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    Hull speed fuel economy

    IN GENERAL, with a planing hull or semi planing hull, with 2 identical cruisers - one with gas power and the other with diesel power,,,,,
    AT HULL SPEED, would these two boats use about the same GPH?
    John
    1986 Marinette 39 Sedan after enjoying our 1984 Marinette 28
    (formerly 89 2755 5.8 OMC & 97 1850SS 4.3L Alpha I)

    #2
    That’s kind of like asking if a Ford or Chev is faster. There are multiple factors involved. However, if both boats were identical, with identical loads, and properly powered, I would expect the diesel boat to burn less fuel. For instance, a 38xx with gas engines at 8 knots would probably burn around 8-9 gallons per hour, the diesel version burns about 4. I expect some of the owners of gas powered 38’s can shed more light on their burn.
    If you are looking at just fuel burn, you are going to be missing the real cost comparison. Diesel boat insurance is a lot less than a gas boat, diesels have much larger oil pans and less common filters, so an oil change is much more expensive, coolant systems are larger, parts, although needed less often are more expensive. Fuel per gallon is cheaper for diesel. If diesel gets on you clothes, I’ve heard the smell can be washed out, but I’ve not had much luck with that.
    Bottom line, if you are trying to save money buying a new boat and have the choice of gas or diesel, you will never recover the cost difference of going to diesel. It will probably make the boat more attractive to a prospective buyer when that time comes. In 2017, we put 180 hours on our 3818 with 175 Hino’s, my moorage cost was more than my fuel cost.
    P/C Pete
    Edmonds Yacht Club (Commodore 1993)
    1988 3818 "GLAUBEN”
    Hino EH700 175 Onan MDKD Genset
    1980 Encounter Sunbridge "Misty Blue" (Sold)
    MMSI 367770440
    1972 Chevrolet Nova Frame off Resto-mod in the garage
    Boating on the Salish Sea since 1948

    Comment


      #3
      The diesel would burn less fuel. Diesel has a higher calorific value than petrol. Diesel burns slower but each drop contains more energy than gas. Consequently, it uses fewer drops per mile.
      Terry (Retired Diving Instructor and Part Time IT Consultant)
      1998 Bayliner 2452. 5.7l V8 - Edelbrock 1409 4bbl - Alpha1Gen2 - Solent UK.
      MMSI 235061726

      Comment


        #4
        I have 2 GM 350 crate engines, when running on one engine no tide flow, no noticeable wind I run an average of 6.5 knots sometimes 7 knots and get by flow scan 1.8 sometimes 2 MPG.
        I had my 4 blade DJX Nibrals tuned for the 2 different ratios of the BW gears. ( I paid 150 for both new Nibrals, high bidder on ebay.)
        My 3 blade spares Nibral 3 blade, $175.00 pair. I love it when they forget to put a minimum sale price on ebay.
        Using 2 engines at displacement speed is a waste of fuel, if you have gears that you can do that with or have a shaft lock.
        Pat says: DO-IT-RIGHT THE FIRST TIME!

        Bayliner 3870 "ALASKA33)
        Twin 350 GM power
        Located in Seward, AK
        Retired marine surveyor

        Comment


          #5
          Not a direct answer but my 3870 gets almost 3 times the mileage running on one engine than my 2950 twin running on one engine. My thought are the diesel by far.
          Btw the best economy is below hull speed. Watch your wake. When you see white water in it, you are pushing water and burning more fuel.
          Doug
          Started boating 1955
          Number of boats owned 32
          Bayliners
          2655
          2755
          2850
          3870 presently owned
          Favorite boat. Toss up. 46' Chris Craft, 3870 Bayliner

          Comment


            #6
            If you want to get rid of that white water.rooster tail. think about adding a pair of tunnel extensions.
            Some have made their own out of SS and one I know of out of fiberglass.
            As soon as the cold streak here in Florida ends I will finish one pair of tunnels for one buyer, but I have 2 more to gel coat that will be available, they work!
            Some say they want to know what the fuel economy is, when you gain 1.5 to 3 knots at fast cruise at the same RPM, that is the fuel economy

            Last edited by boatworkfl; 03-11-2018, 09:03 AM. Reason: add photo of prop pocket tunnel extension
            Pat says: DO-IT-RIGHT THE FIRST TIME!

