Boat Draft Calculator

What is draft?

Draft (or draught) is the vertical distance from the waterline to the lowest point of the hull, keel, rudder or drive leg. It tells you the minimum water depth your boat needs to float without touching bottom. Running aground is one of the most common and most expensive boating accidents, and knowing your draft is what prevents it.

Read this before trusting the number on a keelboat. Archimedes tells you how deep the hull sits, and that is what the formula below computes. It says nothing about a fin keel, a full keel, a rudder or an outdrive hanging below it, because those add almost no buoyancy while adding a great deal of depth. A 35-foot cruiser with a canoe body sitting 30 inches down can easily draw 5 feet 6 inches at the bottom of the keel. Use the keel depth field to add it, or the answer will be dangerously shallow.

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:

The right-hand column is the whole boat, keel included. The formula on its own returns only the first part of that, the hull sitting in the water, so on the two sailboat rows expect the calculated figure to come out well under the published range until you add the keel.

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 about 30 cm.

Notice how close that is to the 30 cm safety margin below. That is not a coincidence, it is the point: on a planing boat in shallow water, speed can consume the entire margin on its own. The calculator turns this around and gives you the speed at which squat eats the margin, so instead of a number to worry about you get a limit to steer by. Below it the margin is real, above it you are relying on water that is no longer there.

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: Add at least 30 cm (12 in) to your calculated draft. Waves, tidal range and an uneven bottom all eat into clearance, and none of them are in the arithmetic. In water you do not know, make it 60 cm (24 in).

Freshwater vs. saltwater: Freshwater density is 1000 kg/m³ against saltwater at 1025 kg/m³, so a boat sits about 2.5% deeper in fresh water. On a small boat that is 1 to 3 cm, on a heavy cruiser rather more.