08 May 2015
May 8, 2015

Why Do Tugs Need Fenders?

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Why Do Tugs Need Fenders?

Tug Fenders

Tug fenders perform multiple roles, all of which need to be properly addressed in the specifications, ideally from an operational, performance and material perspective.

  •  Fenders Absorb Impacts

When a tug contacts a ship this is a controlled collision and the kinetic energy can only be absorbed in one of three ways – damage to the tug, damage to the ship or elastic deflection of the fender. The energy calculations are quite simple, but selecting fenders with appropriate load-deflection characteristics is more complicated. The fenders have to absorb the impact energy not just square-on but also at angles. How fenders are supported also affects the way they absorb energy, so equally important in the design process is the way fenders are attached.

Bow flares, approach angles and other factors affect which and how many fenders absorb the kinetic energy at contact.

Tug Fenders for Tugboat and Workboat

  • Fenders Should Be Gentle

The thrust of the tug is spread over a finite fender contact area commonly called the hull pressure. Fixed fenders in ports and terminals handling large container ships, gas carriers and the like are commonly designed for 20t/m2 or less and these requirements are now appearing in tug specifications.

In contrast, a cylindrical fender will typically exert about 65t/m2 (or much higher locally when sacrificial tyres are fitted). Increasing the fender footprint is one seemingly obvious solution, but the demand for more powerful tugs and lower hull pressures is a growing challenge with current fender technology and materials.

  • Fenders Must Be Hard Wearing

There is constant movement between tug and an abrasive ship hull. Wear rates of the fender depend a lot on the type of rubber (natural, SBR etc), how it is formulated and the quality of processing. Some rubber types have inherently better abrasion resistance. Harder compounds are sometimes more wear resistant but also become more slippery and prone to cuts, tears, brittle failure and fatigue, not to mention higher hull pressures. Soft rubbers will be stickier but can overheat and wear faster in open sea operations.

  • Fenders Should Be Long Lasting

Making long-lasting rubber compounds that perform well in severe marine environments is a specialist skill best left to polymer chemists. Ozone and ultra-violet light (which cause embrittlement and surface cracking in unprotected rubbers) are both abundant where tugs operate but can be resisted with suitable additives in the rubber. There are thousands of possible formulations that need to balance operating performance with environmental resistance, tensile strength, elongation, hardness, tear and abrasion properties. Rubber specifications are already well covered by established standards used for fixed fenders in ports and harbours, but infrequently adopted or adapted for tug fenders. Third party verification of these properties can also be a low-cost precaution to ensure high quality and long-lasting materials are used.

  • What Types of Tug Fender Are Available?

Fenders used on tugs vary in design according to the duty they perform.

Tug Fenders, Tugboat Fenders and Workboat Fenders

(a) Cylindrical Fenders

Cylindrical fenders are popular on the bow and/or stern of tugs, usually for pushing against flared hulls and in open sea conditions. They are typically fitted into a curved recess, held in place by a combination of longitudinal chain and a series of circumferential chains or straps rebated into grooves. Sacrificial tyres are sometimes fitted over the cylindrical fender although the benefits of reducing wear versus greater maintenance and higher localized hull pressures are questionable.

Cylindrical Fender
Cylindrical Fenders are simple to install and operate which makes these units an economical solution for remote locations and for multi user berths...

(b) Pushing Fenders

Pushing fenders present a large flat surface that distribute forces and reduce hull pressures and are fitted to bow or stern depending on tug propulsion.

Pushing fenders evolved from crude square profiles into sophisticated “M” and “W” – shaped cross-sections. These later types provide a more gentle contact face, absorb more energy and eliminate gaps where ropes can snag. M and W Type Fenders can also be fitted around tight radii to give designers more scope with hull form, whilst a simple pin fitting allows quick replacement.

Pushing fenders have evolved from simple keyhole shapes into more sophisticated profiles which better fit the hull and provide lower hull pressures.

Tug Fender
Tug fenders must work harder, for longer and under more adverse conditions than any other fender type. Tug fenders can be divided into three main types...

(c) Side Fenders

Side fenders are mostly used during escort duties and when berthing the tug at unfendered docks. These beltings are most commonly a “D” – shaped profile fixed between parallel flat bars welded to the hull.

Wing Fender
JIER Wing Type Rubber Fenders are developed on the basis of D Type Rubber Fenders. They can be fixed with double line anchors which greatly ...
D Fender
D Fenders can be pre-curved, chamfered and drilled to aid installation at a relatively low cost and can also be cut to the length required ...

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