An outboard motor is a propulsion unit attached to the stern of a vessel without its own power source. It drives a propeller using an engine and is widely used for fishing, commercial operations, and leisure activities. In recent years, demand has grown rapidly due to ease of maintenance and simple removal.

Honda began selling outboard motors in 1964. At the time, the market was dominated by two-stroke engines, which had fewer parts, simpler structures, lower cost, lighter weight, and were capable of higher output. Four-stroke outboard motors were almost nonexistent.

However, Soichiro Honda instructed the development of outboard motors powered by four-stroke engines:

“Don’t pollute the water. If you let unburned oil flow into the water along with exhaust, what do you think will happen to the seas, lakes, and rivers? Others may all use two-strokes, but Honda should go with four-strokes. If they are heavier or more expensive, we just have to find a way to overcome that.” ("Making the Dream Come True: 50 Years of Challenges")

Based on this belief—“watercraft should not pollute the waters they ply”—Honda pursued the development of four-stroke outboard motors. Because outboard motors discharge exhaust gases through the center of the underwater propeller, two-stroke engines, which mix lubricating oil with fuel, release unburned substances into the water. To address this, Soichiro Honda directed development using more efficient and cleaner four-stroke engines.

Following this directive, Honda introduced the GB30 in 1964, its first outboard motor. Based on the 132cc four-stroke general-purpose engine G30 released the previous year, the GB30 adopted a chain-driven propeller structure and delivered 4 horsepower from a 171cc engine. Since then, Honda has consistently produced outboard motors powered by four-stroke engines, culminating in today’s BF350 equipped with a V8 engine.

The GB30 offered unique value: the engine could be removed without tools and used on land as a general-purpose engine. It also featured the advantages of a four-stroke engine, including strong low-speed torque, quiet operation, and fuel efficiency. Its output characteristics suited low-speed trolling, and its clean emissions made it effective for aquaculture such as oyster and seaweed farming, where water quality must be maintained.

However, in an era dominated by inexpensive and lightweight two-stroke engines, Honda’s four-stroke outboard motors—disadvantaged in both cost and weight due to their more complex structure—faced challenges in the market.

Honda continued to expand its lineup, introducing the 2.5-horsepower GB25 and 4-horsepower GB40 in 1967. Up to that point, these models were based on general-purpose engines.

In 1971, Honda launched purpose-built outboard motors: the 45TWIN, featuring a 126cc horizontally opposed two-cylinder four-stroke engine, and the 75TWIN, a 149cc inline two-cylinder engine—the world’s first outboard motor to adopt an OHC mechanism.

In this way, Honda continued its challenge in a market dominated by two-stroke engines by expanding its lineup of four-stroke outboard motors and enhancing product appeal. However, sales performance did not improve significantly at the time.