^ Damen Double Ended Ropax Ferry 8521 (Image Courtesy http://products.damen.com/en/ranges/fast-ropax-ferry/fast-ropax-ferry-8521)
Double-Ended Ferries with Turkish Delight
In 1893, the Society of Naval Architects and Marine Engineers (SNAME) described a ferry as ‘legally, a continuation or prolongation of a highway over a navigable stream.’
Economic importance of ferries has attracted numerous researchers. Ferries played a pivotal role in the development of the Pacific Northwest of the United States. The first double-ended ferries went sailing in the 1930s.
Ferries are water crafts of various sizes that carry (or ferry) passengers, cargo, and vehicles. These are a part of public transport systems between relatively close locations separated by water bodies.
Although slower than trains and automobiles, ferries involve lower capital expenditure. Moreover, they take off large chunks of traffic from already jam-packed roads and railroads.
Double-ended ferries are a special type of ROPAX ship. They are propelled from both longitudinal ends. Each of these ends serves as the bow and the stern depending on the direction of travel.
Turkish shipyards are on a double-ended ferry building spree. Leading the charge is Sefine Shipyards. Cemre and Ada shipyards are not too far behind and neither is Remontowa Shipyard in Poland.
Double-ended ferries usually operate in cramped ferry terminals, short-distance domestic routes, and shallow waters. Most fjord and coastal ferries of Norway are double ended and so are some in British Columbia, Canada and Sydney, Australia.
Famous double ended ferry services include:
- Washington State Ferries
- Staten Island Ferry
- Star Ferry
Turkey-based Sefine Shipyard is building a host of double-ended ferries for multiple clients:
- Two for Basto Fosen, a Norwegian operator, of 142.9m length and 20.7m width with a capacity to transport 600 passengers, 24-30 trucks, and 200 cars
Cemre Shipyard, Turkey is creating one ferry of the same design for Basto Fosen
- Two for the Port of Tallin that can hold 600 passengers, 12 trucks, and 150 cars
Remontowa Shipyard in Poland is making two more such ferries for the Port of Tallin
- One for Norled possesses the capability of conveying 151 passengers, 4 trucks, and 40 cars
- One for Fosen Namsos which can carry 150 passengers, 3 trucks, and 38 cars
Rolls Royce Azipull 100 azimuth thrusters with X3 Propulsion Control System will power all three ferries being built for Basto Fosen. The same Rolls Royce thrusters will also drive two ferries ordered by Fjord 1, another Norwegian operator. Ada Shipyard, Turkey is building these two.
The Staten Island Ferry Operates on the World’s Busiest Ferry Route
Rolls Royce azimuth thrusters provide better maneuverability and steering stability. Loaded with full feathering propellers that lower the power requirements, the thrusters minimize fuel consumption and emissions.
Two Steerprop 2,150kW SP 45 CRP propulsors loaded with Steerprop Push-Pull Contra-Rotating Propellers technology will drive each of the four ferries ordered by the Port of Tallin.
Merits & Design Parameters
Double-ended ferries have symmetrical ends i.e. both ends can serve as the bow as well as the stern. This way, they do not have to turn around while moving back and forth between terminals. And cars do not have to change direction while unloading.
Such operations minimize the terminal time i.e. time needed for:
Through this, double-ended ferries slash fuel consumption and the hazard of mishaps during maneuvers. John W. Waterhouse, PE of the Elliot Bay Design Group lists other merits:
- Excellent Braking through propellers at both ends means such ferries are better at negotiating through crowded shipping lanes
- Better Safety in cross-current-infested waters
- Unchanged Vessel Configuration enables the mariner to focus more on the external effects of tide, wind, and current
- Remarkable Maneuverability particularly with cycloidal or azimuthal propulsion
- Low Resistance
- Protection from Side Damage
Mid-ship symmetry throws umpteen hydrodynamic challenges for double-ended ferry design. Because each end can serve as the bow and the stern, the principle of optimizing hull lines at the fore and aft for good flow properties at the bow and stern respectively is not sufficient.
