Gear Box Selection

Discussion in 'Powerboats' started by MetalHead, May 29, 2012.

  1. MetalHead
    Joined: Feb 2011
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    Location: Alabama USA

    MetalHead Junior Member

    I have read until my eyes are crossed but cant seem to find what I am looking for.

    Here is my boat

    1959 Marinette - http://www.youtube.com/watch?v=Glfu92mZqX4

    Photos - https://skydrive.live.com/redir?resid=E1FDEA11A9AE4258!171&authkey=!AKJId6PKkZDQ1sc

    It currently has a 109 inline 6 Graymarine engine 1:1 ratio transmission.

    I am repowering with a 165 HP Mercruiser L6 (Straight six). Old Gray runs great, but I want better service parts options and will sell the Graymarine 6 after repower to Merc is completed.

    Boat weighs in at about 6000 lbs (Aluminum Construction)

    WOT on the Merc is 4200 but cruise RPM is more like 3000 rpm.

    I have a 1.52:1 Velvet Drive that I got becuase it has all the hardware (Bellhousing , tranny mounts, cooler etc) and I have been trying to figure out if this is a good option for this boat. I was looking at going back with a 1:1 Velvet Drive, but I did not know how the whole shaft RPM/Prop thing had to be calculated.

    Boat Specs
    28 Feet
    9 feet at widest (Front of Cabin area)
    8 feet and the transom.

    Draft (Prop and rudder included) 18 Inches (Basically a flat bottom).

    Planning Hull
    6000 lbs.

    I am looking for fuel economy and will not run the boat WOT all the time.

    Some reading I have been doing puts the L6 in the sweet spot in power/torque at 3000 RPM.

    I know a 1.52:1 will work, but run slower. But if I can have more speed at the same fuel burn rate would the 1:1 be better.

    I am willing to adjust the prop as needed, but I think at this point it is a 13.5x15 prop.

    Thanks for the input.
     
  2. daiquiri
    Joined: May 2004
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    Location: Italy (Garda Lake) and Croatia (Istria)

    daiquiri Engineering and Design

    I'm affraid more info is needed...
    1. What prop do you currently have installed?
    2. What maximum and cruise speeds do you attain and at what RPMs?
    3. Is 6000 lbs the dry weight of the boat or what...?
    Cheers
     
  3. bertho
    Joined: Aug 2006
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    Location: asean archipelago

    bertho bertho

    don't forgot to check the size/quality of your shaft, to increase propeller size/torque is limited by the existing shaft..
    cheer's
    bertho
     
  4. MetalHead
    Joined: Feb 2011
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    Location: Alabama USA

    MetalHead Junior Member

    Shaft size is 1 inch. I do not know the cruise speed as I have never had the boat in the water.

    Others have the 28 foot Marinette at over 30 mph at WOT. But that is on twin engines 225 Crusaders.

    WOT on this motor is 4200.

    Prop size is listed above, but I will try to see if I can get a number off of it tomorrow.

    Using this

    http://www.rbbi.com/folders/prop/propcalc.htm

    I get an unrealistic number using

    RPM - 4200 (WOT) - But I want to run at 3000 which seems to be the peak of the power/torqe curve on the L6
    Ratio - 1.0
    Pitch - 15
    Slip - .15

    Gives me a speed of 50 mph but I am sure this is not possible given 165 HP rating of the engine.
     
  5. CDK
    Joined: Aug 2007
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    Location: Adriatic sea

    CDK retired engineer

    Propcalc.htm is not useful because it ignores prop diameter, boat weight, engine HP and hull shape.
    You could use it only to determine prop slip if speed, rpm and pitch are known.
     
  6. baeckmo
    Joined: Jun 2009
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    Location: Sweden

    baeckmo Hydrodynamics

    It seems the shaft is the limitation now. If the ol'Gray is 109 hp and pulling a 13.5" propeller, the rotational speed is roughly ~3000 rpm, which is possible with AISI 431 material.

    With the 3 tonne boat you would be better off with the Merc engine plus the 1.52 gear and a 15" propeller. BUT: that requires a stronger shaft material than you probably have now. Change to a Duplex 4462 ("Remanite") shaft; then you can keep the 1" shaft diameter in spite of the increased torque.
     
