Boat Design

Displacement

Definitions

Displacement is the truest measure of just how much boat you're dealing with. In a nutshell, it's the actual volume or weight of the vessel.

Of course, there are more than a few ways to state displacement, but only two- the volume displacement and the displacement mass, both at the design waterline- are fair points of comparison between different boats. The rest, although important and helpful when used properly, can be deceptive when seen alone. Always ask which weight is really being discussed, and what's included in it!

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Draught

Definitions

Draught, normal (T)

The depth of water needed to float the boat, when it's loaded to its design displacement and sitting on its design waterline.

Draught, minimum (Tmin)

In a boat with retractable appendages (rudder, daggerboards, outboard or sterndrive engine), the depth needed to float the boat when all these appendages are fully retracted. Again, this measurement is based on the design waterline.

Units: Linear measure (usually metres/centimetres or feet/inches).

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Beam

Definitions

Beam refers to the width of a boat. As with length, there are a few ways to define beam, depending on what you need the number for.

Units: Linear measure (usually metres/centimetres or feet/inches).

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Length

Definitions

There appear to be no less than six commonly used ways to measure the length of a boat. Most of the discrepancies have to do with whether stern platforms and bow pulpits are "integral parts of the hull" or are bolted on separately.

Units: Linear measure (usually metres/centimetres or feet/inches).

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