By Zhou Yuhuan

Tug industry has developed for 180 years so far.With the increasing demand from shipowners and port offi ces for tugs with higher power and higher efficiency, propulsion system and relevant hull design of tugs have seen rapid development. The global tug market is showing a development trend of high efficiency, multifunction, miniaturization and high power.

Looking back on the development history of tug, the original situation of building large-size tugs with installation of high power engine and deep immersion propeller to increase the bollard pull force has changed. The emergence of new type of propulsion system has brought substantial changes to hull design. Its historical change is the Voith tug that was born in Germany in 1952, which adopted vertical axis propeller producing comprehensive driving force, and the bollard pull can be exactly operated at the target position.The born of this type of tug promoted the development of all-round propeller and “Z type gearing”. At present, the four design concepts which adopt “Z type gearing” have been generally approved by tug industry, including ASD, Tugboat with Z type propeller, Rotor Tug and Z-Tech.

The modern ships are getting larger and larger in size and are not easy to operate in port, which frequently need to employ tugs with strong power and fl exible maneuverability to assist the operation. This not only offsets the defect of large-size ships in terms of fl exibility but also reduces their fuel consumption, leaving out the installation of expensive fore and after propellers and achieving considerable economic benefi t .If one tug could work in place of two and more tugs, this means not only reduction of the number of tugs needed, but also improvement of work efficiency on the spot as well as abatement of mutual interference among tugs. Any single tug with increased towing force has signifi cance in both economy and effi ciency. If in future tugs could work more effi ciently by applying new devices,they will certainly have higher competition advantages.

Though smaller than traditional ones in size, the new generation of tugs has stronger driving force and functions.Small tugboats can operate within narrow water areas which are usually difficult for larger ones. Modern tugs should have more multi-functions than the traditional tugs. If the common harbor tugs can have the functions of the salvage vessels, escort tugboats and offshore supply tugs, they will defi nitely win the shipowners’ fl avor. From the perspective of safety and environmental protection, in some ports and regions such as oil ports or sensitive environmental areas in North America and Norway, particularly, the regulations requiring tankers must be escorted by escort tugs will to some degree add to the demand for large tugs. Under the development trend of “four features” including high efficiency, multi-function, miniaturization and high power,for small size ships providing shipping auxiliary and other services for larger ships, operational efficiency and economy are the important factors to be taken into confor ship type design.