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I/O vs. Outboard Fuel Consumption

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  • 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
    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, Orange County NY
    '88 Bayliner 1700 Capri Bowrider, 85 HP Force O/B, "Sea Weasel"
    Want a vessel safety check? Click here. Want to join the Coast Guard Auxiliary? Click here.
    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.

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    • #3
      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

      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
      sigpic

      Comment



      • #4
        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.

        sigpic

        Please, no PM's (Private Messages) regarding boat/engine problems.
        That is what the forums are for.
        Only forum/moderator issues will be answered in PM's.

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        • #5
          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.
          sigpicJames & Deb

          Bad Dog

          If it aint broke your not having enough fun

          my 1988 Riviera Cruzer pontoon rebuild
          http://forums.iboats.com/showthread.php?t=526820

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          • #6
            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.

            Comment



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

              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.

              Comment



              • #8
                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.

                Comment



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

                  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.)

                  Comment



                  • #10
                    Re: I/O vs. Outboard Fuel Consumption

                    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

                    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
                    sigpic

                    Comment



                    • #11
                      Re: I/O vs. Outboard Fuel Consumption

                      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

                      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
                      sigpic

                      Comment



                      • #12
                        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"

                        Comment



                        • #13
                          Re: I/O vs. Outboard Fuel Consumption

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

                          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
                          sigpic

                          Comment



                          • #14
                            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.

                            Comment



                            • #15
                              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

                              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
                              sigpic

                              Comment

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