Draft from Displacement

This relation estimates a ship’s draft by dividing its displacement by the mass of water displaced per metre of immersion, which is the product of water density and waterplane area. It is a useful first-order hydrostatics approximation: more mass must be supported by buoyancy, so the hull sinks deeper until the displaced water weighs the same as the vessel. It works best when the waterplane area does not change much with small changes in draft, so it is mainly a linear approximation near a given loading condition.

Use this when you need a quick estimate of how deep a vessel will sit after loading, ballasting, or changing water density. It is also useful in preliminary ship design and for checking whether a vessel can safely enter a channel, lock, or berth.

• Using displacement in tonnes without converting consistently with density units
• Confusing waterplane area with wetted surface area or cross-sectional area
• Applying the formula in conditions where hull shape changes the waterplane area significantly with draft