Semi-displacement yachts are motor yachts with hull forms designed to operate efficiently at both displacement speeds and in a transitional planing mode, typically cruising at 12–18 knots and capable of reaching 22–28 knots when conditions require. They occupy the middle ground between full-displacement yachts, which are optimised for range and economy at low speed, and planing yachts, which sacrifice range for performance.
The hull form combines a rounded or moderate-V forward section with a flatter, wider stern that generates lift at speed without fully breaking free of the water. At displacement speeds, fuel economy is comparable to full-displacement designs and range on larger examples can exceed 1,500 nautical miles. Pushing into transitional mode increases fuel burn substantially — sometimes by a factor of two or three — so extended passages are almost always run at displacement speeds. The flat stern creates a large cockpit and swim platform, and often accommodates a tender garage.
Smaller semi-displacement yachts below approximately 30 metres are built in GRP; larger examples are typically aluminium. Naval architects use computational fluid dynamics to tune hull performance across the speed range. Propulsion is usually diesel with shaft drives; some builders use Hull Vane stern foils or hybrid systems to reduce fuel consumption at cruising speeds. Gyroscopic stabilisers are common on mid-size and larger builds.
