Re: Gear Ratio VS Prop Pitch
You are missing one piece of information. Props have a range of usefullness (aka efficiency) on typical recreational boats. That is between 17 and 23" of pitch. Above that, you loose the ability to get on plane quickly, but can slowly gain top speed. Below 17 and you will have a tow boat but no top speed. Your engine also has a wide open throttle speed that is top rpm for the engine, but not lugging (overloading) it.
To keep the props in that 17 to 23 range is done by gear ratio in respect to the torque of the engine and the hp.
As an example, with Mercruiser Alpha drives. A SBC V8 will use a 1.5:1 gear ratio, while the 3.0L 4 cylinder will take a 1.98:1 gear ratio. But both may use the same exact 21" pitch prop.