The 1970’s, 80’s and 90’s. By the 1970’s outboard motors had evolved into tried and true basic designs and systems that carried over for decades with minor improvements along the way. Horsepower was always being increased in order top or at least compete with the other manufacturers. Boat propeller performance also had to be improved so the higher horsepower engines worked at their optimum peak. The more powerful engines created more torque on the boat propellers so engineers came up with stainless steel propellers to handle the torque that caused aluminum propellers to flex down in pitch that made less speed. Stainless steel props do not flex, they are about seven times stronger than aluminum props.
Other improvements made on propellers was cupping (curling ) of the tip ends on the blades that got a better bite on the water with less slippage. Increasing the angle of rake on the blades created more bow lift that got more of the boat out and above the water creating more speed also.
Pontoon boats which use outboard motors became very popular and also the low profile go fast style outboard motor bass boats with the wild metal flake colors started flying across the water ways.
Two cycle motors had to have lubricating oil added to the gas manually but automated oil injection started being used and was a great improvement. New fuel injection motors became popular instead of carburated motors because they increase performance and have better gas milage. Many manufacturers that offered only two stroke motors started making four stroke environmentally friendly motors that are quieter and get better gas milage.
Boat propellers kept up with the motor improvements with four and five bladed propellers and more efficient and faster designs.