A reaction ferry crosses a river with no engine because bridles set the hull at about 30-45° to the current and the water pushes it sideways along a cable; its danger is that the current pushes the hull low down while the cable holds it high up, and that couple rolls the upstream side under. For a narrow craft 4 m long and 1.5 m wide carrying 1,200 kg in a 2 m/s current the heeling arm is 0.217 m with the bridle 1.5 m above the water, more than the 0.079 m of righting arm available before the deck edge goes under, so it capsizes; with the bridle 0.3 m above the water it falls to 0.054 m and the craft settles at about 18°.
Part of River Navigation & Ferry Systems
A reaction ferry is a platform on a cable that uses the angle of its hull to the current to turn the river’s push into cross-river motion.
Angling The Hull Turns The Current Into A Sideways Force
The carrier’s mechanism: the ferry hangs from the cable at a trolley that rolls along it; a bridle connects the ferry to the trolley at two points, one forward and one aft; adjusting the bridle lengths angles the hull to the current by about 30-45°; the current pushes against the angled hull and drives the ferry sideways across the river; to return, reverse the bridle angle. The cable restrains the downstream component, so what the cable and anchors must carry is the hydrodynamic force on the hull.
The same force is a danger to the ferry. The water pushes on the hull below the waterline, the bridle holds it at a point above the water. Treat the two as a couple: the lower the hull’s centre of lateral force and the higher the bridle, the longer the arm. The couple tends to tip the upstream side down. If the upstream gunwale dips, water floods across the deck, and the ferry is lost.
What You Need
- A cable, trolley and anchors (see Cable Installation)
- Two bridles of adjustable length, tied low on the hull
- A quick release at the trolley that can be operated from the deck under load
- A platform with enough beam for the load (see Ferry Platform Design)
- A stability check by the inclining test in Loading for Stability
Method: Rig For The Worst Case
Step 1: Compute the broadside force. Angling reduces the load, so the broadside figure is the safe upper bound for sizing. Force = ½ρ v² × (length × draft) × drag coefficient, with the coefficient taken as 1 (an assumed order of magnitude; a real value depends on the hull).
Step 2: Compute the heeling arm = force × (bridle height above the waterline + half the draft) ÷ (weight of the ferry and load).
Step 3: Compare with the righting arm. The wall-sided righting arm is GZ = sin φ × (GM + ½ BM tan² φ), valid until the deck edge goes under.
Step 4: Lower the bridle until the heeling arm is a small fraction of the righting arm at all loads.
Step 5: Set a current limit and test with the quick release.
Worked Example: Narrow Craft And Wide Barge
| Narrow craft | Wide barge | |
|---|---|---|
| Length × beam, depth | 4.0 × 1.5 m, 0.5 m | 6.0 × 2.4 m, 0.6 m |
| Mass | 1,200 kg | 1,800 kg |
| Draft (mass ÷ 1,000 × L × B) | 0.200 m | 0.125 m |
| KG (assumed) | 0.9 m | 0.633 m |
| BM = B² ÷ (12 × draft) | 0.9375 m | 3.84 m |
| GM = draft ÷ 2 + BM − KG | 0.1375 m | 3.27 m |
| Broadside force at 2 m/s | 1,600 N | 1,500 N |
Narrow craft, bridle 1.5 m high: arm = 1,600 × (1.5 + 0.1) ÷ (1,200 × 9.81) = 0.217 m. Deck edge goes under when tan φ = 0.3 ÷ 0.75, φ = 21.8°, where GZ = sin 21.8° × (0.1375 + 0.5 × 0.9375 × tan² 21.8°) = 0.371 × 0.2125 = 0.079 m. The heeling arm is 0.217 ÷ 0.079 = 2.8 times the best righting arm, so there is no equilibrium before the deck edge goes under. With the bridle 0.3 m high the arm is 1,600 × 0.4 ÷ 11,772 = 0.054 m, and the heel balances at about 18°, close to the 21.8° limit. At 3 m/s the force is 3,600 N and the 0.3 m bridle arm is 0.122 m, which also exceeds 0.079 m: the craft capsizes.
Wide barge, bridle 1.5 m high: arm = 1,500 × (1.5 + 0.0625) ÷ (1,800 × 9.81) = 0.133 m, giving a heel of sin⁻¹(0.133 ÷ 3.27) = 2.3°. Beam, not rope, is what keeps a ferry upright; GM goes with the square of the beam. Ferry Platform Design covers how wide a log ferry can be.
Common Mistakes
- Tying the bridle at the top of a tall frame — the lever arm grows with height.
- Sizing the cable for the angled load — the bridle can fail and leave the hull broadside.
- A narrow, loaded craft — stability goes with the square of the beam.
- Never practising the release — it jams when needed.
Safety
A broadside ferry in a current is unrecoverable
If a swamped or heeled ferry swings broadside, the current holds it against the cable and the upstream side stays under. Fit a quick release you have tested under load, tie bridles low, and never carry people on a ferry that has not been tested at the current speed you will use it in. In cold water, see Safety & Capsizing Recovery.
What’s Next
- Learn next: Cable Installation — the cable and anchors that carry the load
- Combine with: Loading for Stability — the stability formulae used here