In-line propellers for tugs with "carousel" towing system

Characterised by a towing winch on a ring that can rotate freely around the tug's deckhouse – a unit referred to as a carousel towing system – the vessel is called the Carrousel RAVE Tug (CRT), using the Dutch spelling of "carousel". The system eliminates the risk of capsizing under a tow load.
In combination with two Voith Schneider Propellers (VSPs) arranged in-line, the tug design allows a slender and highly efficient hull, according to Voith, which says that "new and previously unseen levels of tug operation safety and efficiency" will be reached. The first two CRTs are currently under construction for Novatug BV, a subsidiary of Netherlands-based Multraship Towage & Salvage.
Braking and steering operations can be performed highly efficiently with the new tugs, Voith says, pointing out that utilisation of hull forces rather than propulsor forces results in lower fuel consumption and emissions. The CRT can carry out manoeuvres not been seen before with any other tugboat concept, Voith asserts. During these manoeuvres, the yaw angles of the tug are "virtually limitless".
The VSPs, Voith says, were chosen for their precise thrust control, quick response and insensitivity to variations in inflow direction. Arranged in-line, they allow very smooth and precise control of the tug heading under all possible operating conditions. The towline force can continuously be controlled safely and precisely via the VSPs, according to the company.
With the help of computational fluid dynamics, the Voith ship simulator and model testing, extensive optimisation work has been performed on the design to determine the optimum position of the propellers relative to the carousel system as well as the optimum size and position of the skeg. The result, Voith says, is a very well-balanced tug that is stable and easy to handle in all sea and weather conditions. The VSPs are controlled via a fully redundant Voith electronic joystick control system.
In November 2015, the Netherlands' Damen Shipyards Group began construction of the tugs through its subsidiary Van der Velden Barkemeyer GmbH. Delivery is scheduled for the first quarter of 2017. The CRTs are 32m in length overall and have a maximum operational draught of 6.30m. Propulsion is via two 32RV5 EC/250 Voith Schneider Propellers in combination with 1150 DTL Voith Turbo Couplings, Renk gear couplings and two ABC main engines delivering 2,650 kW at 1,000 rpm.
The slender hull shape of the CRTs and the controllable pitch propeller characteristic of the VSPs allow a free running speed of 14 knots. The bollard pull will be 70 tonnes. Despite the compact size of the CRTs, the steering forces reach 160 tonnes at 10 knots.