calculating prop. Need help.

Discussion in 'Props' started by Joris, Feb 5, 2015.

  1. Joris
    Joined: Mar 2013
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    Location: belgium

    Joris Junior Member

    It's time to start thinking about buying a prop for my project so i asked for prices and advise at three dealers. With very different results.
    I did an attempt to calculate the prop using an online-calculator which is probably outstanding but useless when used by someone with very little knowledge about props. (and boats :D)
    Can anyone suggest a prop or point me in the right direction?
    Boat is a Hacker-replica (20' Gladys) , weight estimated 2200lbs, 85hp@ 4000rpm , 1:1 transmission, max prop diameter 13" if i keep 1" clearance.

    Thanks!
    Joris
     
  2. JSL
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    JSL Senior Member

    At 4000 rpm and 85hp looks like a 10" dia. (approx.) 3 blade. Way too small.
    Suggestion: Get a reduction gear and hire someone that knows what they are doing.
    ps: a 13" prop at 4000 rpm will have too high tip speed and lead to ventilation/cavitation.
     
    Last edited: Feb 6, 2015
  3. tom kane
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    Location: Hamilton.New Zealand.

    tom kane Senior Member

    The easy and cheap way is to see what other similar boats are using for a start point and check out the original design recommendations.
    Prop tip clearance should be at least 2 inches to avoid noise and vibrations.
    I would start with a square prop (equal pitch and diameter) 13x13 in or even a 13x10 in.
    A smaller diameter 12 in will give better prop tip clearance. A square 12x12 in.
    You probably could not find anyone skilled in maths to make any better start.
    Better to have big a diameter with less pitch than small diameter.
     
  4. Joris
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    Joris Junior Member

    Tom,

    I tried to get an idea by comparing different boats with the original specifications from the book that contained the plans (runabout book vol22) . The engine, a Kermath 20hp with a 15 x 20" 3-blade prop which should result in a speed of 14-15 miles per hours but they don't mention the ratio and i don't really know what rpm those engines reached.
    From sales brochures i also found some boots from the 30-ties that were powered by Chrysler Ace-engines (20' runabout, 65hp, 12 x 12 prop)
    After some searching on the net i know the Chryslers revved to 2500rpm for cruising and 3200rpm at top speed but they were available with 3 different ratio's.
    The idea is to find the best solution to make the boat perform with some grace and style. Most dealers seem to be after topspeed and focus on those 4000rpm as if i'm planning a career in smuggling or didn't dare to suggest a different ratio fearing i would ask them which one would be best.:confused:
    I have access to a 12 x 7 prop but if that doesn't perform more or less like i want it too i guess i will have no other choice to start a propeller collection:p

    Joris
     
  5. tom kane
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    Location: Hamilton.New Zealand.

    tom kane Senior Member

    If you have access to a 12x7 that should be a good start point. Lower pitch will give quicker RPM`s and a quicker plane speed. There is nothing wrong with a prop collection if they are cheap and they can be swapped.
    2500 to 3500 RPM is a good range to run a motor in a boat. You can be brutal and trim some diameter of a prop to regain RPM if props are cheap and have to much pitch for the motor. You can guarantee you will want to try a variety of props. Two blade props are more efficient than multi blade props but can make more noise.
     
  6. Joris
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    Joris Junior Member

    Tom,
    I found a Paragon gearbox with 2:1 ratio, identical to my 1:1 Paragon so an easy swap. If the price is right that could be a good step in the right direction and it will make it easier to find secondhand props to test.
    Thanks for explaining a bit what you suggested. I haven't found a dealer who wanted to spend time on it or explain his choice. They go "outboard?" , i go "no...inboard". Basically that's where it ends :roll eyes:

    Joris
     
  7. JSL
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    JSL Senior Member

    a quick comment - (should be verified.)
    for rpm & tip speed limit -
    80 shp at 2000rpm(4000/2) could go to a max of 16" for a 3 blade and about 15" for a 4 blade. Your hull/tip clearance should be at least 15% of prop dia, 20% is better
     
  8. tom kane
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    tom kane Senior Member

    calculating prop.need help.

    A 2:1 box means you can run a larger diameter prop, could you have the prop clearance. Your type of boats I have seen usually run direct drive.
     
  9. Joris
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    Joris Junior Member

    Tom, you mentioned cutting down a props diameter, how much can you trim a prop and are there any disadvantages i should be aware of?

    JSL,
    If i want the 20% clearance i can't go any bigger than 12". But i could run the engine at 3000rpm and still have around 55hp.
    I find many cheap 2ndhand 13" and 14" props with different pitch, it might be worth spending a few evenings milling them down to 12".
     
  10. mr hot rod
    Joined: Jan 2010
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    Location: Chelsea, Quebec, Canada

    mr hot rod Junior Member

    Hi Joris !

    Which engine are you running ? Is this a flat head V8 ? Any idea of its peak torque ?

