL of the ship’s bitts can be the force it takes to “collapse” the bitts. The SWL with a tug line might be 2 x SWL to the pier while moored if I reading this right.


SWL_Bitts 2

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Great information, but I haven’t made a tug fast using figure eights in, well, ever. If it’s a tug’s line, it’s a spliced eye going over the nearest horn of the bit.

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Yes, that’s why the SWL of the bitts with just the eye is 2 X SWL of the rating of the bitts.

Load on the Line = T


Load on the posts is 2 * T so Bitt rating is 1/2 * T

SWL_Bitts 2

Of course there is this as well:


And this:


Here is another source:,-towing-and-salvage/are-ships-deck-fittings-up-to-the-task-of-towing

Ships’ deck fittings are primarily provided to secure the ship to a quay and not for towage operations, their strength determined by the breaking strain of mooring lines. When used for mooring, a figure-of-eight style of securing lines to bitts is adopted, the SWL based on collapsing forces of the component’s vertical levers near the top of the bitt’s legs. If the towing line is placed at the base of one of the bitts however the SWL can in effect be doubled. This misunderstanding can result in tugs not employing the maximum power they can offer.

Very interesting but I’d be afraid to be the guy spreading that piece of info around the harbor…

One incident that I was involved with the survey of occurred with a brand new container vessel. The centre line bitt had been pulled out by a tug with 55 tonne bollard pull tug.
We established that the construction of the foundations had not been completed as per the drawings.
It is pretty unusual to use the bitts to secure to a wharf these days as he lines are on winches.

If we’re going to talk about the SWL of a bitt we at least need to know what it is we are talking about.

But you’re right, a little knowledge can be a dangerous thing. For one from the article and from the photos of damage it looks like damage to fairleads is more common. The load rating would apply to a tug made up center lead aft (CLA) with a chock on C/L as well.

For another the article says the loading in worse case scenario can be up to six times higher then bollard pull and that’s not including environmental loads or poor tug handling.

In that case, perhaps it is wise to stay within the commonly understood SWL (even though it might be twice as high for a single tug hawser) in order to allow safety margin for worst case scenarios, environmental loads and poor tug handling.

I understand exactly what you are saying though with the SWL of a set of bits being calculated for the figure-8 of a mooring line collapsing both bitts inwards. I didn’t know this and appreciate the info.

With the bow thruster I tell them keep it under 80% if you can use everything if you need it.

The same approach might work here. Here is max allowable, keep it under 50% (or whatever) if you can.


I will say that we are briefed on and train extensively to recognize the effects of towline angle and tow mode on deck fittings. At least at our outfit, anyways.