Fuel consumption on 21 foot Cabin boat with merc 4.3 or 5 Litre

[Big Pete]

Considering getting a bigger boat a cuddy or cabin around 21 foot with a 4.3 or 5 Litre mercruiser engine.

What kind of fuel consumption could i expect , anyone have any ideas what sort of Gallons per hour i could expect ?

anyone have any costings etc ?

 

[Big Pete]

Re: Fuel consumption on 21 foot Cabin boat with merc 4.3 or 5 Litre

Surely someone knows what they spend in fuel when they take boats out ?

I am trying to see difference between 3 Litre and 5 litre engine costs ..

 

[truckermatt]

Re: Fuel consumption on 21 foot Cabin boat with merc 4.3 or 5 Litre

fuel consumption really depends on how you drive..... For a bigger boat, you'll be much happier with the bigger motor.. even if you do spend a little more on fuel...

 

[45Auto]

Re: Fuel consumption on 21 foot Cabin boat with merc 4.3 or 5 Litre

See this thread about a test of 23' boats with 4.3L, 5.0L, and 5.7L motors:

http://forums.iboats.com/showthread.php?t=397741

You can also go to Boattest.com, they have test results showing fuel consumption of about any type boat you can think of.

The way you operate the boat will have a HUGE effect on fuel usage no matter what motor you have.

 

[hungupthespikes]

Re: Fuel consumption on 21 foot Cabin boat with merc 4.3 or 5 Litre

x2 auto45

"The way you operate the boat will have a HUGE effect on fuel usage no matter what motor you have."

I run a 265 with a 454/bravo 1, run thru 3-4 tanks a summer running under 2800 rpm/30mph. If we run hard 4000+ , then i need to fill up at lunch and on the way home.

You pay to play

 

[RAT Attack]

Re: Fuel consumption on 21 foot Cabin boat with merc 4.3 or 5 Litre

I have a 19' cuddy with a 5.0 Mercruiser / Alpha 1 outdrive, 2 br carb.. It is a little smaller than what your asking, but thought I would share since it does have the 5.0. As stated by others, it all depends on how you are running it and how well the boat is running. It is a pretty hard thing to determine. I have a 28 gallon tank that last me for a day of fun in the sun. I spend a lot of time hanging out in the party cove, and and a lot of time cruising around about 30 MPH, a couple WOT pulls, and then still many pulls of the wakeboard, kneeboard, skiis, & tube. I usually go through about 24 gallons an outing. I was originally concerned that I would get horrible gas mileage, but all-in-all, I am pretty happy with the fuel consumption. I usually get about the same amount of time on the lake & with real similar driving as my brother gets with his 96' Rinker 19' open bow, 3.0 Mercruiser / Alpha 1, 2 br carb., but his is only a 19 or 20 gallon tank.
I hope this helps some...

 

[QC]

Re: Fuel consumption on 21 foot Cabin boat with merc 4.3 or 5 Litre

See this thread about a test of 23' boats with 4.3L, 5.0L, and 5.7L motors

I went and reviewed that other thread and I am kind of surprised at the results and some missing (and contradictory) data. You're usually better than that. I cherry pick too, but this test shows that the smaller engine wins in fuel efficiency: http://www.stingrayboats.com/products/reviews/tb2_99.html

Quote from the Trailer Boats article:

When dealing with boat weights in the 3500-pound range, we don't think of 133 pounds as being very significant. Yet that extra weight in the stern of the 7.4L MPI equipped boat was enough to prevent the Stingray from planing at 2000 rpm. It struggled at 9.9 mph while the 350 Mag MPI was up and running at over 18 mph. As expected, the small-block engine consumed less fuel. It averaged 10 percent better fuel economy in the slow and midrange speeds, and nearly 20 percent at top speed. Even at the slower speed, the 350 Mag MPI still delivered approximately 5 percent more miles per gallon.

This one is the exact same boat and they swapped the engines. However . . . these are still very unscientific tests as prop, gear ratios, RPM ranges and other stuff do somewhat muddy the water.

Here is a quote of mine from another thread and I think it is still pretty valid when it comes to these discussions:

Due to all of the variables that we are throwing at the discussion, fuel efficiency is almost to the point of voodoo in the case of marine applications . . . Hull design, weight, electronic vs. mechanical fuel systems, 2 cycle vs. 4 cycle, diesel, drive systems, propellers, speed, engine displacement, 4 vs. 6 vs 8 cyl's, planing vs. displacement speeds, application (watersports, idling along, slow cruise, fast cruise, WOT), all of these factors skew the answers . . .

 

[45Auto]

Re: Fuel consumption on 21 foot Cabin boat with merc 4.3 or 5 Litre

Don't have much time right now, but hadn't seen that Stingray test. Makes me want to sell my big block, since the small block was faster AND more fuel efficient!

Kind of interesting that in the exact same boat with the same outdrive and propellor (21" pitch) the 300HP 5.7L could spin it to 4900 RPM at 62.1 MPH while the 310HP 7.4L could only pull to 4700 RPM at 58.4 MPH.

 

[QC]

Re: Fuel consumption on 21 foot Cabin boat with merc 4.3 or 5 Litre

And with 10 fewer ponies . . . I thought they changed gear ratio . . . hmmmmm. Rechecking.

 

[Philster]

Re: Fuel consumption on 21 foot Cabin boat with merc 4.3 or 5 Litre

Don't have much time right now, but hadn't seen that Stingray test. Makes me want to sell my big block, since the small block was faster AND more fuel efficient!

