Semidisplacement

Semidisplacement is a hull form designed to operate between traditional displacement hulls and planing hulls. In its low-speed regime it behaves like a displacement hull, providing stability and interior space. As speed increases, the hull generates increasing hydrodynamic lift, allowing the vessel to rise partially out of the water and reduce drag.

The design aims to balance buoyancy, weight, and waterplane area. It often features a fuller forebody with a sharper aft section and a moderate waterline length to optimize resistance around the target cruising speed. Weight distribution and hull shaping determine how readily the vessel transitions to higher speed operation.

Performance characteristics include good seakeeping and payload at low speeds, with maintained efficiency as speed rises to a semi-planing range. From there, further speed increases may require more power, and the hull behaves more like a planing craft. The overall efficiency is typically better than a pure displacement hull at moderate cruising speeds but not as high as a dedicated planing hull at top speed.

Applications include mid-sized motor yachts, fast ferries, and expedition or offshore vessels that need range, stability, and fuel economy without the extreme planing behavior of high-speed craft. Semidisplacement hulls are commonly chosen when a balance between load-carrying capacity, comfort, and speed is desired.

The term indicates a transitional design philosophy in naval architecture and has been used since the mid-20th century to describe hull forms that lie between true displacement and planing configurations.