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  1. #1
    Seaman Apprentice
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    Default I/O vs. Outboard Fuel Consumption

    I had a couple quick questions about I/Os vs. outboards. I've read up on outboard fuel consumption, and from my own observations, it seems that an outboard will burn approximately 10% of the rated HP in gallons per hour (GPH) at WOT. So does this hold true for an I/O? Or is the fuel consumption closer to that of a car with a similar engine? How much difference in consumption could I expect between a 4cyl and 6cyl? I understand that a good deal of this depends on a hull design and shape, but for comparison we'll say that they are all attached to the same hull. Thanks for any input you have.

  2. #2
    Vice Admiral NYBo's Avatar
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    Default Re: I/O vs. Outboard Fuel Consumption

    There are far too many variables to give you a simple answer. But one thing IS true: The best fuel economy in a boat will be near the worst fuel economy of any car. (Think Lamborghini and 12' tinny with a 10 HP four-stroke outboard.)
    Bob
    '88 Bayliner 1700 Capri Bowrider, 85 HP Force O/B, "Sea Weasel"
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    Disclaimer: Although I am a member of the USCG Auxiliary, the opinions and advice in my replies are my own and do not necessarily reflect CG or CG Auxiliary policy or regulations unless so specified.

  3. #3
    Admiral smokeonthewater's Avatar
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    Default Re: I/O vs. Outboard Fuel Consumption

    If you do a search this gets asked at least once every 3 months.... The 10% rule of thumb carries to pretty much all engines with some variance + or- Even your car, if it makes 100 hp will use close to 10 gallons of fuel in an hour if it is operated at full throttle for the whole hour such as if you were towing 5 more cars up a steep hill the entire time... the huge difference between cars and boats is that a car can run 60 mph all day at 15-30% throttle and a boat of about the same size as the car with the same engine might do 45 mph at 100% throttle.

    For a comparison that you can feel, walk 100 feet on a side walk, then walk twice as fast 100 feet.... next do the same in a swimming pool.... you'll find that the fast walk is more than you can reasonably do

    On dry land, under about 50 mph, double speed takes double hp.... in the water double speed might take 10x hp
    Kevin

    Quote Originally Posted by Tinnie
    But, where are the reevets?
    1981 Wellcraft V-20 CC Fisherman (FREE BOAT)
    1989 Wellcraft Monte Carlo 28(Build thread here)
    1992 Hotsports Jet'n'Cat Yam 701 WR3 drivetrain "SS George Washington"
    90's Yamaha WRIII and a pair of 95 Kawasaki ZXI-900's
    Fresh clean cheap oil is better then old dirty expensive oil any day

  4. #4
    Honorary Moderator Emeritus Don S's Avatar
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    Default Re: I/O vs. Outboard Fuel Consumption

    Gallons Per Hour (GPH) tells you nothing but how many gallons you used in an hour.
    For it to mean anything, you have to turn it into miles per gallon. If you burn 10 gph at 4000 rpm, that doesn't tell you much. But, if you go 40 miles in that hour at 10GPH, that is a lot better than if you only go 20 miles in that same hour at 10 GPH.

    Now you get to take a boat with an outboard at 10 GPH and see how many miles you get compared to a IO running 10 GPH.
    Oh, and different size engines on different sized boats and different rpm, all makes a huge difference in the MPG, while all still burn that same 10 GPH.

    Clear as Mud, right????
    Don S.


    Please, no PM's (Private Messages) regarding boat/engine problems.
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    Only forum/moderator issues will be answered in PM's.

  5. #5
    Rear Admiral MH Hawker's Avatar
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    Default Re: I/O vs. Outboard Fuel Consumption

    That`s one of those things you have to get past real quick. A boat will suck down a lot of fuel period. The bigger the motor the more it burns.
    James & Deb

    Bad Dog

    If it aint broke your not having enough fun

    my 1988 Riviera Cruzer pontoon rebuild
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  6. #6
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    Default Re: I/O vs. Outboard Fuel Consumption

    I usually say a marine engine uses fuel at a rate of "about" 10% of the hp at wot whether pushing a houseboat or a runabout.
    mpg is achieved by getting the best speed for the setup.

  7. #7
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    Default Re: I/O vs. Outboard Fuel Consumption

    Quote Originally Posted by smokeonthewater View Post


    On dry land, under about 50 mph, double speed takes double hp.... in the water double speed might take 10x hp
    Actually, the rules for aerodynamic drag and hydrodynamic drag on a planing hull are pretty much the same. Double the speed takes 4 times the HP.

  8. #8
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    Default Re: I/O vs. Outboard Fuel Consumption

    I do not think a passenger 5.0 L Chevy car engine can EVER come near a 5.0L Mercruiser engine on fuel consumption.

