Help with prop; major difference in experts

Discussion in 'Props' started by Bruce P, Jan 11, 2011.

  1. Bruce P
    Joined: Oct 2010
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    Location: Alaska

    Bruce P Junior Member

    Hello. I am repowering my 32' cruiser (planing hull) with twin Mercruiser 5.0 MPI 260hp (4800 rpm) with Bravo 2 outdrives with a 2.2 ratio. Design displacement is 14,000 lbs.

    Using the most excellent Propeller Handbook by Dave Gerr and using the "Crouch's Propeller Method" I get an ideal prop of 19" dia x 20.6" pitch. This is assuming Gerr's suggested 24% slip (is this too much?)

    The online Mercruiser Prop Selector selects both 17.5 x 23 or 19.5 x 22.5

    The Castle Marine propeller software says to use a 19 x 16.4, but adds that it is using a calculated 15% slip.

    Wow. A lot of answers out there. Are there any experts here that could shed some light on this? I gues if I get no more input I would probably pick 19 x 21 to start.

    Thanks for any input - Bruce
     
  2. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    Most shops will let you try out a set of props, knowing you'll want to pull the recommended RPM. Try the 19.5 x 21 and see what it pulls. Prop shops know there's a fair bit of black magic to prop selection and have a good bit of latitude in this regard.
     
  3. michael pierzga
    Joined: Dec 2008
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    Location: spain

    michael pierzga Senior Member

    Who Knows ? I would suspect that the majority of boats are operating the wrong size, pitch prop.
    Most of the yachts Ive operated over the past thirty years have used variable pitch propellers or Controllable pitch propellors. part of the commissioning regime for these props is to determine, then set the maximum allowable pitch of the propeller. We do this by measuring the exhaust temp. Engine rpms are not reliable...is your tachometer correct...is your throttle adjusted to hit its stops ? can your engine installation achieve full rpm ?
    If possible borrow an exhaust temperature pyrometer from a marine engineering , engine shop then physically test your chosen prop size against maximum exhaust temp. Increase the prop size until optimum exhaust temp is achieved.


    A good description of the technique is found here

    http://www.kastenmarine.com/CPprops.htm
     
  4. Ad Hoc
    Joined: Oct 2008
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    Ad Hoc Naval Architect

    without knowing too much of your hull and if your figures are correct...you should get around 26-27knots.
    At this speed...and your engine HP and ratio...something around 18/19" diameter with a 18/19" pitch. Start with this ball park..and either go up or down in pitch...depending what your engine is doing!
     
  5. Bruce P
    Joined: Oct 2010
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    Location: Alaska

    Bruce P Junior Member

    Thanks all, I do appreciate the responses. Hope I can return the favor some day.

    Bruce
     
  6. gonzo
    Joined: Aug 2002
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    gonzo Senior Member

    21 seems quite high for the boat. In that size range 17-19 is more common. Go to the local marina and check out similar boats.
     
  7. FAST FRED
    Joined: Oct 2002
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    Location: Conn in summers , Ortona FL in winter , with big d

    FAST FRED Senior Member

    The real place to start would be to have the boat weighed in as used condition.

    FF
     
  8. Jango
    Joined: Aug 2005
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    Jango Senior Enthusiast

    The Difference between Crouchs calc. and Castle Marine is 4in of Pitch.(20 vrs 16) Both have 19 in Dia.

    Engine manf' typically rate their engines HP a little High, plus your Boat could very well be Heavier.

    I would start with the 19 x 16,s. If your engines over rev, Increase Pitch etc.

    Increasing Dia should only be necessary to decrease slip, if boat is heavier
     
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  9. yipster
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    yipster designer

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  10. ocean44
    Joined: Jan 2011
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    Location: va

    ocean44 New Member

    Bruce Mercury offers a 18.5 x 19 alum. Based on the sze of the boat It should be close.The only way to no for sure is load the boat and run WOT Should be around 4800-5200
     
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