Flat-bottomed plank boat
FAO's 4.9 m flat-bottomed boat: straight planks, three frames, a bottom planked across and nailed to a thick lowest plank. The simplest planked boat there is, for four people on sheltered water.
Build
Tier 2 · Bronze and Iron Age
Transport
Moderate
Days to weeks with the planks sawn; FAO trains carpenters to build it in a few weeks
Crew 2
Home: On water, Level 2 · Forge, level 2 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
FAO publishes this design, AFR-1, for fishing and transport on lakes, rivers and sheltered coasts, because it needs no building jig, no steamed frames and no trained boatbuilder: the sides are straight planks, the bottom is planked across, and the boat's curve comes from the flare of its sides (FAO Fisheries Technical Paper 117). FAO states its weaknesses too: it slams in waves and drifts sideways without a keel, so it belongs on sheltered water.
Flat-bottomed plank boat, profile and plan
Fig 1
Timber
Flat-bottomed boat, midship section
Fig 2
TimberWaterVoid / air
Keyed parts
| 1 | Lower side plank | 30 × 145 mm: thick enough to take the bottom's nails. |
| 2 | Upper side plank | 20 × 190 mm. |
| 3 | Sheer plank | 20 × 95 mm, along the top. |
| 4 | Frames | Three, of 30 × 70 mm: they set the flare and hold the sides out while building. |
| 5 | Side stiffeners | 30 × 70 mm uprights between the frames, holding the side planks together. |
| 6 | Bottom planks | 20 × 190 mm, laid across the boat and nailed up into the lowest side planks. |
| 7 | Bottom stiffener | 30 × 70 mm along the centreline inside, holding the bottom planks together. |
| 8 | Keel strip and skeg | Outside, along the centreline; the skeg aft keeps the boat running straight. |
| 9 | Stem and transom | A 45 × 95 mm stem at the bow; the transom from planks, at the stern. |
| 10 | Thwarts | Seats across the boat, which also brace the sides. |
Specifications
| Size | 4.90 × 1.22 × 0.40 m | FAO design AFR-1. |
| Weight | about 176 kg | In timber of about 650 kg/m³. |
| Capacity | 4 people | 16.1 ft × 4.0 ft ÷ 15; at full load at least 24 cm of side (0.2 × beam). |
| Planks | lower side 30 × 145, upper side 20 × 190, sheer 20 × 95, bottom 20 × 190 mm | The lowest side plank is thicker, to take the bottom's nails. |
| Framing | 30 × 70 mm | Three frames, side stiffeners, a bottom stiffener, keel strip and skeg. |
| Fastenings | 60 × 3 mm nails; 50 × 5 mm screws in the frames | Hot-dip galvanised in FAO's design; tarred wrought iron at t2. |
| Water | category D | Sheltered water: it slams and drifts in waves. |
Bill of materials
| Material | Quantity | Notes and substitutes |
|---|---|---|
| Side planks | 2 × 30 × 145 mm × 5.0 m; 2 × 20 × 190 mm × 4.9 m; 2 × 20 × 95 mm × 4.75 m | In one length if you can; otherwise joined on butt blocks inside. |
| Bottom planks | about 23 m of 20 × 190 mm | Cut across the boat one at a time, to fit. |
| Framing | about 23 m of 30 × 70 mm | Frames, side stiffeners, the bottom stiffener (4.2 m), keel strip (4.5 m) and skeg (1.5 m). |
| Stem | 0.75 m of 45 × 95 mm | Bevelled to take the side planks. |
| Transom | about 4 m of plank | Cut to the transom template. |
| Thwarts and knees | about 5 m | The thwarts brace the sides as well as seat the crew. |
| Nails | 60 × 3 mm, a few kilograms | Hot-dip galvanised, or wrought iron tarred. |
| Screws or bolts | 50 × 5 mm | For assembling the frames. |
| Caulking cotton, and pitch or bitumen compound | — | For every joint (FAO). |
| Preservative, and bitumen paint | — | Preservative on all surfaces; bitumen paint on the bottom outside. |
| Oars and thole pins or oarlocks | 2 pairs | About 2.2 m, by the 7/25 rule. |
Tools: Saw · Plane · Hammer, and a dolly for clenching · Drill or bradawl for pre-boring · Caulking iron · Bevel gauge · Clamps · Thin board for templates
Construction
Select the timber and fastenings
Medium heavy, medium hard timber that holds nails and does not split easily (FAO), of a species that resists rot; pressure-treat it if you can, otherwise soak it in preservative or tar. Fastenings as in the specification: 60 × 3 mm nails, 50 × 5 mm screws for the frames.
