Encouraging results for Markey’s variable-slip towing brake
Foss Maritime has provided feedback on the variable-slip towing brake, a newly-introduced feature of the Markey towing winches on its three Arctic-class ocean-going tugs.
The recent entry into service of the 100tbp ocean-going tugs Michelle Foss, Denise Foss and Nicole Foss was a newsworthy development for the towing industry, the trio’s progress followed here. Field-testing of one aspect of these tugs reminds us of the importance of what is at the heart of such vessels, the towing winch.
The success of any towing operation is dependent on maintaining continuity of that vital link between tug and tow, the tow line. Sufficient tension must be maintained to allow progress but at the same time snatch or prolonged extreme loads that may test the towing medium beyond its patience, leading to parting must be avoided, a most undesirable situation for the tug captain to say the least.
On a long ocean tow, rendering the tow line when tension increases and then recovering when the load reduces is a well-established process often performed automatically, referenced to a pre-set tension figure. Foss called on its experience with high-response escort and ship-assist hawser winches along with Markey, who supplied the TDSD-40UL towing winches on the trio to develop the first examples for Markey of its water-cooled variable-slip (WCVS) braking feature.
Markey make the analogy with the ‘star drag’ setting on a sports fishing reel for where the captain sets the tension slip-point to compensate for lack of catenary when the tow is on a ‘tight leash’ close to the stern of the tug. This procedure is relevant when crossing a bar or operating in a confined waterway.
The WCVS principle is based on the water-cooled brake and clutch technology that has been proven through years of operation in Markey’s high-response electric escort and ship-assist hawser winches. The Arctic-class tugs are the first towing winches to incorporate this option.
The use of a WCVS brake was also driven by Markey’s familiarity with previous methods used to achieve a level of controllable slip using band brakes, it has offered such a system on Class II hydraulic hawser winches for over 25 years.
Two main advantages are listed for this new application of the WCVS brake in towing winches. The brake itself is integrated into the winch design as a secondary brake; if a problem develops with one of the standard drum band brakes, this brake acts as a fully functional back-up regardless of the length of wire deployed.
The second advantage claimed by Markey is that the WCVS brake offers fine control and high repeatability with minimum readjustment. Due to the nature of band brakes, brake materials and environmental conditions in which they operate, it is very difficult to establish a reliable and repeatable tension set point using that type of system. This is not the case with the WCVS brake due to the combination of materials and water cooling which it is claimed, results in an exceptionally smooth transition from static to dynamic operation.
As with any innovation, Markey and Foss both admit they encountered a learning curve during initial deployment. It has since accrued operating hours in a variety of difficult towing conditions and crews of the Foss tugs speak positively of the new system, Captain Justin Earl stating: “I think the brake is great when used in applications like short wire ocean towing/bar crossing or something like a Panama Canal transit with a heavy load or without much surge gear.” And Captain Raymond ‘Todd’ Zacke added: “I love those slip brakes. And in heavy seas that system is worth its weight in gold for letting out wire safely.”
By Peter Barker
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