Patent ID: 7892056

Claim:
A water cooling apparatus in a power transmission system of a boat propulsion unit comprising: a case arranged to be located behind a hull and supported by a rear portion of the hull; a propeller supported by a lower portion of the case; and a transmission unit housed in the case and arranged to receive a rotational output from an engine, change a speed of the rotational output, and transmit the rotational output to the propeller, the transmission unit including a cylindrical transmission case defining an outer shell of the transmission unit with an axis of the transmission case extending vertically, a transmission device housed in the transmission case and arranged to change the speed of the rotational output of the engine, and a cooling water passage provided in the transmission case for flowing cooling water; wherein the cooling water passage is located in at least one of a side portion or a rear portion of the transmission case in a plan sectional view of the transmission unit.