Kind of interesting that in the exact same boat with the same outdrive and propellor (21" pitch) the 300HP 5.7L could spin it to 4900 RPM at 62.1 MPH while the 310HP 7.4L could only pull to 4700 RPM at 58.4 MPH.

Bigger engines usually have a power curve that drops off at the end. Sometimes the smaller engine with a power curve that doesn't drop off suddenly is the better engine.

A lot of these big blocks are strokers... and they have a lot of purpose pulling down low (in rpm band) with consistency, which is much different that rev-ability, responsiveness and power near redline.

7.4's might pull tree stumps, but their power curves don't work well when responsiveness and redlines are gonna determine their success.

 

[Lrider]

Re: Fuel consumption on 21 foot Cabin boat with merc 4.3 or 5 Litre

Wow with my 17.5' boat 90 hp and 65 gallons I should run all season (if I go slow)

 

[badbowtie]

Re: Fuel consumption on 21 foot Cabin boat with merc 4.3 or 5 Litre

I have a 21 foot cuddy but I have the 5.7 and I have a big tank almost 50 gallons. I was out last weekend saturday for about 6 hours. I skied alittle and wakeboarded alittle. Then took turns pulling the other 5 passangers in the tube. Other than that cruised around about 28-30mph and stopped and eat lunch and swim for about a hour. Then took it back out sunday for about 3 hours and just cruised around about 28-30mph. I still have about a needle with less than the three quarter mark left. When I am out and open up the secondaries allot and have several people taking turns skiing and sloam skiing and am running more like 45-50mph I can burn a whole tank of fuel in about 5-6 hours. I would go with the bigger motor you will be happy you did when you have it loaded down with people and gear and want to still play.

 

[45Auto]

Re: Fuel consumption on 21 foot Cabin boat with merc 4.3 or 5 Litre

The 454 in the test was under-propped, recommended range is 4200-4600 RPM. Probably be about the exact same top speed if they had went to a 23" pitch prop on the 7.4L. Can't say if that 10% taller prop would translate directly into a 10% or more gain in fuel economy, but I doubt if it would lower it any.

 

[QC]

Re: Fuel consumption on 21 foot Cabin boat with merc 4.3 or 5 Litre

Yeah, saw that too, but I don't think that would change cruise efficiency much, but some . . .

The one thing not mentioned yet here, and it will start a whole bunch of other carp for sure, but theoretically (yes theoretically) all throttled (Otto Cycle) engines are more efficient at more open throttle settings. So , also theoretically, if all other things are equal (they never are) the smaller displacement engine would be more efficient at the same boat speed.

 

[Philster]

Re: Fuel consumption on 21 foot Cabin boat with merc 4.3 or 5 Litre

The 454 in the test was under-propped, recommended range is 4200-4600 RPM. Probably be about the exact same top speed if they had went to a 23" pitch prop on the 7.4L. Can't say if that 10% taller prop would translate directly into a 10% or more gain in fuel economy, but I doubt if it would lower it any.

Makes sense.

 

[Big Pete]

Re: Fuel consumption on 21 foot Cabin boat with merc 4.3 or 5 Litre

Thanks for the info guys , it looks like i am going to be going with the 4.3 mercruiser engine boat on the SeaRay 200 cuddy , as long as she checks out on sea trials and compression checks etc.. ..

so i will be able to afford to use the thing i worried over it being so thirsty but seems as long as i am sensible should be ok ..

 

[H20Rat]

Re: Fuel consumption on 21 foot Cabin boat with merc 4.3 or 5 Litre

Yeah, saw that too, but I don't think that would change cruise efficiency much, but some . . .

The one thing not mentioned yet here, and it will start a whole bunch of other carp for sure, but theoretically (yes theoretically) all throttled (Otto Cycle) engines are more efficient at more open throttle settings. So , also theoretically, if all other things are equal (they never are) the smaller displacement engine would be more efficient at the same boat speed.

until you thrown in modern EFI systems. You can run an engine intentionally lean at partial throttle settings, as well as play with the timing. For WOT operation you need to go much richer. Modern car engines at least will see 15:1 or higher during normal cruise. Helps improve emissions as well as mileage. I doubt any boat engine does it, but cylinder deactivation is becoming very popular again also, so that completely changes it.

(yes, car engine, i've only tuned ECU based car engines, can't speak specifically for marine engines, but most newer EFI ones don't stray too far.)


Pure otto cycle theory assumes the same AFR across the board to be true.

 

[QC]

Re: Fuel consumption on 21 foot Cabin boat with merc 4.3 or 5 Litre

I said "all else equal" i.e. EFI vs. EFI, Same intake, same volumetric efficiency, same boost or lack there of, same/same which of course none of them are . . . Stoich or Lean, less throttling (wider throttle) equals better efficiency.

I guess my point being that these engines are all widely different (even the 5.7 to the 7.4) so comparing them and then concluding "bigger displacement is more efficient" is misleading.

 

[H20Rat]

Re: Fuel consumption on 21 foot Cabin boat with merc 4.3 or 5 Litre

I said "all else equal" . . . Stoich or Lean, less throttling (wider throttle) equals better efficiency.

speaking overall efficiency only, an engine running 11:1 at WOT will never be more efficient than the same engine running 15:1 at cruise. You are dumping unburnt fuel out the exhaust at WOT, which more than makes up any efficiency drop you get because of throttling.

 

[QC]

Re: Fuel consumption on 21 foot Cabin boat with merc 4.3 or 5 Litre

Yeah, but 11:1 is past Stoich, so that seems kinda unhelpful . . .

P.S. Edited a little up there while you were replying