repowering to diesel & im getting wishy washy info.

Discussion in 'Powerboats' started by aitkenkingfish, Aug 22, 2011.

  1. aitkenkingfish
    Joined: Apr 2009
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    aitkenkingfish Junior Member

    This may be my fault by not including the info needed but so correct me if im wrong.

    project is a 1985 bayliner trophy 2460 23.6 feet.

    came with a 225 volvo gm 350 small block. outdrive is a volvo 280 single prop

    I have the chance to power with a megatech 4 or 6 cyl.

    4cyl 190 hp@ 3900 rpm 694 lbs
    6cyl 285 hp2 3600 rpm 870lbs

    I can run the stock outdrive or swap it out looks like a duoprop swap would give the best gains but id like to start with the stock outdrive if possible & run a larger prop with more pitch.

    im getting numbers of 30knot cruise using the 4 cyl does that seem correct? ive also got half the people saying the 4 cyl will be perfect & half saying I have to use the 6 cyl or it will be underpowered. I had an underpowered boat before & definatly dont want to end up there again. any help would be appriciated.
     
  2. aitkenkingfish
    Joined: Apr 2009
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    aitkenkingfish Junior Member

    anyone know about diesel convertions?
     
  3. aitkenkingfish
    Joined: Apr 2009
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    aitkenkingfish Junior Member

    I like the design it fits my needs & I have no plans of selling it for the next 10 years. so 8 grand for a diesel conversion sounds like money well spent. 10 years 2 120 mile trips a week

    well if i did my math right 2 trips per week 120 miles doesnt include moving around or trolling. fuel costs
    diesel would run $8611.20
    gas would run $20,020 if i did the math correct it would pay for itself in the first year.
    diesel 4.6 gph
    gas 11 gph
    416 hours based on a cruise of 30 for the deisel
    520 hours based on aq cruise of 24 for gas
    1913.6 diesel at $4.50= $8611.20
    5720 gas at $3.50=20,020.00
     
  4. CDK
    Joined: Aug 2007
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    Location: Adriatic sea

    CDK retired engineer

    You're at a disadvantage because you live in a country where they sell you the cheaper diesel fuel at a higher price. In most parts of the world diesel is cheaper, even more so when it is dyed red.

    The engine sure is more economical than the GM 350, but not as dramatic as they want you to believe. With a state of the art diesel 4.6 gallons will provide you with approx 75 hp. That may be just enough to keep your boat planing, but certainly not at 30 knots, because you'll need almost full throttle for that.

    It still may be a wise decision if you look at 10 years of safely and dependability. I also expect the price of gas to rise faster in the future.
     
  5. FMS
    Joined: Jul 2011
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    FMS Senior Member

    How goes it aitkenkingfish?

    I would like to know how you have arrived at your gph and cruise speed numbers.

    I was starting to post the numbers in post 3 above are overly-enthusiastic, but then read CDK expressed the same already. You may likely get 30 or 40% more out of diesel than gas, but in post 3 you estimate a 59% fuel savings in gph AND 25% more speed at the same time? How?

    In any case, would like to hear more about your conversion. Where are you now with it now?
     
  6. gonzo
    Joined: Aug 2002
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    gonzo Senior Member

    You couldn't run the stock outdrive with any of the setups you mention. The gear ratio needs to be changed. A 350 longblock is $2600.00 with a two year warranty. It will cost yo about that to regear.
     
  7. Quatsino Boater
    Joined: Feb 2011
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    Location: Port alice BC

    Quatsino Boater Junior Member

    Hi Aitkenkingfish, I have a 23 foot Campion sedan( Hard top and enclosed alaska bulkhead). Mine is powered by a Volvo 4.3 gl with a 1.7:1 sx drive. the volvo is a 6 cyl GM . The boat is around 4392 lb unloaded and around 4600 lb with fuel and gear. I get around 30 knots at 4300-4500 rpm with a 15x19 prop. This is with the sx leg all the way down and my Bennet trim tabs tweeked.

    I see some pretty big waves /swells up here ( 6 to 9 foot) in the open Pacific. Getting up on plane fast is a must if you are going to plane the boat at a 45 degrees to the waves or ride in the troughs. Running 10 to 15 miles offshore the sea state can change dramatically in as little as a half hour so you need to head back in in a hurry. I definately would not go to a 4 cyl.

    On a side note they recomend my boat should have 225hp max. They do manufacture my boat now with an option of a cummins deisel and a bracket set up for outbards as well. I often thought of the bracket idea for more cockpit space for fishing.
     
  8. aitkenkingfish
    Joined: Apr 2009
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    aitkenkingfish Junior Member

    sorry guys the posts have been going to junk mail. well I purchased the 6cyl on friday I can machine an adapter plate to except the flywheel off my 5.7 volvo which will except the volvo lower unit. Im considering changing my 280 single prop no trim lower unit to a newer duoprop with trim. the numbers came from gas with single prop diesel with duoprop i think the percent differnece is the difference between the single prop & duoprop drive. the new duoprop is a 178-1 ratio when I enter this in a prop calculator it keep telling me a lower ratio will be more efficiant. Im not sure i understand the diesel will turn slower which makes me think i want a higher ratio to spin the prop quicker?
     
  9. aitkenkingfish
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    aitkenkingfish Junior Member

    Quatsino Boater is that a gas motor?
     
  10. gonzo
    Joined: Aug 2002
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    gonzo Senior Member

    The higher the ratio, the more reduction and the slower the propeller will turn. A diesel turns slower than a gas engine and the formula will give you a ridiculously high pitch with the same gears.
     
  11. aitkenkingfish
    Joined: Apr 2009
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    Location: jacksonville florida

    aitkenkingfish Junior Member

    thanks gonzo now it makes sense I had it backwards.
     

  12. Quatsino Boater
    Joined: Feb 2011
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    Quatsino Boater Junior Member

    Yes that is the stock factory set up. 2000 -Volvo penta 4.3GL gas engine and a Volvo penta SX 1.7:1 single prop.
     
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