A reaction ferry is a cable ferry that uses the reaction of the current of a river against a fixed tether to propel the vessel across the water. Such ferries operate faster and more effectively in rivers with strong currents.
Some reaction ferries operate using an overhead cable suspended from towers anchored on either bank of the river at right angles to the current flow. A "traveller" with pulleys runs along this cable and is attached to the ferry with a tether rope. This can divide into a two-part bridle which defines the angle of the ferry to the current. Sometimes two pulleys and tethers are used. Sometimes a single tether is attached to a bar that can be swung from one side of the ferry to the other. This type also uses a rudder in order to set the angle of the ferry to the current flow from zero - it is then stationary - to the best angle for maximal crossing speed. Ferries without a rudder change the relative lengths of the bridle or multiple tethers in order to steer. The lateral force of the current moves the ferry across the river.
A now rare type of reaction ferry uses a submerged cable lying on the bottom across a river or tidal water. This can be a wire rope or a chain and is pulled to the surface by the ferry or its operator. It passes through moveable pulleys or belaying points whose location sets the ferry's angle. In order to set off, manual work is required to initially pull the cable and also to manoeuvre, especially during the turning of the tide.[1]
The ferry may consist of a single hull, or two pontoons with a deck bridging them. Some ferries carry only passengers, whilst others carry road vehicles, with some examples carrying up to 12 cars.
A reaction ferry operates as a sailing craft where the traveller pulleys represent the wheels of a land yacht and the moving fluid is the water current rather than the wind. In the case of a reaction ferry with an anchored tether, the analogy can also be to a kite. In both cases the ferry's hull itself represents a sail and is angled to the apparent water current in order to generate lift in the same way a sail is set at an angle to the apparent wind.
With an overhead cable stretched across a river at right angles to the current, the ferry is, in sailing terminology, sailing on a reach with the true current exactly at right angles to the direction of crossing. For the anchored-tether type ferry this is valid when the tether is parallel to the current, near the middle of crossing. In sailing, the speed is governed by the lift-to-drag ratios (L/D) of the sail and the hull including centerboard or keel and rudder.[2]
For reaction ferries, L/D ratios also apply except that one is very high, for example typically 30 for a traveller on a steel rope, as visible in aerial photographs, and the other can vary from low, e.g. 1-2 without a centerboard, to 3.5 with one.
A diagram is shown which follows the standard force diagram for sailing.[3] [4] It is drawn with a traveller L/D of only about 6 in order to make it clearer. The ferry L/D is drawn at 1.5. The lift L acts at right angles to the direction of the apparent current, the vector sum of the true current and the current component due to the crossing speed. The drag D acts parallel to the apparent current. The vector sum of L and D is the resultant force R. This force can only exist because the tether exerts an opposed force of the same magnitude (see Newton's laws of motion), in this simplified two-dimensional projection of what is really a three-dimensional situation. R can be resolved in a drag component directly downstream and a component in the direction of crossing, the thrust T which drives the ferry. This is balanced by the opposing drag of the traveller pulleys. The amount of lift required is set by the angle of incidence of the ferry to the apparent current (here 10°), often done with a rudder (not shown).
In the figure the crossing speed is the same as the speed of the true current. With a centerboard or keel, the hull's L/D could increase several times. This would increase the crossing speed also several times, but according to the drag equation the forces increase with the square of the speed and put a great load particularly on the overhead cable. With the anchored-tether type ferry, such high speeds would be unobtainable because its tether drags in the water or is supported by buoys that do and this drag would also increase with the square of the speed.
At one time over 30 reaction ferries crossed the rivers of British Columbia, primarily the Fraser River and the Thompson River. Those still operating include:
In Quebec, the small Laval-sur-le-Lac–Île-Bizard Ferry operates seasonally across the Rivière des Prairies from Laval-sur-le-Lac to the Île Bizard.
Reaction ferries cross the rivers Sava and Drava.
A number of reaction ferries operate in Germany, particularly across the rivers Elbe and Weser. Between the 17th and 19th centuries, they were quite common on the Rhine. Currently operating ferries include:
The is a historic reaction ferry across the Adda River at Imbersago. It is reputed to have been designed by Leonardo da Vinci.[6]
A number of reaction ferries operate:
Four passenger ferries cross the Rhine in Basel.[11] [12]
Three such ferries cross the Aare in Bern.
A small traditional ferry, the last on this river, crosses the Doubs.[13]
The Hampton Loade Ferry, which carried passengers only, crossed the River Severn at Hampton Loade in Shropshire until 2017. It was operated partly by the current and partly by punting.
Several reaction ferries crossed rivers in the Ozark Mountains of the central United States during the first half of the 20th century. The Akers Ferry across the Current River near Salem in Missouri remains in operation. Menor's Ferry in Jackson Hole, Wyoming, was a dual-pontoon reaction ferry built in the 1890s and operated until 1927. A replica was constructed by the National Park Service in 2009.[14] [15]