    Cars simply do not get a high enough LOADING of the engine long enough............The transmission would die.

    Boats do get the engine fully loaded.

    To answer your topic. The testing done by marine engine companies are PROBABLY as truthfull as automobile companies.

    Is a 2 stroke better or worse ? Who COULD perform a honest unbiased test ?

    2 strokes do NOT have a true camshaft. So they can not vary the valve timing for better power or mileage.............4 strokes can vary the camshaft timing. & most do.

  9. #9
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    Default Re: I/O vs. Outboard Fuel Consumption

    Quote Originally Posted by Mischief Managed View Post
    Actually, the rules for aerodynamic drag and hydrodynamic drag on a planing hull are pretty much the same. Double the speed takes 4 times the HP.
    Except at slower speeds the losses in the tires and transmission are bigger factors than aerodynamic drag. At some point the graphs cross over and aerodynamics become the major influence. (I'd have to guess its probably below 50 mph, but its going to be vehicle specific.)

  10. #10
    Admiral smokeonthewater's Avatar
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    Default Re: I/O vs. Outboard Fuel Consumption

    Quote Originally Posted by smokingcrater View Post
    Except at slower speeds the losses in the tires and transmission are bigger factors than aerodynamic drag. At some point the graphs cross over and aerodynamics become the major influence. (I'd have to guess its probably below 50 mph, but its going to be vehicle specific.)
    what he said.... from say 5 mph to 10 mph is a negligible difference in a car but in a boat it is HUGE
    Kevin

    Quote Originally Posted by Tinnie
    But, where are the reevets?
    1981 Wellcraft V-20 CC Fisherman (FREE BOAT)
    1989 Wellcraft Monte Carlo 28(Build thread here)
    1992 Hotsports Jet'n'Cat Yam 701 WR3 drivetrain "SS George Washington"
    90's Yamaha WRIII and a pair of 95 Kawasaki ZXI-900's
    Fresh clean cheap oil is better then old dirty expensive oil any day

  11. #11
    Admiral smokeonthewater's Avatar
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    Default Re: I/O vs. Outboard Fuel Consumption

    Quote Originally Posted by cyclops2 View Post
    2 strokes do NOT have a true camshaft. So they can not vary the valve timing for better power or mileage.............4 strokes can vary the camshaft timing. & most do.
    not entirely true... ever hear of a power valve? I don't know first hand of one on a marine engine but two stroke engines can vary "valve" timing

    So far I have only ever owned one 4 stroke engine with varible valve timing... it is in my bmw.... I've never even seen any boat up close with variable valve timing

    Anyhow for the O/P's question... there isn't enough difference from engine to engine w the same hp to get excited.... sure a brand new 4 stroke outboard with fuel injection will use less gas than a 20 year old 2 stroke or I/O of the same HP but the cost of the engine will far outweigh any fuel saving for about the first 80 years of ownership for the average user.
    Kevin

    Quote Originally Posted by Tinnie
    But, where are the reevets?
    1981 Wellcraft V-20 CC Fisherman (FREE BOAT)
    1989 Wellcraft Monte Carlo 28(Build thread here)
    1992 Hotsports Jet'n'Cat Yam 701 WR3 drivetrain "SS George Washington"
    90's Yamaha WRIII and a pair of 95 Kawasaki ZXI-900's
    Fresh clean cheap oil is better then old dirty expensive oil any day

  12. #12
    Vice Admiral Tail_Gunner's Avatar
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    Default Re: I/O vs. Outboard Fuel Consumption

    A few thing's to consider...a outboard engine has had a lot of RD that be research. Each mfg is highly competitive so the bench mark is always being moved and the price point..better econ and power yes. In additon the leg on a OB is far more hydrodynamic and can be placed to a Optimum setting to each individual boat. Combine those both and you have your answer..but at a price sometimes outrageous ..but that's a opinion.


    75 HP MERCURY OPTIMAX $6,875 200 HP MERCURY OPTIMAX COUNTER $13,425
    90 HP MERCURY OPTIMAX $7,325 225 HP PRO XS MERCURY OPTIMAX $14,850
    115 HP MERCURY OPTIMAX $7,650 225 HP PRO XS MERCURY OPTIMAX TORQUEMASTER 20" $14,900
    115 HP PRO XS MERCURY OPTIMAX $7,975 250 HP PRO XS MERCURY OPTIMAX 20" $15,650
    125 HP MERCURY OPTIMAX 20" $8,325 250 HP PRO XS MERCURY OPTIMAX TORQUEMASTER 20" $15,725
    125 HP MERCURY OPTIMAX 25" $8,550 250 HP MERCURY OPTIMAX 25" $15,675
    150 HP PRO XS MERCURY OPTIMAX $10,375 250 HP MERCURY OPTIMAX COUNTER 25" $15,825
    175 HP PRO XS MERCURY OPTIMAX $11,675 250 HP PRO XS MERCURY OPTIMAX 25" $15,775
    200 HP PRO XS MERCURY OPTIMAX 20" $12,450 250 HP MERCURY OPTIMAX 30" $16,075
    200 HP MERCURY OPTIMAX $13,075 250 HP MERCURY OPTIMAX COUNTER 30" $16,225.............................
    Stickly a opinion your milage may vary.

