Current issues and compromises

Designers of roro ferries are faced with a choice of maximising lane metres or minimising risk of damage to the sideshell through contact with harbour walls. Designs to maximise lane metres have flat sides above the fenders; those minimising risk of damage to the sideshell have sides above the fenders that slope inwards toward the centreline. Each design has different elements of risk attached to it for the owner.

Roro ferries with flat sides tend to be built for flexibility of operation between services, with maximising revenues for both owner (charter) and operator (cargo) in mind. There tends to be no long-term commitment from the owner to a particular service, nor to owning the ship. The service on which the ship is operated is deemed to be the one that maximises its earning potential at any particular moment in time. This flexibility also makes such roro ferries relatively easy to sell – especially in their early years – should, for example, the operator?s plans not match expectations. However the flat sides are exposed to potential damage through, for example, contact with harbour walls during manoeuvring operations. This damage can present problems in later life.

Roro ferries built to minimise risk of damage to the sideshell, at the expense of some lane metre capacity, tend to be designed for a specific route. The owner takes a long-term commitment to a particular service, or area of operation, and plans for the ship to stay on that service for many years. While having an interest in maximising revenue from that service, the owner also has an interest in continuity of service and the condition of the ship in its later life, because the owner still plans to operate the ship then. The operator loses some lane metre capacity and therefore potential revenue taking this long-term condition approach. The route specific nature of such designs can also make them difficult to sell if conditions on the service for which they are designed change and therefore require it.

Designers of roro ferries are also tasked with producing a ship that consumes less fuel. For many ferry services, achieving a reduction in the required power (and therefore fuel consumption) to give a certain speed is not a simple case of increasing the ship?s length to beam ratio to give a lower block coefficient and Froude number. Ferry length is restricted by the diameter of the turning circle in many harbours. This means designers can be faced with a decreased length to beam ratio, in order to allow the ship to load more cargo.

In recent years ferry designers have achieved a reduction in the required power to achieve a given speed, through incorporating features in their designs such as a more efficient bulbous bow, a reshaped ducktail and trim wedge at the stern, and lower drag openings in the hull for equipment such as bow thrusters and fin stabilisers.

This in turn has enabled the equipping of ferries with smaller main engines, consuming less fuel and requiring a lower capital outlay. These smaller main engines can also allow a reduction in size of the machinery space and a corresponding increase in roro and/or passenger space. The design features have also benefited new generation high-speed roro ferries, although these ships still require a massive increase in power to achieve an incremental increase in speed.

The turning circle diameter restriction on ferry length has required ferries to be highly manoeuvrable as they approach the maximum dimensions for unassisted port operation. The solution commonly adopted today is for a twin-screw vessel with CP propellers and twin flap rudders with tunnel thrusters at the bow. However, operators now have the option of adopting azimuthing pods or thrusters at the stern to give the required manoeuvrability.

A number of operators have looked at sidestepping the need to turn around in port altogether through a double-ended ferry solution. This is in common use on very short crossings. It has the advantage of removing time consuming turning manoeuvres in port and has the potential to allow increased ferry length, giving more cargo loading potential and, with an increased length to beam ratio, different powering requirements. However the design?s adoption is still commonly viewed as an unacceptable technical risk, especially for longer crossings for which the economics are uncertain and unproven.