Single Screw Tugs

At the historic tug-of-war in 1845 when HMS RATTLER (driven by a screw) and HMS ALECTO (fitted with paddles) were tried against each other in a spectacular sea trial, the former succeeded in towing her rival at a speed of nearly three knots.  Since that time, screw propulsion has been the backbone of both harbour and deep sea towage for over a century.

During the 19th, and right up to the early seventies in the 20th century, single screw tugs were in general use in ports in both the Northern and Southern hemispheres. Deep sea towage was also undertaken by increasingly powerful single screw tugs.

In harbour towage there were exceptions, one of the most notable being the Red Funnel Group at Southampton, who, stretching back to the late 19th Century, consistently built twin screw tugs - feeling they gave greater manoeuvrability. But most tug owners preferred to stick to single screw, as these were cheaper to build. As time went on, horsepower and bollard pull increased from modest levels of 500/600 hp to 1000-1500 hp for harbour tugs.

Single screw tugs are used extensively in ice conditions around the world, where the ice is required to be broken for ships to safely berth without packing the ice. Single screw tugs are fuel efficient and are excellent for this service. They are also used to clear ice from Teminal quays where the ice packs between the ship and the quay. The tugs are not used for berthing, only ice clearing. During the summer months, they are used as work boats and short towage services.

With the increasing size of vessels requiring more powerful tugs and turn-arounds becoming tighter, pilots and shipowners were demanding greater manoeuvrability. By the 1970's Voith Schneider tugs were being introduced. In an attempt to improve matters, some tug operators who operated what they considered still reasonably modern single screw tugs, fitted powerful bow thrusters.

An even bolder innovation was the fitting of retractable azimuth units in a number of tugs, which improved both bollard pull and the ability to skirt under the bow or stern of container ships. These were named 'Combi Tugs'. This, however, had only limited appeal, since the majority of the major port operators had made large capital investments in ASD, ATD and Voith propulsive tug units, thereby displacing the conventional single screw units.

Large numbers of single screw tugs have found homes in the developing world. Most were built of robust construction in traditional British or Continental shipyards and, being well maintained, have found no lack of purchasers.