    Speed is how fast you hit the wall
    Torque is how far you move the wall"

  13. #13
    Admiral smokeonthewater's Avatar
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    Default Re: I/O vs. Outboard Fuel Consumption

    Yup... $15,000-$20,000 will buy a WHOLE FREAKING LOT of gas........
    Kevin

    Quote Originally Posted by Tinnie
    But, where are the reevets?
    1981 Wellcraft V-20 CC Fisherman (FREE BOAT)
    1989 Wellcraft Monte Carlo 28(Build thread here)
    1992 Hotsports Jet'n'Cat Yam 701 WR3 drivetrain "SS George Washington"
    90's Yamaha WRIII and a pair of 95 Kawasaki ZXI-900's
    Fresh clean cheap oil is better then old dirty expensive oil any day

  14. #14
    Seaman Apprentice
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    Default Re: I/O vs. Outboard Fuel Consumption

    First off I love the banter on these boards. Second off, comparing effeciency between say a '77 Johnson 70 and an '86 170 Mericruiser I understand that obviously the 70 will burn less fuel per hour. But, in the 80's did outboards not become more efficent? I feel that with the energy crisis of the late 70's people were more focused on more economic engines and less gas consumption coinciding with higher fuel prices. Also "Gallons Per Hour (GPH) tells you nothing buy how many gallons you used in an hour.
    For it to mean anything, you have to turn it into miles per gallon. If you burn 10 gph at 4000 rpm, that doesn't tell you much. But, if you go 40 miles in that hour at 10GPH, that is a lot better than if you only go 20 miles in that same hour at 10 GPH."
    I realize the difference between MPG and GPH, MPG is determined by the speed and efficency of the engine, and the efficency of the engine can effectively be rated by its GPH consumption. But this isn't an economic forum nor a pissing contest, so to cut to the point you're all saying that the 10% rule of thumb will still apply to an I/O regardless of whether it's a four-banger or a straight six? Sorry I'm 23 and have somehow developed this money-eating hobby.

  15. #15
    Admiral smokeonthewater's Avatar
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    Default Re: I/O vs. Outboard Fuel Consumption

    Sadly yes.... the engines got more efficient... that let a certain boat get 3 mpg instead of 2.8 mpg
    Kevin

    Quote Originally Posted by Tinnie
    But, where are the reevets?
    1981 Wellcraft V-20 CC Fisherman (FREE BOAT)
    1989 Wellcraft Monte Carlo 28(Build thread here)
    1992 Hotsports Jet'n'Cat Yam 701 WR3 drivetrain "SS George Washington"
    90's Yamaha WRIII and a pair of 95 Kawasaki ZXI-900's
    Fresh clean cheap oil is better then old dirty expensive oil any day

  16. #16
    Seaman Apprentice
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    Default Re: I/O vs. Outboard Fuel Consumption

    Lol, they got us there eh?

  17. #17
    Captain JoLin's Avatar
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    Default Re: I/O vs. Outboard Fuel Consumption

    Forget any comparison of a boat to a car. There's simply no commonality between the two. Having run three very different types of boats, the simple answer is that whatever you run you'll use a lot of fuel. Here are some real world examples (mine) with engines in good state of tune. My normal, comfortable cruise speed regardless of the boat I'm running always seems to be the same and is GPS-based, so it's a good basis for comparison. I never had a fuel flow meter so calculations are approximate, but they're very close. I'm kinda geeky and spent a lot of time on this :-)

    21' ProLine Walkaround cuddy with carbed 175 hp Johnson 2-stroke outboard
    - normal cruise speed app. 25 mph @ 4000 rpm
    - consumption at cruise 9-10 gph.

    24' Chris*Craft express cruiser with single fuel injected 5.7 V-8 (280 hp) and duo-prop I/O
    - normal cruise speed app. 25 mph @ 3200-3400 rpm
    - consumption at cruise 10-11 gph

    27' Four Winns express cruiser with dual fuel injected 4.3 V-6's (205 hp each) and single-prop I/O's
    - normal cruise speed app. 25 mph @ 3200-3400 rpm
    - consumption at cruise app. 16 gph

    The big surprise for me was when I went from 21' to 24'. The increased efficiency of the fuel injected 4-stroke and duo-prop outdrive in the bigger and much heavier boat made it a wash in terms of fuel cost. I spent months double checking my calculations.