            Bayliner 3870 "ALASKA33)
            Twin 350 GM power
            Located in Seward, AK
            Retired marine surveyor

            Comment


              #7
              Originally posted by dmcb View Post
              Not a direct answer but my 3870 gets almost 3 times the mileage running on one engine than my 2950 twin running on one engine. My thought are the diesel by far.
              Btw the best economy is below hull speed. Watch your wake. When you see white water in it, you are pushing water and burning more fuel.
              Doug
              Doug,
              So at hull speed or below, you got almost 3 times the fuel mileage with the one diesel over the gas?

              Thank you for the input.
              John
              1986 Marinette 39 Sedan after enjoying our 1984 Marinette 28
              (formerly 89 2755 5.8 OMC & 97 1850SS 4.3L Alpha I)

              Comment


                #8
                1. Diesel as a fuel has higher energy content than Petrol. Diesel contains 38.6 Mega Joules per litre of energy as compared to 34.8 MJ per litre of petrol. So, each litre of diesel produces more energy than an equivalent litre of petrol. This would mean you need to use less diesel to achieve a certain amount of power than you would with petrol. This amount is typically about 25% lesser diesel than equivalent petrol.
                2. Diesel engines can operate at higher compression ratio around 17-21 . While petrol engines can operate only at compression ratio around 8-12. The efficiency of engine is directly proportional to compression ratio. The high compression ratio in a diesel engine means that it needs heavier parts and longer pistons and connecting rods to compress air better. It has a higher thermal efficiency than a petrol engine. This increased length of pistons also means it has a better mechanical advantage than a petrol engine and develops higher torque. Torque is the turning force that the engine generates – and since more of this comes at a lower rpm in a diesel engine, it is easier to move away without revving the engine too much and this also saves fuel.
                3. Diesel can auto ignite at high temperatures and this is the principle on which a diesel engine works. When this temperature inside the cylinder in an engine goes higher than 210 degrees centigrade (the auto ignition temperature of diesel), a small spray of diesel is injected into the cylinder which then ignites spontaneously. As in diesel engines, diesel is minutely directly sprayed into a cylinder, lesser fuel is used. Petrol GDi (Gasoline Direct Injection) engines also have direct fuel injection in the combustion chamber, but most petrol engines are MPFi (Multi Point Fuel Injection).
                4. Diesel has cetane (six-sided) rings in its makeup instead of octane (eight-sided hydrocarbon rings), so its burning characteristics are better – it is more slow burning, allowing for a longer burn time and better efficiency. This is also the reason why diesel engines don’t reach high rpm ranges.
                Terry (Retired Diving Instructor and Part Time IT Consultant)
                1998 Bayliner 2452. 5.7l V8 - Edelbrock 1409 4bbl - Alpha1Gen2 - Solent UK.
                MMSI 235061726

                Comment


                  #9
                  Originally posted by shade2u2 View Post

                  Doug,
                  So at hull speed or below, you got almost 3 times the fuel mileage with the one diesel over the gas?

                  Thank you for the input.
                  I get on one gas engine at hull speed 1.8 to 2 mpg, no diesel can get 3x that.
                  He is comparing his diesels to his 29' gas boat, the 38 has more flotation than the 29' boat..
                  After I install my tunnels I will break the 2 gallons per mile number at displacement speed on 1 engine.
                  It will be interesting to find out what the gain will be for my gas 3870 compared to the diesel 3870 that I will help install tunnels on in Alaska this spring and do a test run.
                  I will post the data.
                  I wish this extra cold weather would get back to normal here in Florida, cannot layup fiberglass when it is this cold.

                  I made an error, I was getting 1.8 + miles per gallon on one engine
                  Last edited by boatworkfl; 01-20-2018, 05:27 PM. Reason: error
                  Pat says: DO-IT-RIGHT THE FIRST TIME!

                  Bayliner 3870 "ALASKA33)
                  Twin 350 GM power
                  Located in Seward, AK
                  Retired marine surveyor

                  Comment


                    #10
                    Apples to apples the diesel engine will burn less fuel at any speed, but at hull speed the number is much more significant.


                    KEVIN SANDERS
                    4788 LISAS WAY - SEWARD ALASKA

                    Comment


                      #11
                      I have a 32 foot with a planning hull with 350's inboards. I get 2.1 mpg at about 7 mph.

