Abstract: Relates to attachments to a motor boat for reducing the speed of the boat below the idling speed sufficient to permit trolling for fish. The speed controlling device includes a plate member mounted for movement from a vertical position immediately aft of the propeller and across the path of the propeller wash, thereby to slow the speed of the boat, to a second position away from the propeller and out of the path of the propeller wash. Hydraulic mechanism including a piston-cylinder combination unit is connected to a fixed support, such as the propulsion unit of the boat, and to the plate member for moving the latter between the aforesaid two positions of movement. Ports are provided in the cylinder for admitting fluid under pressure to either side of the piston to impart movement of the piston in either direction in the cylinder and to impart corresponding movement to the plate member.