    Hope this helps a little.
    John and Linda
    Escapade II
    1997 Four Winns 278 Vista
    Twin V-P 4.3 Gi w/SX Drives

  18. #18
    Admiral jigngrub's Avatar
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    Default Re: I/O vs. Outboard Fuel Consumption

    Quad carbed 50 hp Merc 4 stroke @ 32 mph divided by 5 gph = 6.4 mpg.
    I consider overkill a job well done... plus a little extra.

    There's as many ways to do something as there is people to tell you how to do it... but not all of them are the right way.

    The drinking and debauchery will continue until morale improves... and maybe for a little while after.

  19. #19
    Lieutenant coolbri70's Avatar
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    Default Re: I/O vs. Outboard Fuel Consumption

    fact, 2 stroke engines are not fuel effecient by design. fact, 4 stroke engines are heavy by design. in a boat weight cancels out hp and uses more fuel to do it so i'm thinking either way boating aint cheap

  20. #20
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    Default Re: I/O vs. Outboard Fuel Consumption

    Well we all seem to agree.

    Marine engines of all types are never going to deliver anything close to a car on MPG.

    They also have such horrible shapes that EAT horsepower.

    They usually are overloaded with people & gear having tons of fun. Which no car can come near to doing.

    So do not bother with a MPG test.

    If it carries your friends safely & laughter is plentifull ? The engine is delivering fantastic MPG.

    Rich

  21. #21
    Commander NHGuy's Avatar
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    Default Re: I/O vs. Outboard Fuel Consumption

    Cyclops hit it right on the head. In a boat or your cool Saturday night cruise car you are looking for smiles per gallon.

    ps. Happy Holidays & Merry Xmas
    1989 Baja Sunsport 196, Mercruiser 5.7, Alpha 1
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  22. #22
    Admiral dingbat's Avatar
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    Default Re: I/O vs. Outboard Fuel Consumption

    Fuel is the cheap part of boating. If you're worried about the fuel cost, you better look for a different hobby
    Grady White 226
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  23. #23
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    Default Re: I/O vs. Outboard Fuel Consumption

    Ding bat has me pegged.

    I use MAYBE 40 to 50 gallons of regular a year in the Chaparral SSI 5.0 L.
    The 16' Lowe with a 9.9 hp & electric start. 8 gallons a week. May 10 thru September 15. Weather permitting. No typing error.

  24. #24
    Admiral smokeonthewater's Avatar
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    Default Re: I/O vs. Outboard Fuel Consumption

    Quote Originally Posted by dingbat View Post
    Fuel is the cheap part of boating. If you're worried about the fuel cost, you better look for a different hobby
    Strange... fuel has always been the single biggest cost for me...... from my first 12' tinny 25 years ago to my 30' cruiser now, fuel has always been the one thing that hurts the wallet more than anything else.

    With my current boat, if I just ease out to the island that I can see from the ramp, and spend the day before easing back, it still costs me $50 and if I pull tubes it's $200- $500...... I pretty much never pull tubes.
    Kevin

    Quote Originally Posted by Tinnie
    But, where are the reevets?
    1981 Wellcraft V-20 CC Fisherman (FREE BOAT)
    1989 Wellcraft Monte Carlo 28(Build thread here)
    1992 Hotsports Jet'n'Cat Yam 701 WR3 drivetrain "SS George Washington"
    90's Yamaha WRIII and a pair of 95 Kawasaki ZXI-900's
    Fresh clean cheap oil is better then old dirty expensive oil any day

  25. #25
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    Default Re: I/O vs. Outboard Fuel Consumption

    Another thing, the discussion always centers on fuel/speed/distance/engine performance at WOT. That maybe what lake boaters do all day, but not open water boaters, especially not larger ones (because you are on bigger water; the 12' jon/9.9 will usually be run at top end).
    Just like cars, there is a fuel efficiency at lower speeds. but there is also the GPH factor where going too slow (car or boat) will burn more fuel for the miles than going faster. For small cars that speed is about 48 mph I read somewhere.

    So a recreational boater on a WOT lake may use more fuel per outing than one with basically the same boat on the bay.

    Even when it's flat I don't usually run WOT; the motor is loud and I feel like it's working too hard (tach says it's not). An OB has a sweet spot where the motor sounds good and pushes you along smooth quick adn quiet. Cars are like that, too.

    I'm not smart enough to figure out if my 150 HP running at 3/4 throttle for 30 mph uses the same fuel as a 115 at WOT at 30 mph. That's not what I have a boat for; it's what I have a boat to get away from.
    A man of constant boat tinkering.

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