Ferries transport a mix of wheel-based cargo, cars and passengers. The ferry types used today cover everything from high speed crafts and small double end ferries to large RoPax vessels and luxury cruise ferries.
Ferries usually operate on short routes with a fixed itinerary and regular departures. The speed is tailored to suit the route length so that desired departure and arrival times can be offered. Displacement ferries often operate at Froude numbers between 0.25 and 0.35. This relatively high hull speed makes hydrodynamic optimisation extra important. The block coefficient should be kept low. Lengthening the waterline is often very beneficial for ferries, and they profit from the use of ducktails. Other stern applications such as trim wedges and interceptors can also give very good results.
Ferries often have a low draft and twin screw propulsion is most commonly used. The twin shaft lines result in large drag, and appendage design can have a big impact on the power demand. Some of the newer propulsion concepts such as CRP and wing thrusters without any open shaft lines will offer considerable power savings.
These advanced propulsion solutions can be ideally combined with combined dieselelectric and diesel-mechanical (CODED) machinery. The electric propulsion gives high efficiency at low speeds and while manoeuvring. The mechanical booster with low transmission losses is engaged at transit speeds. This arrangement is especially suitable for ferries that typically operate on routes containing shallow water areas and speed restrictions, and which sometimes give a most varying operating profile and make optimisation more difficult.
Ferries have frequent port calls, and reducing the turn-around time in port allows for a clear reduction of speed at sea. Good manoeuvring and efficient cargo handling is therefore of utmost importance. Cargo can be loaded using two level link spans for fast loading. High power tunnel thrusters or steerable thrusters offer good manoeuvring to reduce port time.
The payload functions can be divided into two distinct parts, the passenger facilities and the RoRo spaces. The split between passenger and cargo functions affects the way the ship is designed and optimised. Large passenger areas give rise to high hotel loads, so various means of reducing power consumption by accommodation functions, such as lighting, HVAC and the galley, can result in large power savings.
Reducing the weight of the ferry gives clear power savings, since the speed is often relatively high. The lightweight (LWT) of a ferry, ie. its weight without cargo, fuel etc., represents a large part of its displacement. Reducing the construction weight by using lightweight materials can therefore have a big impact on the total displacement.
Alternative fuels are also an interesting option. A regular service between fixed ports makes bunker logistics easier to arrange. LNG in particular could be an ideal fuel: this could significantly reduce not only emissions but also energy consumption. The coolness of LNG can be used to cool the air conditioning, eliminating the need for conventional heavy fuel heating