Make templates for the frames and transom
Draw the frames and transom full size on a floor or on thin board, from FAO's drawing, and cut templates. The frames set the flare of the sides, and with it the whole shape of the boat. Mark the bevel on every edge a plank will lie against.
Build the frames, transom and stem
Make the three frames, the transom and the stem complete with bevels, from the templates. Screw the frame joints.
Join the lowest side planks to length
If the 30 × 145 mm planks are not long enough, join them on butt blocks inside. Mark the position of every frame and side stiffener on them.
Assemble on a flat floor
Set the two lowest side planks, the frames, the stem and the transom together on a flat floor. When this is done, the shape of the boat is given (FAO): the rocker of the bottom comes from the flare and the curve of these planks, and no jig is needed.
Side stiffeners and the rest of the sides
Fasten the side stiffeners to the lowest planks, then fit the upper (20 × 190 mm) and sheer (20 × 95 mm) planks. Nail through each plank into every frame and stiffener; pre-bore where it splits.
Turn it over and plank the bottom
Turn the boat upside down and check the bevel on the lowest side planks so the bottom planks bed flat on them. Plank the bottom across the boat, starting at the stem and working aft. Lay a strand of cotton and bitumen compound in the joint between side and bottom. Nail each bottom plank up into the side planks with 60 × 3 mm nails, pre-bored. Softwood bottoms: leave a gap the thickness of a hacksaw blade between planks, never fitted tight.
Bottom stiffener, keel strip, skeg and thwarts
Fasten the bottom stiffener along the centreline inside across all the bottom planks, and the keel strip outside with the skeg aft. Fit the thwarts and their knees.
Caulk, preserve and paint
Drive caulking cotton into any open seam with a caulking iron and fill it with compound. Treat every surface with preservative; paint the bottom outside with bitumen paint, which FAO calls the best and cheapest protection.
Float, test and mark
Keep the boat wet for a day so the planks swell. Then: the swamp test (full of water, it floats level); the offset-load test (the crew on one side, the side stays clear); and four aboard with at least 24 cm of side. Cut the capacity and category D into the transom.
Tolerances that matter
| Dimension | Target | Why it matters |
|---|---|---|
| Bevel on the lowest side planks | the bottom planks bed flat all along | A gap at the chine is the first leak every flat-bottomed boat gets. |
| Gap between softwood bottom planks | a hacksaw blade | Fitted tight, they swell and buckle (FAO). |
| Nails | at least 1/7 of the plank across, 2.5 × it long | FAO's minimum: 3 × 50 mm for 20 mm planks. |
| Side above water, four aboard | at least 24 cm | FAO's 0.2 × beam. |
How this fails
Softwood bottom planks fitted tight: they swell and buckle open along the chine.
Nails clenched along the grain split the plank; clench across it.
No preservative: rot in fresh water ends these boats (FAO).
Taken into waves: it slams and drifts sideways. Sheltered water only.
Overloaded: past four people, the side drops under 24 cm.
How you know it worked
VerificationAfter a day afloat the seams have stopped weeping; the boat floats level swamped; the offset-load test leaves the side clear; with four aboard it keeps at least 24 cm of side; and capacity and category are cut into the transom.