Why do outboards run higher pitch props than inboards for the same horsepower?

Question

Why do outboards run higher pitch props than inboards for the same horsepower?

Answer 1

Its not just gear ratios, most inboards have a marine reduction gear. The plane fact is an outboard can't swing the same size prop as an inboard and so they increase the pitch.  Also since they tend to run at higher rpm the prop is designed to run at higher speeds. Thats why outboards typically have poor low speed handling (thrust delivery) when compared to inboards. Notice when you put an outboard in reverse? It takes a long time to see a reaction. Compare this to an inboard that reacts much quicker, almost instantly on a diesel inboard.

Answer 2

Gearbox ratios. The pitch of the prop is determined partly by the rpm at the prop shaft, and this depends on the ratio of the gearbox. A prop with the correct pitch will allow the motor to achieve maximum rpm at the 'red-line' or rev limit for the motor at Wide Open Throttle (WOT).

Too low a pitch will allow the motor to over rev; if the pitch is too high then the motor cannot achieve maximum rpm because it is working too hard. This applies whether it is an inboard or an outboard.

Many inboards have a direct drive (1:1) and all outboards have a reduction gearbox. Naval architects have formulas for calculating the correct pitch for a particular boat/motor/gearbox combination.

Answer 3

The pitch of the prop is measured by how far the prop will move forward in one revolution. Some I/O s have the same pitch as some outboards. It just depends on if you want speed, power or some of both.