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Okie2
02-22-2010, 09:35 PM
Just a generic question....Taking an average Donzi owner/driver (not an average "boat owner"...I mean people who RUN their boats)...throw in a classic 22 w/ 454 vs. a classic 16 w/ 350....under the same water conditions...same driving techs....and driving say 50 miles per weekend....how much more gas will the 22 burn over the 16? In other words...is it that much more expensive to operate a 22 over a 16? Just curious. Just generally speaking...not getting to particular about add ons & modifications. Scott

Phil S
02-22-2010, 09:48 PM
...sounds like fodder for the classic "we really need to get a bigger boat honey" argument to me....:lookaroun:

I'm in ! :yes:

...typical conversation at my house..."I've heard that, up on plane, a 45 ZR uses less fuel than a canoe......no seriously, I read that somewhere....ok, so I wrote it down, then I read it...what's your point ?

Ask me how well that's working...:confused:

Planetwarmer
02-22-2010, 09:48 PM
I would say under normal conditions and 50 miles of driving, I may use 20-25 gallons. I guess I havnt really added it up, but that is about how far I usually drive and how much I burn in a weekend.

22C 454/330hp Alpha 1 32:1/ 21P Cleaver

Okie2
02-22-2010, 10:01 PM
The reason...I don't mind dropping $100 in a weekend for gas....don't want to drop $1,000.

True story from my prop guy from one of his customers...who had tons of $$$$$$$$. "So I bought this big "cig" type boat....lots of big engines...gave my foreman a credit card to fill up with gas. Drove a couple of days & gave foreman credit card to fill again. Foreman returns with boat & credit card & owner asks for receipt...sees the bill for over $3,000....Sells boat."

I don't want to get caught with my pants & wallet down...Just don't know how much gas a BBC burns running up & down the lake. I do know what the 16 w/ modded 355 will cost. Just checkin'. Scott

Dr. David Fleming
02-22-2010, 11:25 PM
Generally the less throttle you open the less gasoline the engine burns. There are exceptions however. If you read the performance boating magazines you will see where various models perform with regards to speed, and acceleration as well as specific fuel consumption.

From my observations they indicate the the performance boats get the best distance and speed between 3000 rpm and 4000 rpm at cruise. This might seem high, 4000 rpm, but the engine is at its most efficient operating range and the boat at is most efficient hull speed. So you go the farthest on the gas you burn - this is efficiency.

The gasoline engine gets its greatest fuel economy and performance at the point in its operation where the "volumentric efficiency" and "thermal efficiency" are at the peak. These are engineering terms - "volumentric efficiency" is the operating condition when the cylinders are most completely full of fuel. This is usually at a particular rpm range when the cam timing and cylinder head efficiency all match to fill the cylinders completely without waste. Some rare race engines can border on 100% when the tuning of the exhaust and intake fills the cylinder completely. In your engine look for this "volumetric efficency" at the rpm where the engine peaks its torque curve - usually 3,500 - 4,000 rpm. It makes the most torque because the cylinders are most filled.

Don't confuse torque peak with power peak because horsepower is a measure of maximum work done in a given time - not most efficient operation.

"Thermal efficiency" is a term pertaining to the amount of energy extracted from the gasoline. Engineers know that the higher the compression the more thermal energy is recovered from the burned gasoline. They try to set the compression as high as possible but the quality of the gasoline restricts this. For this reason a 12:1 compression engine is substancially more efficient than a 8:1 compression engine. Most BBC marine engines are around 8.5:1 because of the generally poor quality of marine gas. If you used racing fuel you could run the 12:1 ratio but this would require special pistons in the engine - not an easy change to make. This change would give you substancially more power and distance on the fuel you burn - hence the term "thermal efficiency."

Say you are out on the water the tank is almost empty and you have a long way to go. Throttle back, but keep the hull speed high as possible with the engine in the 3000 rpm to 4000 rpm. This is most effective distance for the gas burned. Slowing down to idle results in very little fuel burned but very little distance covered.

Hope this helps,

zelatore
02-22-2010, 11:34 PM
Say you are out on the water the tank is almost empty and you have a long way to go. Throttle back, but keep the hull speed high as possible with the engine in the 3000 rpm to 4000 rpm. This is most effective distance for the gas burned. Slowing down to idle results in very little fuel burned but very little distance covered.


