HP question

[Donzi111]

Currently have a 225HP Johnson on a 23' boat - if I switch to a 150HP with same size prop and rpm - seems like I could maintain same speed and use less fuel? Or is there something I am missing?

[boats.net]

75 HP less wont run same speed and could use same amount of gas if boat is under powered.

[Donzi111]

HP vs Speed

I think you may be right (as most equate HP with speed) but..............if prop is same size and rpm is the same, how can the speed be less?