Caterpillar and Sanmar develop hydraulic hybrid system

A RAmparts design will be the first reference for the new hybrid drive A RAmparts design will be the first reference for the new hybrid drive

Caterpillar Marine and Sanmar Shipyards have entered into a collaboration agreement to build a tug with an innovative hydraulic propulsion system furthering principle of a hybrid tug design.

Seldom a month passes where news related to alternative-fuelled or so-called green tugs is not reported here. Despite something of a hiatus which some suggest is related to a period of low conventional fuel prices it is clear this latest revolution, particularly with shiphandling and escort tugs continues to develop.

Owners are attracted by the various benefits associated with green tugs including increased fuel efficiencies and subsequent reductions in running costs, not to mention pressure to improve the environmental footprint of what by the nature of the job can be a somewhat ‘smokey’ activity. The bottom line of course is the cost of the investment or put another way the pay-back time and this alternative to the more normal hybrid arrangement involving electric motors and batteries presents an interesting development.

The new and patented Marine Advanced Variable Drive (AVD) harnesses Caterpillar’s experience with integrated power systems along with Sanmar’s expanding tug building business and involves a fully integrated hydro-mechanical propulsion system which it is claimed, provides ‘significant improvements’ in both fuel efficiency and vessel performance.

Caterpillar state that different from a typical power take-in solution, the AVD incorporates a planetary gear set allowing seamless clutch engagement of main engines, auxiliary engines, or both to provide a scalable power installation to meet operational needs in terms of maximum vessel speed, power, or bollard pull. This also allows propeller speed independent of engine speed with optimal engine efficiency leading to fuel savings of fifteen to twenty percent, essentially all the benefits of a variable speed diesel-electric propulsion system, Caterpillar claim at a fraction of the cost and size.

The AVD system’s flexibility can accommodate multiple configurations whereby auxiliary engines can be utilised to provide low load or transit operations extending the time between overhauls. Electric motors can be used instead of hydraulics if desired and diesel engines can be substituted by natural gas as the system provides superior vessel performance regardless of engine load acceptance. Main engines can also be downsized with supplemental power supplied via auxiliaries or generators if electric motors are used.

The vessel selected for the first Caterpillar AVD system will be a Robert Allan Ltd-designed RAmparts 2400SX tug. The machinery specification is unsurprisingly dominated by Caterpillar, centred on: two 3512 main engines, a C32 auxiliary and Cat MTA 627 fixed-pitch thrusters. The AVD system will include fully-integrated controls with customised operating modes and display panels in the wheelhouse. Compared to conventionally-powered examples with equivalent bollard pull (70t) the return on investment is estimated at three years or less based on projected fuel and operating cost savings.

By Peter Barker

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