Double propeller drive for boats

In a steerable double propeller outboard drive unit for a boat having a transom and means mounting said unit on said transom for steering said unit about a generally upright steering axis, said unit including a first propeller having a plurality of blades, a first propeller shaft drivingly connected to said first propeller, and a second propeller having a plurality of blades, a second propeller shaft mounted concentrically with said first shaft and drivingly connected to said second propeller, power means including a substantially vertical output drive shaft coupled to said first and second shafts for rotation of said first and second propellers in opposite directions, the improvement comprising the after propeller of said first and second propellers being of a predetermined diameter smaller than the diameter of the fore propeller, and said after propeller including at least one blade more than said fore propeller so that said first and second propellers provide approximately equal thrust to minimize the reaction steering moments on said unit.