A cable ferry is a flat-bottomed platform tied to a cable spanning the river, so it cannot drift downstream; a reaction ferry is driven across by angling its hull to the current so that the water itself pushes it, while a hand-hauled ferry is pulled along the cable by people. The river does the work only while it flows, and the force on a ferry rises with the square of the current speed, so a river that rises from 2 to 3 m/s loads the cable and anchors 2.25 times as hard.
Part of River Navigation & Ferry Systems
A cable ferry is a river crossing for people, animals and loads in which a platform is held on a fixed cable and moved across by the current or by hand.
The Cable Holds The Ferry; The Current Or The Crew Moves It
A river is a barrier for anything that cannot swim, and a bridge is expensive. A ferry is the cheaper answer. Left to itself a boat drifts downstream as it crosses; a cable removes that problem and also makes the crossing predictable. The carrier describes the reaction type: a cable spans the river, the flat-bottomed ferry is attached to it through a trolley and a bridle at two points, the bridle lengths set the hull at about 30-45° to the current, and the current pushes the hull across. To return, reverse the bridle angle. The river supplies the energy for as long as it flows.
Where the current is too slow to drive a reaction ferry, the same cable and platform can be used with a crew pulling hand over hand along it. The parts are the same, and the safety reasoning is the same.
What You Need
- A cable and two anchors, each strong enough for the loads in Cable Installation
- A platform (see Ferry Platform Design)
- A trolley and a bridle on the cable (see Reaction Ferry)
- A crossing with firm banks on both sides and a current you have measured
Method: Decide Whether And How
Step 1: Measure the current with a float (see Reading the River) at the crossing in normal and high water.
Step 2: Check the span and bank heights: the cable must clear flood level.
Step 3: Choose the drive: reaction if the current is reliably strong enough to push the loaded hull; hand-hauled if not.
Step 4: Compute the loads for the highest current you will operate in, and the highest you can expect (the next two articles do the arithmetic).
Step 5: Set an operating limit and a flood rule, and tell everyone who will use the ferry.
Worked Example: Same Ferry, Two Rivers
A scow 6.0 m long drawing 0.125 m has a side area below the waterline of 6.0 × 0.125 = 0.75 m². Taking a drag coefficient of 1 (an assumed order of magnitude), the broadside force at 2 m/s is ½ × 1,000 × 2² × 0.75 × 1 = 1,500 N, and at 3 m/s it is ½ × 1,000 × 3² × 0.75 = 3,375 N. The ratio is 3,375 ÷ 1,500 = 2.25 = (3 ÷ 2)². Doubling the current quadruples the force; a river that rises by half raises it 2.25 times.
Common Mistakes
- Designing for the average current — design for the flood.
- A cable hung low over the water — it traps boats and swimmers.
- A ferry without a limit on load or current — set a number and mark it.
Safety
A ferry pinned broadside in a current can be pulled under
If the platform swings broadside and the bridle is attached high, the current pushes the lower hull downstream while the cable holds the top, and the upstream side dips under. Attach the bridle low, fit a release you can operate from the deck under load, and stop operating before the river reaches the limit you set.
What’s Next
- Learn next: Reaction Ferry — the mechanism and its stability
- Combine with: Cable Installation — the cable and anchors that carry it