The long-tail boat, known as Ruea Hang Yao (เรือหางยาว) in the Thai language, is a type of watercraft native to Southeast Asia, which uses a common automotive engine as a readily available and maintainable powerplant. A craft designed to carry passengers on a river may include a lightweight long canoe hull, up to 30 metres, and a canopy. There is much variation among these boats, some have evolved from traditional craft types, while others have a more improvised look—the sole defining characteristic is a secondhand car or truck engine.

This engine is invariably mounted on an inboard turret-like pole which can rotate through 180 degrees, allowing steering by thrust vectoring. The propeller is mounted directly on the driveshaft with no additional gearing or transmission. Usually the engine also swivels up and down to provide a "neutral gear" where the propeller does not contact the water. The driveshaft must be extended by several metres of metal rod to properly position the propeller, giving the boat its name and distinct appearance.

Advantages to the inboard engine with a long driveshaft include keeping the engine relatively dry. Following the basic design pattern allows a variety of engines to be attached to a variety of different kinds of hulls. This flexibility simplifies construction and maintenance while sacrificing the efficiency and comfort that might be expected of a typical mass-produced product.