is the 7.4 really bad on gas consumption?

[UncleWillie]

... Running a smaller motor at its rated capacity is always more efficient than running some huge monster at 5% of rated output. Its just a fact of life...

That is the key difference to consider when operatiing at less than full puwer.

The link above has a wealth of comparative data.
I processed all the engine into one chart for comparison.
Performance wise there is not a lot of difference.

Engine	CID	HP	GPH	HP/CI	HP/GPH
MerCruiser 3.0 TKS	181	135.0	10.5	0.75	12.9
MerCruiser 4.3 TKS	262	190.0	16.0	0.73	11.9
MerCruiser 5.0 TKS	305	220.0	18.2	0.72	12.1
MerCruiser 4.3 MPI	262	220.0	18.0	0.84	12.2
MerCruiser 4.5 MPI	275	250.0	19.7	0.91	12.7
MerCruiser 5.7 L	350	260.0	21.0	0.74	12.4
MerCruiser 5.0 MPI	305	260.0	18.5	0.85	14.1
MerCruiser 5.7 MAG	350	300.0	23.0	0.86	13.0
MerCruiser 7.4 MPI	454	310.0	28.5	0.68	10.9
MerCruiser 6.2 MAG	377	320.0	24.5	0.85	13.1
MerCruiser 8.1 MAG	496	375.0	30.5	0.76	12.3
MerCruiser 8.2 MAG	502	380.0	31.0	0.76	12.3
MerCruiser 8.1 HO	496	425.0	33.5	0.86	12.7
MerCruiser 8.2 HO	502	430.0	37.5	0.86	11.5
			Mean	0.80	12.4
			S.Dev	0.07	0.76
			Max	0.91	14.1
			Min	0.68	10.9

The GPH data is crowdsourced and may not be very accurate until a large data set is collected.
The 5.0 and 7.4 numbers look suspicous.
Mercs, just invented, 4.5 design, displays the improvements you might expect.
Otherwise you are going to get about 12.5 HP per gallon per hour, more or less, no matter what engine you have.
That number is inherent to gasoline, and thermodynamics. You can't argue with Physics.

 

[jimbo_jwc]

Just plugged in my numbers to see what kind of range I have for my 3.0L and found out what RPM I'm going to run at Lake Cumberland .You have to like that site for people get to compare to others . I wished when they voted the would list their boat make .1500 RPM was my best sorry wouldn't copy n paste.
MPG ESTIMATOR

Enter speed value (Miles Per Hour)
MerCruiser 3.0/181 - 135 hp

RPM	MPH	GPH		MPG

 

[UncleWillie]

Just plugged in my numbers to see what kind of range I have for my 3.0L and found out what RPM I'm going to run at Lake Cumberland .You have to like that site for people get to compare to others . I wished when they voted the would list their boat make .1500 RPM was my best sorry

It doesn't matter what boat the engine is in. Your 3L is going to burn ~10.5 gph at WOT running it in a barrel
The data is for GPH not MPG. The data is comparing engines not hulls.

You didn't say what boat you have either. But 1500 rpm is likely your WORST MPG!
That will place most boats just an the verge of getting on plane. Worst place to be.
3000 to 3500 rpm is typically the sweet spot. Well up on plane and at about half full speed.
Yes! More GPH, but Best MPG.

 

[redneck joe]

Lots of number and Mather so I'll redneck it up fo rn you. It you spend 100 day now you'll spend 125 with the big one. Or close to that math

 

[philipp10]

The best way to rate a boat/engine combo, is to look at mpg. I did when I bought a boat, comparing a 3.0 L 4 cylinder with a 4.3 L 6. At low speeds around 1000 rpm, the 3.0 will do 8.5 mpg, the V6 will do about 4.5 mpg. At higher speeds, say 3000 rpm, the difference is not near as great. This is all from a multiple Boat Test magazine articles I have read. Its not complex, its always better mileage to go with the smaller motor if you want efficiency but of course you lose the ability to go really fast. That's the trade off. And BTW, boats and cars are the same as far as this subject is concerned.