Mooring Bitts for Towage

April 15th, 2006 by JCB
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MOORING BITTS FOR TOWAGE

During the last few years tug companies have upgraded their fleets to include more powerful tugs and many of these new tugs are rated with a bollard pull of over 60 tonnes. In addition to the problems identified in the previous MAIB report, pilots also now need to be aware of the load specifications of the mooring bitts of a vessel which may also used for towing. In recent years it has become mandatory to mark the Safe Working Load (SWL) on the bollards and this has been useful in guiding us as to how much tug power can be used.  In view of this I was interested to learn from the specifications that the actual SWL for towing is actually twice the bitt rating and this is explained in the following extract from the construction rules:

The British Standard refers to the methods of belaying the rope as ‘mooring’ for figure-of-eight belaying, and ‘towing’ for loop belaying. The ‘towing’ SWL is twice the mooring’ SWL. The reason that the SWL depends on the method of rope belaying is that certain belaying methods tend to pull the two posts together and thus induce a

higher stress in each barrel than that produced by an eye laid around a single post. With figure-of-eight belaying, the loading in each post corresponds to the sum of all forces in the successive rope layers, which can be higher than the maximum rope load. Experienced mariners are aware of this phenomenon and have devised methods that

effectively distribute the external load over the two posts (for instance, by taking one or two turns around the first post before starting to belay in figure-of- eight fashion). Nevertheless, IS0 takes a conservative approach by assuming that some mariners may lack this knowledge. There is only one problem with this and it is a major one. Experience indicates that ship owners usually take the maximum rating from the bitts and then use this figure to calculate the rating for the fairleads so, unless the fairlead is clearly marked at the higher rating then it is prudent to only use the marked SWL rating as the maximum for towing. This factor possibly explains why there are increasing reports of fairleads rather than the bitts failing whilst towing! I have also been advised that the load ratings are set by the manufacturers of the fairleads and bitts but the SWL marked is not necessarily applicable to the fixing to the deck or bulwarks!