By definition, efficiency would be when the boat is on plane, trimmed
out and running without effort. That is not at WOT throttle - more
often it's at 2/3s throttle. On my Ranger for example (it's the
closest boat I have to a 17 foot Center Console) it comes up fully on
plane around 3,000 rpm at 20 or so mph and settles in (same throttle
setting) at 4100 rpm and 30 mph. WOT on this boat is 5500 rpm and 45
mph. At cruise (4100 rpm/30 mph) it eats very little fuel - I can run
all day at that speed. The engine is a 200 hp Evinrude FICHT.

Thanks for the correction. 2/3 throttle is what I will aim at.

You run your 200HP motor at 2/3 throttle to achieve 30 mph. Does this
mean that I will need something like a 150HP motor at 2/3 throttle to
achieve 20 mph? I have an impression that yours is a bass boat, and
bass boat tends to have a very oversized motor. Therefore, I need to
discount that to get back to something more appropriate for a center
console.

Seem like I need to check the maximum HP rating of the boat and not
going to much below the max. rating. The other alternative is to see
what the manufacturer offered in their boats. If the max is 150HP and
the highest HP of the motor that they bundled with a boat is 115HP, I
would assume that 115HP is fine and not need to go for a 150HP motor.

I know that you are attempting to lessen the economic impact of a boat
purchase, but at some point, it makes no sense. The monetary
difference between a used 40 horse and a used 90 horse isn't that
significant. Considering the engine load you states (people, gear,
fuel) I would consider 90 hp the bare minimum.

Yes, saving upfront cost is a major consideration. The other
consideration is not to spend more than required. But if higher
horsepower is required to get the boat on plane reliably with 4 persons
on board, I will have to get a higher horsepower.

Yes, the price of a used motor seems to have a lot to do with how old
it is instead of how powerful it is.

Does your 90 HP minimum have taken into the account of 600-lb of
men/woman on board? If 90HP is good enough, I will have more selections
of motors available instead of being narrowed down to only 115HP.

Jay Chan