looking at buying a 1993 21' Crownline and am a little leary about it as it has the 7.4 .With gas so expensive here in Canada I'm a nervous that the fuel bill will bring some regrets? any input would be great...cheers
All of this sounds good and is pretty much spot on if that is the carb or Fi is tuned properly. I have a buddy who burns fuel in a 454 in 21' boat at rates that would stagger your mind..however the carb is a original carb 1989 vintage it appears to have never been opened up the paint on the carb is pristine. I have tried with no avail to get get a new carb on that boat but it still runs decent so he refuse's to even think about a new carb....how he can think it can still meter fuel right at that age and how it is still running stagger's my mind...Dont be one of those guys that assumes if it run's thing's are ok... I dont have the heart to tell him the fuel consumption rate he is going thourgh now could easliy pay for a new carb and i would no longer pass his old tub in my 4.3..:faint2:
you don't boat all day at high speeds. You do way more idling than you realize (no wake zones, just wanting to go slow etc). It is at these speeds where the 7.4 will burn way more fuel than the smaller motor. Same thing happens in a car. The V8 stuck in traffic idling uses probably 1.5 times the 4 cyl.
In general, the bigger engines will use more fuel at idling speeds and at the higher throttle ranges (because they can). In terms of the OP's question, it will not be 'bad' versus what may be perceived, due to the power usage factors discussed in the previous posts.
I recall reading an article on this very topic in a boating magazine quite a few years ago . . . 7.4 vs. 5.7's . . . same boats, but properly matched gear ratio and prop pitch . . . the 7.4's used about 10-15% more fuel than the 5.7 overall, even though the engines were 30% more displacement.
You may be right about the idle fuel flow being more for a 7.4L engine vs a 3.0L engine..........but to operate a 21ft boat at "no wake" with a 3.0L engine would probably be using similar fuel flow because it would need to be operated at "OFF-idle" throttle settings to get the same "no-wake" speed.
Also, I don't think the OP is talking about getting that 21ft Crownline with a 3.0L engine (if one even exists.....I don't think Crownline offered ANY 21' boat with a 3.0L engine....it appears that the 4.3L V-6 is the smallest )
He's likely referring to the difference between a 7.4L engine and maybe a 5.7L V-8.
I suspect the difference running at idle between the two, is minor.
I would be willing to bet if (2) identical 21' Crownline boats (5.7L, 7.4L) were operated similarly for a day, the 7.4L boat would likely use less than 10 gallons more and possibly less than 5 gallons(more)
If you ran them around at WOT a LOT and did a lot of FULL-throttle launches, then the 7.4L powered boat would of course use considerably more.
When I go to the lake for a day of wake-boarding, I NEVER need full throttle launches.
Because of the Bravo III, it's just not necessary. Even pulling a skier, full throttle is not required (out of the hole) OTOH, It probably would be if I had the same boat with a 5.7L engine...............and full throttle absolutely would be needed with a 4.3.......
A 3.0l may use less fuel at idle but it is also doing less work. Car and boat engines have nothing in common when it comes to fuel efficiency.I wasn't trying to say a 21 ft Crownline would run a 3.0 L motor. My point was, a larger motor always uses more fuel at low throttle positions. At idle, it takes a lot more fuel just to overcome engine friction on the larger motor. Thats where you will use more fuel along with the weight penalty of a larger motor over a smaller lighter one. It take lots of fuel to push that extra weight. Why do you see the car makers now using smaller 4 and even 3 cylinder engines in cars? CAFE is forcing that. BTW, take a look at the most efficient boats of all, a sailboat. They run 10 hp OB and get incredible mileage. 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 said, like an earlier poster said, the difference between a 5.7 and a 7.4 is not going to break the bank.