Most double ended ferries have one propeller at each longitudinal end. Smaller and shorter ferries that run through unfavorable water conditions also have one at each longitudinal end, but diagonally arranged.
Larger ferries that need greater power and/or have to ply in somewhat shallow waters have two propellers at each end. This improves redundancy and maneuverability. Ferries that sail in one main direction have two propellers at the stern end and one at the bow end.
Force Technology, Denmark has been researching diverse facets of double-ended ferries for over two decades and recommends the following designs:
- lowering hull resistance
- defining optimum hull lines for specific ship speeds (Froude Number) and water depth
- choosing proper propulsion system
- optimizing power distribution between forward/aft propeller groups
- aligning thruster units (thruster legs) with the local flow
- streamlining of appendages such as fins – fixed flat and bulbous fins, skegs, streamliners, rudders, and thruster head-boxes
That apart, separation between passengers and vehicles is obligatory in ferries carrying over 1000 passengers. The symmetrical structure means even seasoned passengers can lose their way. Visual signs prevent such confusion that can prove lethal during emergency evacuations.
Vehicle deck design determines the vessel’s main dimensions and vertical access structure. Number and width of car lanes, casings, and passageways establishes the ferry’s width. Terminal size and vehicle width dictate the shape of the ends.
Wider ferries are more stable and cut down vehicle loading-unloading time. Structural system of double-ended ferries must:
- hold vehicle loads
- support passenger decks
- provide unobstructed views to passengers
Froude Number is the ratio of inertia force of a fluid element to its weight. A dimensionless quantity, it determines the resistance of an object moving through water. It enables us to compare objects of different sizes and understand how scale models work in relation to finished products.
Double-ended ferries usually have their Longitudinal Center of Buoyancy (LCB) centered at the mid-ship regardless of the ferry’s design speed to maintain symmetry of hull lines about the mid-ship.
LCB is the longitudinal distance from the mid-ship to the center of the displaced volume of water when the hull is stationary. The LCB must align with the vessel’s longitudinal center of gravity for the hull to be in equilibrium.
Designers prefer excellent course-keeping over maneuverability, dynamic-trim-free operations, and wider vehicle deck area. Inherently, double-ended ferries require low maneuvering in the terminal.
Trim is the difference between draughts at the AP (Aft Perpendicular, the aft end of the design waterline) and at the FP (Forward Perpendicular, the forward end of the design waterline). Dynamic Trimming is the adjustment of ship height to minimize water resistance.
Builders locate the propellers, thrusters, and rudders of double-ended ferries so as to:
- submerge the propeller adequately and minimize ventilation risk
- assure ample inflow on to the propeller and rudder
- ensure good maneuverability when using wing thrusters
Fitted at the lower part of the hull at the ends are skegs that support the hull in dry dock and the shaftline. Their shape and volume heavily influences the bow waves created by the vessel’s bow. Larger bow waves slow down ships and endanger shore infrastructure and moored ships.
Unlike conventional vessels, large skeg area does not improve course keeping in double-ended ferries beyond a certain limit, for it creates side forces that augment yawing. Instead of increasing skeg area, provide additional fins.
The balance between fuel efficiency, reliability, operability, and maintainability determines the propulsion system. Locate the propeller as low as possible when using wing thrusters. This neutralizes ventilation risk.
Double-ended ferries can be built with hard chime or round bilge. The double-curved, round bilge is tougher to forge but gives smoother flow. You can easily create single-curved hard chimes, but have to align them to prevent resistance-boosting separation around sharp corners.
V-hull forms offer intact and damage stability. A two-compartment subdivision prevents flooding while large openings at the end and the side quickly dissipate heat and smoke.
Specialist shipbuilders say the art of shipbuilding is a fine balancing act. The immense economic importance of double-ended ferries means the art will continue to evolve.
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