RoRo vessels are designed to transport wheel-based cargo or cargo loaded with wheel-based equipment. Efficient loading and unloading methods are characteristic of RoRo vessels and the cargo is usually moved over large shore ramps. The most common categories of RoRo vessels are Short Sea RoRo, Pure Car (and Truck) Carriers (PCTC), deep sea RoRo and combined RoLo vessels. Short sea RoRos often operate on fixed routes with regular departures, while deep sea RoRo and PCTCs may have more varying itineraries.
Short route RoRos have frequent port calls, and reducing the turn-around time in port allows for a clear reduction of speed at sea. Good manoeuvring is therefore important for efficiency, and so is efficient cargo handling, which applies to all RoRo vessels.
A PCTC may look large and bulky from the air. The underwater hulls, on the other hand, are usually fairly slender and have low block coefficients compared to the modest Froude numbers they operate at. This gives low resistance and the propulsion efficiency of the large, lightly loaded propeller is usually high. To further reduce propulsion power demand, advanced propeller designs and rudder hull interaction can be sought. A contra-rotating propeller could reduce some rotational losses and further improve efficiency. The efficiency of scale also offers great potential for savings in RoRos.
The large, bulky superstructure of a PCTC and all the ventilation fan structures can result in high wind resistance. A more streamlined superstructure therefore results in a clear reduction in the power demand.
The need for large deck areas has lead to multi-deck vessels with high centres of gravity, resulting in a situation were many PCTCs have fixed ballast to achieve sufficient stability. PCTCs also have quite a long operating range, so the fuel carried onboard forms a large part of the deadweight compared to the relatively low density cargo, such as private cars. Reducing the construction weight – by using lightweight deck materials, removing ballast through design changes and restructuring the bunker logistics – will reduce the displacement of the vessel and thereby have a positive impact on the power demand.
The large electricity consumers in RoRos are usually the ventilation fans for the RoRo spaces, since frequent air changes are needed during cargo handling. A new option for power generation is a hybrid power plant consisting of a combination of diesel generating sets, shaft generators, fuel cells and batteries. This plant can be optimised for the power profile of a PCTC, with high electric loads in port and lower power demand at sea. The hybrid plant could also include solar power. The solar cells could then advantageously be located on the large surfaces on the superstructure of a PCTC.
The world wide operation of a PCTC can also benefit from an energy-saving culture. The crew can affect power consumption, for example by optimising the speed to arrive just in time, shutting down unnecessary electric consumers and ensuring the right trim, etc.