I haven't tried measuring fuel burn on small boats like the classics, so maybe they will actually get better economy once on plane. But don't try that with a big boat! EVERY boat over about 30' I've actually taken fuel burn readings on does far better at displacement speeds than on plane. The worst economy usually occurs just before plane - that should be obvious by the huge wake. Then there's a rise in economy as speed increases and the boat gets further on plane, followed by a decline (more or less linear with diesels, but almost logarithmic with gas) as you head for WOT.

But, uh, that still doesn't help tell you how much fuel a 16 vs a 22 burns...I just know my 22 burns so much less than my 32 it's a relative joy to fill it up!

MOP
02-22-2010, 11:51 PM
Interesting question, but I will guess close to 1/3 more. You have about 900lbs difference in weight and a lot more planing surface. I have a small block 22 having run several events I usually wait out on the bay while the BB gas up, I know there is a fair difference between BB & SB and mine is a 383 with a little juice.

handfulz28
02-22-2010, 11:53 PM
Or more simply, expect to burn 15-25 gals per hour...depending on all kinds of variables. :bonk:

MOP
02-23-2010, 12:22 AM
25 is a real steep number even the BB boys will come back on those numbers, running pretty hard I don't come near that. I run off my 40 gal rear tank and get through the two days of the Donzi run without refueling. You are talking larger boat numbers which I have a pretty good handle on having been a marine tech and then a yacht broker. An example that is easy to check out, a SB 34 Silveton at cruise burns in and around 25-27 gph @ cruise. The later BB 34 burns 30-35+ same speed and conditiions, the later boats are a little heavier and the hull is a bit different. Most all of my old clients that upgraded to the late model had a fit, even the old Silverton 40 double cabin did better and hovered just above 30 kind of a bummer having a way smaller boat using that much fuel. These numbers are running 3-3,200, a totally different type of running then most classic do.

yeller
02-23-2010, 01:11 AM
Had a 16 with 350 jet drive. Bought a 22 with 496HO.

Let's just say, I spend a lot more time 'drifting' than I use to.

.....probably 3 times the fuel used.

osur866
02-23-2010, 03:16 AM
True story, I have an 18 with mx6.2 engine stock except stainless marine exhaust, running -2" lower, ran AOTH 08' along some of the BB 22 boats to and from lee's ford, the ones that didn't get lost, and when returned back to Jamestown still had just under 1/2 tank it's a 41 gal. Tank and on the way back ran pretty hard as Grizz can atest to, most of the bb 22's that didn't get lost think where down to less than 1/4 and well some got towed back the ones that missed the turn. I think in order to do a true comparsion to this would be to take a bb boat and a sb boat that are equally stock and run them side by side for a day and then fill up at the end of the day. I've been very pleased on how efficent mine has been when you compare performance to fuel useage, nothin like runnin around @ 3600 RPM's at a 53 mph cruise, try that in most other boats of our size with similar power.

jl1962
02-23-2010, 05:35 AM
My Ski Sporter with a 302 Ford and a 21" prop will use about 8 gal/hr at a fast (for a 16) cruise - usually 3500 - 4000 rpms = 40 - 45 mph. One year at the Dust-Off - I ran the entire Poker Run +/- 60 miles on about 13 gallons. Not exactly a Prius but about half of what the big blocks used.

Not a material savings in the cost of ownership however - as we know it's all relative. I remember trying to calculate fuel consumption the year the owner of the 38 ZR took 50 of us for fast rides for several hours and ran out of fingers and toes!

JL

Pismo
02-23-2010, 05:36 AM
My 22 502 uses gas like obama spends money. Big blocks chew it up hugely. 2500rpm or less seems to get the best mpg.

oledawg
02-23-2010, 06:52 AM
My mildly modified C22 454 takes about 12 - 15 gallons to do a round trip up my lake at rpms varying from 3,000 to 5,000 so given that is about a 30 mile run I figure smileage at 2 - 2.5 mpg or since it usually takes a little over an hour, the same 12-15 gph of run time. Figure $45 - $70 per hour for gas depending on how hard and long you run. And of course the price of gas at any given moment, I assumed $3.50 which may be way too low this coming summer. :bonk:

VetteLT193
02-23-2010, 07:24 AM
a 22 with what kind of 454 is the other question? base 330 HP you'll do pretty good. Even a later MPI model you will do good. More power though... not so good. I think on the high power end of the scale you'll get down to the 1 mile per gallon scenario. I think the most efficient 22 Big block will see 3 MPG.

