To provide a propulusion unit for a small boat in which assembling work can be simplified to reduce cost and which can prevent large size when a four cycle engine is used and also effect to accessories due to exhausted heat can be avoided.

A four cycle engine 22 and a propulsion unit 20 are coupled such that a crank shaft 46 and a propulsion shaft 24 are in parallel when a propulsion device 19 of a water vehicle 1 (small ship) is formed so as to provide propulsion force by applying pressure to water suctioned from a ship bottom 2a and jetting to rearward by rotational driving the propulsion shaft 24 by engine power and front end of a crank shaft 46 and the propulsion shaft 24 are respectively coupled by a drive gear 49 (power transmitting mechanism) and a timing chain 54a for forming a cum shaft drive mechanism in a rear side portion is disposed by the power transmitting mechanism of the crank shaft 46.