Prop question...

Discussion in 'Sterndrives' started by Ambitious, Oct 3, 2009.

  1. Ambitious
    Joined: Jan 2009
    Posts: 23
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    Location: Serbia, Belgrade

    Ambitious Naval Architect

    Dear all,

    We have just finished testing of our 32 feet mahogany boat.

    Lwl=7.7 m
    Bwl=2.5 m
    D=4.4 t

    Engines&propulsion:
    2 x yanmar 6byz 260hp@4000 rpm
    bravo one diesel X. ratio 1.5
    prop: mirage plus 14 3/4" X 21 "

    Top speed is 42 kn at 4080 rpm with three persons on board.

    I think props might have slightly higher pitch. So we can match 4000 rpm.
    What do you think about 4 or 5 blade props maybe?

    thanks
     

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  2. dwwcanada
    Joined: Feb 2010
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    Location: Ontario, Canada

    dwwcanada New Member

    Prop /RPM

    It seems you have matched your WOT RPM for that engine. Going to a higher pitch will most likely drop you below the recommended 4000 RPM. You can go for the Mercury HighFive Stainless Steel prop, 5-blade designed to replace any Quicksilver/Merc prop pitch for pitch. The 5-blade will reduce vibration at top speed, and great in rough water.
     
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  3. Jango
    Joined: Aug 2005
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    Location: Mid Atlantic

    Jango Senior Enthusiast

    You are getting about 10% slip with your present 14 3/4 x 21P. PROP which is very good for the speed (42K) and weight of your boat. Increasing Pitch will decrease RPM as previously mentioned and is probably not advised. Going to a 4 blade prop will decrease Slip slightly but will probably drop motor speed by 200 - 300 RPM,s. As long as Hole Shot and Acceleration is adequate, I would stay with your present Prop.

    Jango
     
  4. gonzo
    Joined: Aug 2002
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    Location: Milwaukee, WI

    gonzo Senior Member

    You seem to be right on the money. Any modifications would only be to get lower speed and better economy.
     
  5. Ambitious
    Joined: Jan 2009
    Posts: 23
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    Location: Serbia, Belgrade

    Ambitious Naval Architect

    Thanks for reply.
    If I get the point you think that we have a good combination at the moment regarding engine+drive+prop.
    I would be happy to get some better acceleration and to keep top speed. Is it doable with 4 or 5 blade prop with same diameter and pitch?

    Thanks again
     
  6. Jango
    Joined: Aug 2005
    Posts: 519
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    Location: Mid Atlantic

    Jango Senior Enthusiast

    A 4 or 5 Blade Prop will decrease engine RPM. Max Speed will be reduced, since your current Slip is Low. The Only way to increase Acceleration is to decrease Pitch which will most likely also decrease Top Speed

    Jango
     
  7. Frosty

    Frosty Previous Member

    That extra 80 RPM will soon be eaten up by the extra weight that you will no doubt put on it now that it is finished.

    For instance were you fully fuel'ed and watered.

    As said you are spot on there.
     

  8. Ambitious
    Joined: Jan 2009
    Posts: 23
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    Location: Serbia, Belgrade

    Ambitious Naval Architect

    We were fully loaded with four persons on board. But you`re probably right because hull surface was perfect clear. After couple of days in water hull will become dirty.....
     
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