A raft is the simplest way to move heavy cargo on water: logs or sealed drums lashed to a frame, with a raised platform and a long sweep oar or pole for steering. What it can carry depends on the buoyancy of its floats: a cubic metre of seasoned softwood at 400-500 kg/m³ lifts 500 to 600 kg when fully submerged, while a cubic metre of green wood at 800-1,000 kg/m³ lifts only 0 to 200 kg, which is why dry wood or sealed drums are the floats to choose.
Part of Coracle & Small Watercraft
A raft is a floating platform made by joining buoyant logs or containers, the quickest way to carry loads across a river or along it.
Buoyancy Is The Water Displaced Minus The Float’s Own Weight
A float supports a load equal to the weight of the water it displaces, minus its own weight. For a log fully under water, that is (1,000 − ρ) kg per cubic metre of log, where ρ is the wood’s density in kg/m³. Using the density table in Logistics & Load Planning:
| Wood | Density (kg/m³) | Lift per m³ when fully submerged |
|---|---|---|
| Seasoned softwood | 400-500 | 500-600 kg |
| Seasoned hardwood | 600-800 | 200-400 kg |
| Green (wet) wood | 800-1,000 | 0-200 kg |
Fully submerged is not usable: the raft would then have no freeboard. In practice a raft rides with about half to three quarters of each log under water, which cuts the usable lift well below those figures. Log Raft Design works through the numbers, and Barrel & Drum Raft covers sealed containers.
What You Need
- Floats: dry logs of similar diameter (20-30 cm and 3-4 m long for the carrier’s design), or sealed drums
- Rope and cross-poles for the frame and lashing (see Lashing Techniques)
- Planks for a raised platform
- A sweep oar or poles
- A flotation device for everyone aboard
Method: Build And Test
Step 1: Float-test a sample log: a short length of each log, to see how it rides.
Step 2: Lay the floats parallel with the largest on the outside, and lash them to cross-poles.
Step 3: Add the platform above the floats.
Step 4: Launch empty, then add load gradually and mark the waterline.
Step 5: Plan the crossing so the current does not carry you into hazards, or tie the raft to a rope or cable (see Cable Ferry Systems).
Worked Example: The Lift Of One Log
A log 25 cm across and 4 m long has a volume of π × 0.125² × 4 = 0.196 m³. At 450 kg/m³ it weighs 0.196 × 450 = 88 kg and, fully submerged, supports (1,000 − 450) × 0.196 = 108 kg of load. At 800 kg/m³ the same log weighs 157 kg and supports 0.196 × 200 = 39 kg. Eight logs therefore carry at most 8 × 108 = 864 kg of load when dry and 8 × 39 = 314 kg when green, and that is with the logs fully submerged and the deck awash.
Common Mistakes
- Using green logs — some barely float; test a sample.
- Counting the full displacement as cargo — the raft’s own weight comes first.
- Lashing with a single cross-pole — the logs spread and the raft folds.
- Treating a raft as a controllable boat in current — it drifts; a rope or cable is the only reliable way to steer a crossing.
Safety
A raft in a river drifts into whatever lies downstream
A raft cannot hold a line in a current, can be pinned on a strainer or rock, and its logs can roll apart in rough water. Stay out of rapids, weirs and fast water, wear flotation, and if it breaks up hold a log; see Safety & Capsizing Recovery.
What’s Next
- Learn next: Log Raft Design — capacity and lashing in detail
- Combine with: Barrel & Drum Raft — sealed containers as floats