Estimate your boat's draft based on displacement and hull dimensions.
Deep fin keel for upwind performance
Shallow draft for beaching
Full keel for stability
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Download NowDraft is how deep your boat sits in the water, and it is the number that decides whether you clear a shoal, a marina or the trailer bunks. It falls straight out of displacement: the heavier the boat, the more water it must push aside, and the deeper it settles for a given hull shape.
A fat, boxy hull reaches a given displacement with shallow draft, while a fine, narrow hull needs more depth to displace the same weight. That is why a keel gives a sailing yacht deep draft: it trades depth for the low ballast placement that keeps it stiff.
Enter the hull length, beam, displacement and a block coefficient, and the calculator returns the loaded draft, a few useful ratios, the wetted-surface estimate and the volume of water displaced.
Volume ∇ = Displacement ÷ 64; Draft = ∇ ÷ (L × B × Cb)| Block Coeff (Cb) | Draft (ft) | Hull Type |
|---|---|---|
| 0.70 | 0.84 | Very full pontoon / scow |
| 0.55 | 1.07 | Full workboat |
| 0.45 | 1.44 | Typical cruiser |
| 0.30 | 2.31 | Fine fin-keel |
Draft for 8,000 lb displacement on a 24 ft × 8 ft hull by block coefficient.
Formula verified against standard naval-architecture references · Updated October 2026
Draft comes from the water a boat must displace to float. Divide the displacement in pounds by 64 to get the underwater volume in cubic feet, then spread that volume across the hull's length, beam and fullness to find how deep it sits. The fullness is captured by the block coefficient, so a finer hull digs deeper for the same weight, while a fuller hull stays shallower.
Yes, by a simple proportion: each extra 64 lb sinks the hull by roughly one more cubic foot of displaced water, spread over the waterplane area. The heavier the boat, the deeper it floats. Adding gear, tanks or a bigger engine therefore eats freeboard and increases draft, which is why lightweight construction pays off throughout a design.
Moulded draft is the depth of the hull itself. Total draft adds everything below the hull, principally the keel, rudder and any centreboard or dockbox. For a fin-keel sailboat the appendage can dwarf the hull draft, so always quote total draft when checking bridges, shoals and launch ramps.
You can shed displacement, which is the cleanest route, or accept a fuller hull form for a given weight. Appendage options include a shoal keel with a bulb, a lifting centreboard or a swing keel. Beware that shallowing a keel without a low ballast focus reduces righting moment, so draft reduction and stability must be balanced together.
The block coefficient describes how much of the surrounding box the hull actually fills. A high Cb means a full, blunt shape that displaces a lot of water in little depth, giving shallow draft; a low Cb means fine lines that need more depth for the same volume. Choosing Cb is therefore choosing the trade between draft, interior volume and drag.
Indirectly. Deeper hulls with appendages usually carry more wetted surface, which raises frictional drag at low speeds and hurts light-air sailing and fuel economy. Deeper keels do, however, improve windward grip, so a boat may point higher and make good its course with less leeway. The speed effect depends on whether you are measuring raw drag or speed over the ground upwind.
Read the deepest point honestly rather than the number on the spec sheet, which usually quotes a clean hull at light displacement. Add anything fixed below the keel line, the shaft log, the sonde, a bolt or a fouled patch, and then subtract for the tide and the water density you are actually in. A boat that draws four feet in the marina may carry four and a half once you allow for weed on the bottom and a full fuel tank, and fresh water floats her higher than the salt bar you are about to cross. When in doubt, sound the shallowest spot at the top of the tide and note your speed, because a fast boat over a bar is a boat with no margin left to stop.