system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do marine engine utilize only one gear for their transmission instead of multiple gear ratios in a automobile?

I don't know if I'm wording this correctly but it's something that has always confused me.  Why is it a marine engine will basically have a single gear instead of multiple gears you would find in a automobile transmission?
gpt
In a car the speeds of the engine and the wheels are directly connected by the gearbox. The engine needs to be within a relatively narrow band of speeds to run efficiently and generate the best power, so you use a gearbox to allow the wheels to turn at a wide range of speeds while the engine stays within its preferred range.

In a boat the propeller can move at a single speed and be providing forward thrust whatever speed the boat is moving at, because it can "slip" through the water. You don't need to turn the propeller really slowly to get the boat moving and then speed it up as the boat speeds up, so you don't need a gearbox allowing the engine to turn the propeller at a wide range of speeds.