                      The bigger the boat the more economical diesel will be.
                      1989 Avanti 3450 Sunbridge
                      twin 454's
                      MV Mar-Y-Sol
                      1979 Bayliner Conquest 3150 hardtop ocean express.
                      Twin chevy 350's inboard
                      Ben- Jamin
                      spokane Washington

                      Comment


                        #12
                        I have never tried to compare gas to diesel.
                        If I had the money I would have 2 300 hp Perkins in my boat.
                        that being said, my gas engines take me anywhere I need to go, a tune up on my gas engines is a pittance compared to a diesel.
                        I can buy a new long block crate engine for $2200.00. I paid $1100.00 for my 3870 from the ins. company, stripped it, re-wired it completely and I keep working on it, it works for me
                        I paid $2800 for 2 gas engines and all brand new manifolds, the engines have not yet needed to add oil between changes.
                        There some diesel engines that I would rather have gas engines than diesel. Some Bayliner owners like their gas powered 38XX Bayliners.
                        It is all about what you are willing to pay for. Bayliner sold gas engine 38XX boats to those that wanted them.
                        I do not knock diesels, but there is a market for both, and they both make a boat go.
                        Pat says: DO-IT-RIGHT THE FIRST TIME!

                        Bayliner 3870 "ALASKA33)
                        Twin 350 GM power
                        Located in Seward, AK
                        Retired marine surveyor

                        Comment


                          #13
                          Originally posted by boatworkfl View Post
                          I have never tried to compare gas to diesel.
                          If I had the money I would have 2 300 hp Perkins in my boat.
                          that being said, my gas engines take me anywhere I need to go, a tune up on my gas engines is a pittance compared to a diesel.
                          I can buy a new long block crate engine for $2200.00. I paid $1100.00 for my 3870 from the ins. company, stripped it, re-wired it completely and I keep working on it, it works for me
                          I paid $2800 for 2 gas engines and all brand new manifolds, the engines have not yet needed to add oil between changes.
                          There some diesel engines that I would rather have gas engines than diesel. Some Bayliner owners like their gas powered 38XX Bayliners.
                          It is all about what you are willing to pay for. Bayliner sold gas engine 38XX boats to those that wanted them.
                          I do not knock diesels, but there is a market for both, and they both make a boat go.
                          I've also had both gas and diesel and I prefer gas for various reasons. Unless it's a long distance cruiser it my opinion gas is probably better. Simple to work on and cheap. For example needed.a alternator on my 330 Cummins on my 3988. Just the rebuilt alternator was $500 that was 15 Years ago.
                          1989 Avanti 3450 Sunbridge
                          twin 454's
                          MV Mar-Y-Sol
                          1979 Bayliner Conquest 3150 hardtop ocean express.
                          Twin chevy 350's inboard
                          Ben- Jamin
                          spokane Washington

                          Comment


                            #14
                            Originally posted by shade2u2 View Post

                            Doug,
                            So at hull speed or below, you got almost 3 times the fuel mileage with the one diesel over the gas?

                            Thank you for the input.
                            Yes. Both boats have twins but my remarks are running on one engine on each boat.

                            Pat, the white water I am talking about isn't the rooster tail from prop tunnels. It is the water along side the boat than all boats have. And I can more than double your best mileage indicated.
                            Gas vs diesel may be closer with a fuel injected gasser. Don't know that but suspect it would. At slow speed a carb engine isn't the most efficient invention ever.
                            Doug
                            Started boating 1955
                            Number of boats owned 32
                            Bayliners
                            2655
                            2755
                            2850
                            3870 presently owned
                            Favorite boat. Toss up. 46' Chris Craft, 3870 Bayliner

                            Comment


                              #15
                              Originally posted by yachtman View Post

                              I've also had both gas and diesel and I prefer gas for various reasons. Unless it's a long distance cruiser it my opinion gas is probably better. Simple to work on and cheap. For example needed.a alternator on my 330 Cummins on my 3988. Just the rebuilt alternator was $500 that was 15 Years ago.
                              I just bought 3 Delco 10SI 110 amp alternators for about $60.00 each
                              Pat says: DO-IT-RIGHT THE FIRST TIME!

                              Bayliner 3870 "ALASKA33)
                              Twin 350 GM power
                              Located in Seward, AK
                              Retired marine surveyor

                              Comment

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