Propellor sizing

Discussion in 'Props' started by corglass, Jun 6, 2010.

  1. corglass
    Joined: Jun 2010
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    corglass Junior Member

    Hi there,

    This is my first ever post so please be kind.

    I'm posting from sunny Ireland btw.

    I'm currently building a 19 Glastron speedboat that will be powered by a volvo 940 2.4 6 cly turbo diesel engine (140hp) via an Alpha one (Gen II) stern drive.

    I've never really done anything like this before so I've some questions. The boat will only be use on freshwater btw. I want help sizing a propeller. The stern ratio is 1:1.84. I plan on coupling directly to from the engine so my overall ratio will remain 1:1.84.

    I want a boat capable of 40mph ish, so I want help
    - Sizing the propeller. (pitch diameter etc)
    - Any useful info on maranising my diesel engine.
    - Any info on a where to source a female splined alpha one drive coupler?
    - Any info on where to source a second hand instrument panel from ANY boat?

    Thanks in advance!


    Irish Conor

    Boat pic:
     
  2. Jango
    Joined: Aug 2005
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    Jango Senior Enthusiast

    Had to make some assumptions:

    1.Diesel engine, 3600 max RPM
    2. Gross weight w. Driver and half fuel, 2200 lbs

    Since max RPM is fairly low with weight of 2200 lbs with 1.84 gear, recommended prop would be a 18 x 23, 3 blade. If weight is less and/or max RPM is higher, prop Dia can be smaller and pitch less.
    Max speed w. 140 HP and 2200# is about 38 MPH
     
    Last edited: Jun 7, 2010
  3. powerabout
    Joined: Nov 2007
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    powerabout Senior Member

    but you can only put a 15" diameter prop on an Alpha...
     
  4. Jango
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    Jango Senior Enthusiast

    Unless the max motor RPM is Higher, You will need to use the largest 4 Blade prop that will fit, or use a lower gear ratio for higher prop RPM. This will decrease Both Dia and Pitch.
     
  5. corglass
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    corglass Junior Member

    Hi,

    Thanks everyone for all the replies.

    Can someone please explain why I'd go for a 4 blade prop over a 3 blade?

    A dealer suggested a 15" dia with 15" pitch today, on a 3 blade aluminum prop. How does this sound?

    The price is 180 dollars (150 euro), does this seem right?

    Thanks again,

    conor
     
  6. gonzo
    Joined: Aug 2002
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    gonzo Senior Member

    A 15x15 would be adequate for a gas engine running at 4500 RPm. Your engine may turn too slow for it.
     
  7. Jango
    Joined: Aug 2005
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    Jango Senior Enthusiast

    With the 1.84 gear, your max prop RPM is only about 1950 rpm (3600 / 1.84 ). In order to obtain the required Blade area, since you can't go bigger than 15 dia, you need to Add blades. If for instance you had a 1.4 gear, you could get by with a 15 x 18, 3 blade ( 2570 prop rpm ) , about 37 mph.
    A 15 x 15 3 Blade with 1.84 gear will give you a max speed of about 22 - 25 mph with High slip.
     
  8. corglass
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    Location: Ireland

    corglass Junior Member

  9. Jango
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    Jango Senior Enthusiast

    NO, should be at least a 22 or 23 Pitch, 15 dia, 4 blade with 1.84 gear
    Top speed with the 15 x 16, 4 blade will be about 25 mph @ 3600 rpm.

    Best solution is to change gearing to something less than 1.4 : 1 allowing less pitch, smaller dia, 3 bladed prop.
     
  10. corglass
    Joined: Jun 2010
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    corglass Junior Member

    thanks Jango,
    i think changin the gearing would be alot of work and a lot of dosh. I'd be very very greatful if you could have a look on the previous site i linked to and suggest a propellor from them.


    If there is none suitable then i might go down the gearing route.

    I appreciate the help,

    Corglass
     
  11. Jango
    Joined: Aug 2005
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    Jango Senior Enthusiast

    ALL previous recommendations were based on max rpm of 3600 and 2200 gross weight, which are probably correct

    Varify the Max RPM ( @ 140 HP ) - look at some manuf. info and ALSO find Total Weight w. Driver and Half Fuel, everything included. motor, outdrive, hull etc. Otherwise the Best Prop could be chosen with inaccurate info.

    If you need to - weigh each part seperately and ADD together. Better to varify before you commit.

    If total weight is less than 2000 lb with 3600 + RPM, The 14 1/2 x 18 would be the best prop out of the three, but far from ideal. Gear change is not a big deal and is the best solution.
     

  12. Fiber Artz
    Joined: Jun 2010
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    Fiber Artz Junior Member

    Prop Size For Ivb

    [​IMG]

    I'm trying to determine what the appropriate diameter and pitch for this boats propeller would be its 23 ft. and its based off the hickman sea sled with a merc 135 v6 as its power plant any suggestions
     
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