I'd figure on 2 miles per gallon as the going rate.

I'd get about 3.5 MPG in the Minx with a small block.

The ZXO with the old props got a bit over 2 MPG. I think with the new wheels I'll beat that considerably.

RE: zelatore... just about every small boat will get better MPG on plane at a certain speed / RPM. but like you said, big boats are different!

Just Say N20
02-23-2010, 08:13 AM
1979 18 2+3
Standard Mercruiser 260 hp/350 Chevy
Alpha outdrive
43 mph @ 3,600 rpms (just before the Quadrajet secondaries open)

Topped off tank in St. Joseph, Michigan

Ran in calm water along the coast at a constant 43 mph from St. Joseph to Pigeon Lake (63 miles total including channels, if I remember right).

Topped off the tank, tilting the trailer so it would give the boat the same position as when the boat was sitting in the water, and it took 11.5 gallons.

Gave me 5.4 mpg for the return trip.

I'm anxious to see how the 16 does. Smaller boat MIGHT make it marginally better, but Volvo 290 outdrive might make it marginally worse than the Alpha.

Okie2
02-23-2010, 08:34 AM
Osur had the kind of info. I was looking for....AOTH would be a great way to compare....running to Lee's Ford & back....most run similiar speeds (not including the hot stuff like the Cig 20 guys, etc)....That would be a good test....fill up...run the crap out of the boats... come back fill up again & access the damage. Kind of what I thought...hot 22's suck a little more gas....I had quite a bit when I came back.

Ghost
02-23-2010, 11:06 AM
For laughs, let me try a different math than we've kicked around so far.

Optimal air/fuel mixtures are pretty constant. So, assuming two motors are carbureting or injecting properly, and that their timing/tuning is similar, fuel burn at a given RPM ought to be pretty proportional to the ratio of their displacements.

454/350 = about 1.3, or a 30% increase.

Given the stock 454s I have seen run a little lower RPM than stock 350s, maybe adjust this slightly. So, scaling based on a top RPM of 4600 RPM for a 454 and 5000 RPM for a 350, that shifts the scale so that you're comparing slightly different RPM at the same % utilization of the power of your motor.

Doing that, (454/350) x (4600/5000) = about 1.2, or a 20% increase in consumption of one over the other.

Based on this, assuming you ran the motors about as hard in each boat, I'd think you'd burn 20-30 percent more gas in the big block. Thinking about what I've read and observed, I'm not sure those numbers aren't pretty good.

At least for those two motors in the same hull. A 16 with a stock 350 versus a 22 with a stock 454 might be different enough in performance that you simply wouldn't run them the same way. I'll leave that for those who know the boats. I don't know the boats. But I suspect the "wahooooo" factor requires a 22 to go a bit faster than a 16, for the same adrenaline rush.

FWIW,

Mike

thehow33
02-23-2010, 07:31 PM
My estimate is based on data given so far in this thread and some common sense factors.
The difference between motors alone is quite substantial now factor in weight and the length of the hull, which I believe would have even less drag in a 16. So I'm think a 454 22C uses appr. 40% more fuel to go just as far as a 350 16.

Planetwarmer
02-23-2010, 09:16 PM
At least for those two motors in the same hull. A 16 with a stock 350 versus a 22 with a stock 454 might be different enough in performance that you simply wouldn't run them the same way. I'll leave that for those who know the boats. I don't know the boats. But I suspect the "wahooooo" factor requires a 22 to go a bit faster than a 16, for the same adrenaline rush.

FWIW,

Mike

Just think of the fuel savings and the "wahoooooo" factor in an 8' 125 cu/i Classic!

Okie2
02-23-2010, 09:22 PM
I do like that woooo factor....most of my passengers usually say "Oh, ****"....a little 16....377 hp...& waves & air.....Ain't it a blast! Planet...you be around Tenkiller this summer? Never did catch up with you last season. Scott

Planetwarmer
02-23-2010, 10:58 PM
Im sure gonna try! Im desperately working towards a doctorate in osteopathic medicine, and I got a loooong way to go. Ill take some time out on occasion though. PM me before you head out, and I will try to make it.