    You haven't told us how fast you want to go with this boat. You would normally prop the boat so that it hits the engine's rated maximum RPM eg. 4000 RPM.

    The first step to finding the right prop is to determine the optimum propeller diameter. Our Excel spreadsheet uses the Diameter-HP-RPM Formula from Dave Gerr's Propeller Handbook to find optimum propeller diameter from HP and RPM.

    The spreadsheet can be downloaded from our website :

    Here's some output from a modified spreadsheet using the following input values :

    BHP = 85
    RPM = 4000
    Gear Ratio = 1:1

    [​IMG]
    (Click image for PDF version)
    This is a light boat and should not require a reduction gear. As soon as we know what your required speed is, we can enter the value into our Shaft HP VS Speed Calculator. With 85 HP on tap, you may not have enough HP (and torque) to meet your requirements.

    ____________________
    Paul Kane
    Kane Custom Boats Ltd.
    Chelsea, Quebec

    Building the Glen-L Hot Rod : http://www.kanecustomboats.com

    Glen-L Boat Video Directory : http://www.kanecustomboats.com/pages/video_directory.html
     
  11. Joris
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    Joris Junior Member

    Hi Paul,
    I actually already downloaded a few things from your page (and did a lot of reading!) but apparently this is not something you pick up in a few days :D

    The engine is an older model DOHC Ford 2litre inline 4. I can only guess at the torque and power as this is an engine i'm only familiar with from car racing. If i take 2/3rd of what i had in the car it should easily produce 60hp @ 3000rpm, 85hp @ 4000, 105hp @ 5500rpm.
    I'm not after the highest possible speed. The original Hackers did 15mph with a 20hp engine so i was hoping with three times the power i should be able to get close to 25-30mph at 3000rpm? Anything beyond that is a bonus but i promised my wife the crazy days are over. :D i might revv it a bit further once in a while just for fun if it wants to.
    I tried your calculator and 60hp @3000rpm should get me 25mph with a 12 x 15 prop. Is that correct?

    Joris
     
  12. gonzo
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    gonzo Senior Member

    If you are going to underprop the engine, there should be a RPM limiter. Otherwise, if you give it full throttle, it may destroy itself.
     
  13. tom kane
    Joined: Nov 2003
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    Location: Hamilton.New Zealand.

    tom kane Senior Member

    Altering any propeller from it`s original specs is not a good practice better to keep it original
    if possible and hopefully the prop will be marked with diameter and pitch. If not it can be measured. However a prop performance can benefit from and may often need "tuning" or "tweaking" which can make big differences.
    New posts are giving good information.
     
  14. mr hot rod
    Joined: Jan 2010
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    Location: Chelsea, Quebec, Canada

    mr hot rod Junior Member

    Hi Tom ! Nice to meet another Kane half way round the world ...

    Joris : Now that we know what your performance expectations are, we can move to the next step.

    Here's the ouput from our Shaft HP VS Speed Calculator.
    We used the following input :

    Column 1 : BHP=85, RPM=4000, LB=2200, MPH=30
    Column 2 : BHP=105, RPM=5500, LB=2200, MPH=35

    Shaft HP VS Speed (PDF)
    To determine shaft HP to attain a desired speed, refer to the SHP column in the 'Shaft HP Required for a Given Speed' table. For example, 66 HP would be required to attain 30 MPH, 185 HP would be required to attain 50 MPH.

    Analysis : Required speed 30 MPH, 85 HP @ 4000 RPM, Gear Ratio 1:1, 11x12 prop
    Required speed 35 MPH, 105HP @ 5500 RPM, Gear Ratio 1:1, 9x10 prop

    The spreadsheet can be downloaded from our website :
    Feel free to PM me if you have any questions about the spreadsheet.

    ____________________
    Paul Kane
    Kane Custom Boats Ltd.
    Chelsea, Quebec

    Building the Glen-L Hot Rod : http://www.kanecustomboats.com

    Glen-L Boat Video Directory : http://www.kanecustomboats.com/pages/video_directory.html
     

  15. Joris
    Joined: Mar 2013
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    Location: belgium

    Joris Junior Member

    Tom,
    Thanks for the info. I was hoping it was possible to reduce the diameter by 1/2" or maybe 1" because it isn't easy to find props in the 10-11" range. Would it be acceptable just for a few shake-down runs?

    Gonzo,
    rev limiter in place;)

    Paul,
    Thank you for your work on this. I will be on the lookout for a 11x12 or something close to that. I read that one inch on the diameter equals 2-3 on the pitch. Just to get an idea about how this works; does this mean that if i find a 12x10 i would only loose on topspeed but the difference at 3000rpm would be minimum? And would it be worth trying a 12x8 or is that too far off?
    Reason i'm asking, i haven't found anything in 10-11" but 12 and 13" turn up at prices 20% of new...
     
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