Boat Draft Calculator
Calculate your boat's draft based on hull type, displacement, and load.
Know your minimum water depth for safe navigation.
What is draft?
Draft (or draught) is the vertical distance from the waterline to the lowest point of the hull (or keel). It tells you the minimum water depth your boat needs to float without touching bottom. Running aground is one of the most common and costly boating accidents — knowing your draft prevents it.
Types of draft:
- Static draft: Draft with no movement, normal load
- Loaded draft: Draft with full fuel, water, passengers, and gear
- Dynamic draft (squat): Additional sinking at speed due to hydrodynamic effects
Draft estimation by hull type:
Archimedes’ principle: a floating vessel displaces water equal to its own weight.
Displacement (kg) = Waterplane area (m²) × Draft (m) × Water density (kg/m³) × Block coefficient
Solving for draft: Draft = Displacement / (Waterplane area × Water density × Block coefficient)
Block coefficients by hull type:
| Hull Type | Block Coefficient | Typical Draft Range |
|---|---|---|
| Flat bottom (jon boat) | 0.80–0.90 | 10–30 cm (4–12 in) |
| V-hull powerboat | 0.45–0.55 | 40–90 cm (16–36 in) |
| Deep-V (offshore) | 0.35–0.45 | 60–120 cm (24–48 in) |
| Sailboat fin keel | 0.30–0.40 | 120–250 cm (48–100 in) |
| Sailboat full keel | 0.35–0.45 | 100–200 cm (40–80 in) |
| Catamaran | 0.40–0.50 | 30–90 cm (12–36 in) |
| Pontoon boat | 0.85–0.95 | 25–60 cm (10–24 in) |
Squat effect at speed: When a boat moves through shallow water, it sinks lower. The squat formula:
Squat (m) ≈ Speed² (knots) × Block coefficient / (100 × √(Water depth))
At 10 knots in 3 m of water with Cb = 0.5: Squat ≈ 100 × 0.5 / (100 × 1.73) = 0.29 m. Your effective draft increases by ~30 cm.
Worked example: A 6 m V-hull powerboat weighing 1,200 kg loaded, beam 2.2 m:
- Waterplane area ≈ 6 × 2.2 × 0.7 = 9.24 m² (70% of L×B is a common estimate)
- Block coefficient: 0.50 (V-hull)
- Water density: 1025 kg/m³ (saltwater)
- Draft = 1200 / (9.24 × 1025 × 0.50) = 0.253 m ≈ 25 cm (10 in)
Safety margin: Always add at least 30 cm (12 in) to your calculated draft for safety — waves, tidal changes, and bottom irregularities can all cause grounding. In unfamiliar waters, add 60 cm (24 in) or more.
Freshwater vs. saltwater: Freshwater density is 1000 kg/m³ vs. saltwater at 1025 kg/m³. Your boat sits about 2.5% deeper in freshwater — usually 1–3 cm for small boats, more for larger vessels.