Boat draft
Short answer: Draft is the vertical distance from the waterline down to the bottom of the boat. It is not a fixed number: more weight aboard sinks the hull and the draft increases. Consumer guides usually quote it to the boat’s deepest point.
21 ft outboard runabout Drawn to scale, three loads Checked
The numbers at each load
| Measure | Light | Design | Heavy | With load |
|---|---|---|---|---|
| Draft, changes with load | 0.32 m 1 ft 1 in | 0.38 m 1 ft 3 in | 0.46 m 1 ft 6 in | Changes |
| Beam, fixed | 2.50 m 8 ft 2 in | 2.50 m 8 ft 2 in | 2.50 m 8 ft 2 in | Fixed |
| Freeboard, changes with load | 0.70 m 2 ft 3 in | 0.64 m 2 ft 1 in | 0.56 m 1 ft 10 in | Changes |
| Depth, fixed | 1.02 m 3 ft 4 in | 1.02 m 3 ft 4 in | 1.02 m 3 ft 4 in | Fixed |
| LOA, fixed | 6.40 m 21 ft | 6.40 m 21 ft | 6.40 m 21 ft | Fixed |
| LWL, changes with load | 5.49 m 18 ft | 5.60 m 18 ft 4 in | 5.73 m 18 ft 10 in | Changes |
Numbers are for this drawing's 21 ft outboard runabout. Read your own boat's figures off its spec sheet or capacity plate. Light: Tanks low, one person aboard: the drawing’s highest waterline. Design: The waterline the hull is drawn to, with a typical day’s load. Heavy: Full tanks, several people and gear: the drawing’s lowest-riding waterline.
What the number does not tell you
Terms often confused
DraftDepth
Draft is measured down from the waterline; depth is the height of the hull from keel to deck, wherever the water sits.
DraftDraught
Draught is the British spelling of the same measurement.
DraftAir draft
Draft is measured down from the waterline; air draft is the height from the waterline up to the boat’s highest point, the figure that matters under bridges.
Boat draft in more detail
Draft is the vertical distance from the keel up to the surface of the water. British usage spells it draught, and the ITTC dictionary uses that spelling for the same measurement. The verb is draw: a boat drawing 18 inches needs that depth of water to float in. The explorer above draws one runabout at three waterlines, light, design and heavy, and prints the draft for each.
One boat, three drafts
Watch the red line as you switch loads. Weight added aboard sinks the hull deeper and the draft grows with it; take the weight off and it shrinks back. Trim works on the ends of the boat: a hull sitting bow-up draws more water at the stern than at the bow, even with the same load aboard.
Where the published number comes from
A spec sheet gives one draft figure, and consumer boating guides usually take it to the deepest point of the boat rather than to the keel. Which point that is depends on the drive. An industry guide says it is usually the drive’s skeg by the propeller with the motor lowered, and the keel with it raised, so outboard and sterndrive boats often publish two drafts. The same guide says an inboard boat’s draft usually runs to its propeller or rudder, and a jet-drive boat’s only to its keel. A boating-course glossary puts draft in practical terms, as the depth of water a vessel needs to float.
Draft and depth
The two words sound alike and measure different things. Depth is the height of the hull from the keel to the deck, and loading the boat does not change it. Draft is the part of that depth sitting under water. The rest, above the waterline, is freeboard, so whatever increases the draft takes the same amount off the freeboard.
Sailboat draft
A sailboat’s draft runs to the lowest part of its keel; a boat with a centerboard has two drafts, board up and board down. The keel page draws five keel types over one waterline.
Hull shape and draft
Course guides list a shallow draft among the traits of flat-bottomed hulls, and the explorer’s runabout, a moderate V, draws more than the flat-bottom hull in the hull atlas.
Common questions
What does draft mean on a boat?
Draft is how far the boat reaches below the water surface. A boating-course glossary puts it as the depth of water a vessel needs to float.
How is the draft of a boat measured?
Draft is measured vertically, from the waterline to the lowest point of the boat. Consumer guides usually take it to the deepest point, so outboard and sterndrive boats often publish two drafts, drive down and drive up.
Does draft change when you load a boat?
Draft changes with load. Added weight sinks the hull deeper, so the draft grows, and the freeboard shrinks by the same amount.
What is a shallow-draft boat?
A shallow-draft boat is one whose hull reaches only a little way below the water. Course guides list a shallow draft among the traits of flat-bottomed hulls.
What is the draft of a sailboat?
The draft of a sailboat is the depth to the lowest part of its keel. A sailboat with a centerboard has two drafts, board up and board down.