A ferry platform must be flat-bottomed, wide and stable, with a deck large enough for its load (about 2 x 4 m for one ox cart), hinged ramps at both ends, side rails, and at least 30 cm of freeboard when loaded. Two log pontoons 0.6 m across and 6 m long in dry softwood carry only about 128 kg of load at that freeboard and green logs carry none, whereas a flat-bottomed barge 6 m by 2.4 m carries 3,720 kg, so the pontoons the carrier lists suit a footbridge-weight load, not a cart.
Part of River Navigation & Ferry Systems
A ferry platform is the floating deck that carries people, animals and vehicles across a river, supported by pontoons or a barge hull.
Freeboard Sets The Payload, And The Hull Sets The Freeboard
The carrier’s checklist is: a hull of two pontoon logs or sealed drums under a flat deck, or a proper flat-bottomed barge; a deck of a size for the load (a single ox cart needs about 2 × 4 m); hinged ramps at each end for animals and vehicles; side rails so animals cannot walk off; and at least 30 cm of freeboard above the waterline when loaded, since with less, waves come over the deck. The freeboard is not a free choice. It is whatever the buoyancy leaves after the structure and load are counted, and that can be computed before anything is built.
For a log pontoon the buoyancy per metre of length depends on how deep the log sits. The area of a circle segment of radius r submerged to depth h is r² arccos((r − h) ÷ r) − (r − h) √(2rh − h²). The freeboard of the deck is then the log’s diameter minus h, plus the deck thickness.
What You Need
- A hull: logs, sealed drums, or a barge of planks (see Barrel & Drum Raft, Log Raft Design)
- Deck planks and cross-beams, and ramps on hinges
- Rails strong enough for a frightened animal
- Wood densities for your own logs: float-test a short length (see below)
- A load line, a mark painted on the hull at the maximum safe draft
Method: Size The Hull For The Load
Step 1: List the load. Cart, cargo and animal; the example assumes 1,400 kg (an assumed total), plus 400 kg for the deck and frame (assumed).
Step 2: Choose the freeboard: 30 cm minimum from the carrier.
Step 3: Compute the displacement at that freeboard for each hull option.
Step 4: Subtract the hull’s own mass and the deck, to get the payload.
Step 5: Compare the payload with the load, and enlarge or change the hull until it exceeds it with margin.
Step 6: Mark the load line on the hull at the draft you computed.
Worked Example: Logs Against A Barge
Seasoned softwood is 400-500 kg/m³ and green wood 800-1,000 kg/m³ per Logistics & Load Planning. Take logs of 0.6 m diameter and 6 m long (r = 0.3 m), a deck 5 cm thick on top, and freeboard 0.30 m. The log must then sit with 0.25 m of its diameter above water: h = 0.35 m.
One log’s submerged area at h = 0.35 m (r − h = −0.05 m) is 0.3² × arccos(−0.05 ÷ 0.3) − (−0.05) × √(2 × 0.3 × 0.35 − 0.35²) = 0.09 × 1.738 + 0.05 × 0.2958 = 0.1712 m². Displacement per log = 0.1712 × 6.0 × 1,000 = 1,027 kg, so two logs displace 2,055 kg. Own mass at 450 kg/m³ is π × 0.3² × 6.0 × 450 = 763 kg per log, 1,527 kg for two.
| Hull | Wood | Displacement | Own mass + deck 400 kg | Payload at 30 cm freeboard |
|---|---|---|---|---|
| 2 logs, 0.6 × 6 m | 450 kg/m³ | 2,055 kg | 1,527 + 400 | 128 kg |
| 2 logs | 800 kg/m³ | 2,055 kg | 2,714 + 400 | none: the logs alone outweigh the displacement |
| 4 logs | 450 kg/m³ | 4,110 kg | 3,054 + 400 | 656 kg |
| Barge 6.0 × 2.4 m, 600 kg | planks | 4,320 kg | 600 | 3,720 kg |
Barge: draft = 0.60 m depth − 0.30 m freeboard = 0.30 m, displacement 1,000 × 6.0 × 2.4 × 0.30 = 4,320 kg, minus its own 600 kg (an assumed hull mass) gives 3,720 kg. Against the assumed 1,400 + 400 = 1,800 kg load for a cart and deck, only the barge qualifies. At 3,720 kg of cargo it draws 0.30 m; each 1,000 kg of cargo adds 1,000 ÷ (1,000 × 6.0 × 2.4) = 0.069 m of draft, so a load line is practical: it falls 6.9 cm for each 1,000 kg.
Common Mistakes
- Using green logs — they float poorly, and some barely at all; float-test a length.
- Freeboard measured to the top of the log — measure to the deck.
- A deck 2 m wide on a 1.5 m hull — the narrow beam gives a low GM (see Loading for Stability).
- Ramps that are not secured when raised — they drop on a person.
Safety
Freeboard disappears quickly
A ferry loaded to a 5 cm freeboard ships water over the deck in the wake of a floating log, and the animals panic. Mark the load line, refuse overloads, and never carry a load that exceeds your computed payload. Fuel drums or chemical containers used as floats must never be cut or heated; see Barrel & Drum Raft.
What’s Next
- Learn next: Cable Installation — the cable the platform hangs from
- Combine with: Reaction Ferry — the stability check with the current