Analyze hull fullness and performance characteristics
Fine-lined racing hull
Balanced cruising hull
Full-bodied motor yacht
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Download NowThe block coefficient is the simplest single measure of how full a hull is: it compares the real underwater volume to the volume of a rectangular box with the same length, beam and draft. A number close to 1 is a barge; a number near 0.4 is a fine-lined yacht.
Fullness drives almost everything downstream. A blunt, high-Cb hull carries more weight on a given draft and has roomier interiors, but it makes more wave drag. A fine, low-Cb hull slips through the water but demands depth to float the same load.
Enter length, beam, draft and displacement and choose freshwater or seawater, and the calculator back-solves the underwater volume from the weight of water displaced and returns the block coefficient with a plain-language read on the hull form.
Cb = ∇ ÷ (L × B × T), where ∇ = Displacement ÷ water density| Vessel Type | Cb Range | Character |
|---|---|---|
| Racing / lifeboat | 0.30 - 0.45 | Fine, low drag |
| Sailing cruiser | 0.45 - 0.55 | Balanced |
| Motor yacht | 0.55 - 0.65 | Fuller, roomy |
| Trawler / barge | 0.65 - 0.85 | Very full, capacious |
Typical block-coefficient ranges by vessel type.
Formula verified against standard naval-architecture references · Updated October 2026
The block coefficient (Cb) is the ratio of a hull's underwater volume to the volume of a rectangular block having the same length, beam and draft. It is the most basic expression of how full or fine a hull is. A Cb of 1 would be a solid box; a typical sailing yacht sits around 0.45 to 0.55, and a blunt barge or trawler can exceed 0.7. Lowering Cb means a slimmer, lower-drag but less capacious form.
A higher Cb means a fuller hull that generates more wave-making and form drag at a given speed, so it needs more power or sail to reach a target pace. A lower Cb means finer lines and less drag, ideal for speed, but it forces the boat deeper to float the same weight. Designers pick the Cb that suits the intended speed range, since a hull optimised for one band performs poorly in another.
Neither is universally better; it depends on the job. Workboats, trawlers and cruising yachts favour a higher Cb because it gives interior volume, stability and load-carrying on a modest draft. Racing and high-speed hulls favour a lower Cb because reduced drag translates directly into speed for the same effort. Choose fullness for utility and livability, and fineness for performance.
Displacement determines the underwater volume the hull must produce, and Cb compares that volume to the enclosing box. For fixed length, beam and draft, a heavier boat has a higher Cb because more of the box is filled. Conversely, to hold a low Cb while carrying more weight, you must increase at least one of the dimensions, usually by deepening the draft or widening the beam.
Early in a project, designers choose a target Cb to match the vessel type and speed, then use it to estimate the displacement a given set of dimensions will float, or the dimensions needed to float a required load. It appears in many empirical drag and power formulas, so an accurate Cb makes engine sizing, speed prediction and stability checks far more reliable.
Density decides how much volume the boat must displace to float. In denser seawater the same weight needs less volume, so the computed underwater volume, and hence the Cb, is slightly lower than in freshwater. Using the wrong density shifts the result by a couple of percent, so match the assumption to where the boat will normally float.