How fuel efficient are the inboard/outboards?

I know you can only find inboard/outboards (which I assume are 4-strokes) on a few of the larger aluminum boats, but how fuel efficient are they compared to 4-stroke outboards of the same horsepower? Just asking for curiosity sake.

That's an interesting question. The last (non commercial) aluminum fishing boats that I recall coming out of the factory with an I/O (that wasn't a jet drive) were the Lund Tyee and Baron, but it seems that they are only making them with outboards now.

This link is kind of interesting https://www.fishing.net.nz/asp_forums/how-hungry-is-a-135hp-30l-mercruiser_topic69815.html

I think that the new big horse power outboards are fuel efficient enough now that the space trade off of the I/O isn't worth it any more.

IIRC the starcraft 16' holiday could be gotten with the merc 140hp I/O

Not very if u look at the weight factor. Plus your motor sits below the water. They are cheap however

I went and looked at the boat tests reports on the Mercury Marine website While the information is still comparing apples and oranges, the results seemed closer than expected. The major difference when looking at the Verado 250 outboard and the 4.7 Liter Mercrusier that also puts out 250 hp is the outboard seems to have greater acceleration, runs at higher rpms, and is a bit more thirsty. The Mercrusier seemed to be faster at around 5200 rpm (I'm assuming that is around its max rpm) than the outboard at a similar rpm. The tests show that outboards seemed to go faster when they run in the 6000's rpm but fuel consumption increased exponentially as expected.

For what you lose in any boat they are installed in they are not worth it in my opinion.

You lose a tremendous amount of usable space in the boat!
They are more difficult to work on
They are more prone to failure as they have more systems, more seals, more gear sets, etc.
They absolutely have to be properly winterized in areas that experience prolonged freezing temps.
They are more expensive to properly service, most people skimp on necessary services due to this.

At mid-range RPM they are pretty darned efficient. How's that for a positive

They are efficient at mid range RPM. Say, 3000-4500-ish. Generally speaking. They're biggest advantage: Quiet operation. Thats why many dedicated pleasure boats are I/O. I've had several. Maintenance intensive, compared to an outboard and also compared to an inboard-which isn't a lot different other than not having an outdrive to keep up with.

for pleasure use only in deeper waters, an I/O is hard to beat. OB's are less maintenance but noisier and use a little more fuel, and cheaper to work on if/when that time comes. Though, I will say this: Since the 4 stroke stuff has been getting more advanced, the differences in noise have been getting slimmer. But I still prefer an I/O just because that's what I grew up around. Last one I had was a 4.3L V6 GM engine. At idle it was almost silent. At cruise speed (~3000 RPM) it was quiet enough to hold a normal conversation without raising our voices much. I loved that boat.