  7. MetalHead
    Joined: Feb 2011
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    Location: Alabama USA

    MetalHead Junior Member

    But what speeds could I expect from a 3000 rpm motor, 15x15 prop and a 1.52:1 transmission? That is about 1974 Propshaft RPM.

    Again WOT on the motor is 4200 but the power curve peaks at about 3000.
     
  8. MetalHead
    Joined: Feb 2011
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    Location: Alabama USA

    MetalHead Junior Member

    Ok found these markings on the prop

    Columbia D13 x P12 R (I think the last letter was an R)

    Hydroflow (Or hydro something)

    Was a Graymarine a Clockwise engine?
     
  9. CDK
    Joined: Aug 2007
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    Location: Adriatic sea

    CDK retired engineer

    The difference in output doesn't translate into a higher speed, just the same speed at a lower throttle setting, which is a good thing if you aim for better fuel economy.
    For higher than hull speed, you need twice the power of your Mercruiser engine.
     
  10. FAST FRED
    Joined: Oct 2002
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    FAST FRED Senior Member

    Better look up the ability of a 1 inch shaft to handle power.

    A modern engine larger than stock with a reduction gear may require a very different shaft.

    FF
     
  11. MetalHead
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    Location: Alabama USA

    MetalHead Junior Member

    The boat really isnt for speed anyway. I was just looking to find out given what I have , which transmission would be best for the size boat I have.

    Graymarine is a 109 and the Mercruiser is a 165

    Can 1974 shaft RPM (I'll figure out how large it needs to be next) push a 6000 lb boat 25 to 30 mph ?

    Also with a practically flat bottom from midpoint back, what speed would make that hull plane?
     
  12. baeckmo
    Joined: Jun 2009
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    baeckmo Hydrodynamics

    The prop data you gave sounds more reasonable than the 13,5 x 15 you mentioned before!

    Unfortunately you get things all mixed up; the rpm's is not what pushes your boat, it is ALL ABOUT POWER! There are three limiting factors:

    1/ The torque your shaft can tolerate. See my note above.
    2/ The power/weight ratio (and bottom shape) gives the possible velocity. In your case, something like 20 - 22 knots (provided the bottom shape and center of mass is reasonable).
    3/ The propeller disc area must match the power at this velocity. More Hp per disc area leads to reduced efficiency AND possible thrust breakdown due to cavitation.

    So:
    1/ If you want to stick to the 1" shaft diameter with increased power, you have to use a stronger shaft material.

    2/ Unless you limit the power output of the new engine to 109 hp @ 3000 rpm, the operating point will differ.

    3/ You can NOT use 1:1 with 4200 rpm, unless you reduce the prop diameter, and if you do so, the prop can NOT handle the power; speed goes down and fuel bills go up.

    If you want to stick with the shaft and prop as is, then your only option is to lock the throttle to give ~3000 rpm with the existing components. But since you have a reduction gear available, you will have MUCH better performance and mileage if you invest in a stronger shaft and a new propeller to fit the new engine.

    Remember that the cost of a propeller is a small fraction of your fuel costs; a good, correct propeller pays for itself in reduced fuel costs in short time!


    NOTE!
    You wrote that the Merc has its power peak at 3000 rpm; I doubt that, knowing the max rpms are 4200. That would turn your existing shaft into a corkscrew, even with a 1:1 transmission. Again, if you want to use the existing shaft and propeller, you have to lock the output of the new engine to 109 hp @ 3000 rpm.
     
  13. FAST FRED
    Joined: Oct 2002
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    FAST FRED Senior Member

    you will have MUCH better performance and mileage if you invest in a stronger shaft and a new propeller to fit the new engine.

    The prop will probably be larger in diameter , so it wont simply drop in.

    FF
     
  14. MetalHead
    Joined: Feb 2011
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    MetalHead Junior Member

    I am willing to do that. Shaft and prop. But I am still trying to figure out if I should go 1:1 or 1.52:1 .
     

  15. Carteret
    Joined: Jan 2004
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    Carteret Senior Member

    based on the info you provided, boatdiesel.com's calculator states that a boxed wheel, ie a 13x13 propeller is the most efficient with a 1 to 1 ratio gear for your hull. I would size up the shaft if I were you. It is the wise thing to do.
     
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