Log raft
Eight dry logs, six cross-poles and cord: a raft that carries 400 kg across or down calm water, rated by sum and proved by a load test.
Home: On water, Level 1 · Foothold, level 1 of 6 in its track. Serves 1 step.
Every build on this site has hazards that injure or kill — quicklime, carbon monoxide, molten metal, collapsing ground. Read the safety notice before starting, and read this build's own failure modes below.
What it is and why this shape
A raft is the most stable and the most forgiving boat a group with an axe can build, and it carries more than anything else of its size. It is also slow and steers badly, so it belongs on calm water: crossing a lake, ferrying loads over a river, or drifting down one with a steering sweep. Almost everything that decides whether it works is decided before the first lashing: how dry the logs are.
Log raft, in plan
Fig 1
Timber
Log raft, section at a cross-pole
Fig 2
TimberWater
Keyed parts
| 1 | Logs | 8 dry logs, 25 cm × 4 m, the front ends trimmed to a slant underneath. |
| 2 | Top cross-poles | 3, 10 cm × 2.4 m, seated in notches 2–3 cm deep, 0.3 m from each end and in the middle. |
| 3 | Bottom cross-poles | 3, under the logs at the same stations, so each station clamps the logs between two poles. |
| 4 | Clamp lashings | One between every pair of logs at every station, binding the top pole to the bottom one: 21 in all. |
| 5 | Deck | Poles or split planks over the middle 2.5 m, lashed to the top cross-poles. |
| 6 | Steering sweep | 4–5 m, pivoting in a forked stick lashed upright at the stern. |
Specifications
| Size | 4.0 × 2.0 m | 8 logs of 25 cm, side by side. |
| Logs | 8 × 25 cm × 4 m, dry | 1.57 m³; at about 450 kg/m³, about 710 kg. |
| Lift, just awash | about 830 kg | 1,570 kg of displacement, plus the lower poles, less the logs, poles and deck. |
| Rated load | 400 kg | About half the lift: four adults with light gear, or two and 240 kg of goods. |
| At 400 kg | log tops about 8 cm clear | The deck about 20 cm above the water. |
| Water | category D | Sheltered water, wind up to force 4, waves under 0.3 m. |
Bill of materials
| Material | Quantity | Notes and substitutes |
|---|---|---|
| Logs, dry softwood | 8 × 4.0 m, 22–28 cm | Straight, sound, no rot; each passes the float test in stage 1. |
| Cross-poles | 6 × 2.4 m, 10 cm | Three for the top, three for the bottom. |
| Cord | about 150 m, 8–10 mm | Natural fibre, bark or rawhide. It stretches on first wetting: re-tighten after an hour afloat. |
| Deck poles or split planks | about 20 × 2.2 m | Over the middle 2.5 m. |
| Steering sweep | 1 × 4–5 m | A long pole with a flat blade at one end. |
| Poles | 2 × 3–4 m | For shallow water and for fending off. |
| A float for everyone, and a throw line | — | See boats and safe water travel. |
Tools: Axe · Saw, if there is one · Knife · 3 short logs as rollers · A measured cord
Construction
Choose dry logs, and prove it
Use dead standing trees, or logs seasoned under cover for months: they have already lost most of their water. Float test: cut a 50 cm length from each log and float it. Accept logs that float with at least half their thickness above the water: about 500 kg/m³ or lighter. A freshly felled log floats with only about a quarter out and makes a raft that sits awash.
Cut to length and shape the bows
Cut all eight to 4.0 m. At the front, trim the underside of each log to a slant over 30–40 cm, so the raft rides up over floating wood and small waves instead of ploughing into them. Lay the logs side by side on three rollers at the water's edge, the thickest in the middle, butts and tops alternating so the raft comes out square.
Notch the stations
Mark three stations, 0.3 m from each end and in the middle. At each, cut a notch 2–3 cm deep and 10 cm wide across the top of every log, so the top cross-pole seats in all eight and cannot slide.
Clamp the logs between pairs of poles
At each station, lay a top pole in the notches and pass a bottom pole under the logs directly beneath it: work in knee-deep water, or with the raft up on its rollers. Between every pair of logs, bind the top pole to the bottom pole with 6–8 turns of cord pulled hard, then take 3–4 frapping turns round those turns between the logs to tighten them. That is 21 clamp lashings: every log is held top and bottom in three places.
Deck, sweep and poles
Lay the deck poles across the top cross-poles over the middle 2.5 m, and lash each end. Lash a forked stick upright at the middle of the stern for the steering sweep. Tie the sweep and both poles on with lanyards, so they are not lost overboard.
Re-tighten after an hour afloat
Float the raft empty for an hour. Natural cord stretches when it first wets: re-tighten every lashing before loading, and check them again every morning of a trip.
Load test and capacity mark
In calm water no deeper than your waist, load 400 kg: four adults, or sacks and water drums weighed on a scale. The raft must float level with the log tops about 8 cm clear. If they are not, the logs are wetter than you thought: lower the rated load until they are. Cut the rated load into the stern log.
Tolerances that matter
| Dimension | Target | Why it matters |
|---|---|---|
| Log diameter | 22–28 cm, thickest in the middle | Uneven logs make a raft that lists and a deck that rocks. |
| Log tops at the rated load | at least 8 cm clear | Less, and every wave washes over the load. |
| Lashings | tight after the first hour afloat | A loose station lets the logs spread apart. |
How this fails
Green logs. The same raft of freshly felled wood has about 250 kg of lift left fully awash: it cannot carry four people. Float-test every log.
Lashings left loose after the first soaking. Re-tighten before loading.
Loading one corner. The raft sinks at the loaded corner; keep the load in the middle.
Weirs, fallen trees and floods. A raft steers badly and cannot stop: scout the river first, and carry round anything you cannot see past.
Travelling after dark, or in wind above force 4. Tie up first.
How you know it worked
VerificationEvery log passed the float test; the raft has carried 400 kg level with the log tops at least 8 cm clear; every lashing was re-tightened after an hour afloat; and everyone aboard wears a float.
Scaling and variants
More load: add logs, not load. Twelve of the same logs lift about half as much again. A second layer of logs laid crosswise makes a stiffer, higher raft for rougher river water. Bamboo: the same method, with culms closed at every node, which float higher than any log; lash two layers crosswise.