Marine Engineering and Naval Architecture

One of the major concerns during a towing operation is how much load is being put on the towline and the fittings used to secure it. There is a way of approximately calculating this – and it will at least give an indication of when the tow line is getting close to or has reached its safe working load. Maximum safe working load is usually taken to be 50% of the rated strength of the rope. The rated strength or break load being the load at which the line will probably part. Any rope manufacturer or supplier should be able to give not just the rated strength of a rope, but also its ‘elongation at break’ or ‘stretch at breaking’. For example Polyester 3 strand 20 mm Diameter - break load 6300kgs - elongation at break 30%. Using the above example a cheap & easy ‘eyeball’ method of gauging loads on the towline can be set up.   If the elongation at break is 30%, then the elongation at the ropes max safe working load (50%) will be approx 15%. A preset length is measured an

There are many different types of rope both in terms of the material and construction used. Outside of the more specialised ropes available the most common materials used are; Polyester.   Polypropylene. Nylon. Materials Polyester and Polypropylene are the most common materials used for towlines. Nylon isn’t generally used by Coastguard as a towline, not because of any issues of strength – in fact Nylon is very strong, and often used in commercial towing. The problem is Nylons capacity for stretch. While a towline needs to be able to stretch to absorb some of the dynamic loading imposed on it, Nylon rope depending on its construction can stretch to an excessive degree (60% + in some cases). The greater the stretch the greater the recoil when a line parts. The danger is not just from the towline, there is also the risk of a fitting such as a cleat being pulled out but remaining attached to the recoiling line. Polyester Stretches approx 25 to 35% of its length. Does

A tow line is used with a tug (tugboat or towboat) which is a type of vessel that maneuvers other vessels by pushing or pulling them either by direct contact or by means of a tow line. Tugs typically move vessels that either are restricted in their ability to maneuver on their own, such as ships in a crowded harbor or a narrow canal, or those that cannot move by themselves, such as barges, disabled ships, log rafts, or oil platforms. A tow line is the essential link between tug and ship. This applies to tugs operating alongside a vessel in the push-pull mode as well as to towing tugs fastened with a tow line to the bow or stern of a ship, also called `tugs towing on a line‟. Basic towline requirements A towline must fulfil two basic functions, firstly to function as the load-carrying link between tug and ship and secondly to cope with the dynamic loads resulting from the relative motion between tug and ship. This leads to the following basic requirements for towlines for harbo