Gear ratio for pedal drive

Discussion in 'Props' started by Astrocat505, Feb 18, 2021.

  1. Astrocat505
    Joined: Feb 2021
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    Astrocat505 Junior Member

    What would be an ideal gear ratio for an 11 foot trimaran hull kayak using a dual 6 inch three blade counter rotating boat propeller set with a 1.16 pitch? I'm currently at a 1 : 13.33 ratio with a cog adjustable chain drive. Any ideas?

    I've got a range of reaching 1 : 18.75 gear ratio with the correct crank cog and shaft cog...
     
    Last edited: Feb 20, 2021 at 1:43 PM
  2. gonzo
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    gonzo Senior Member

    Depends on your physical condition and the time you expect to be pedalling.
     
  3. Astrocat505
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    Astrocat505 Junior Member

    Thanks for your reply. But is that the ideal gear ratio? I'll just stick with the 1:13.33 for now and see if it moves my boat.
     
    Last edited: Feb 20, 2021 at 10:20 AM
  4. BlueBell
    Joined: May 2017
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    BlueBell Ahhhhh...

    1 : 7 is more typical but that's with large, single, two-bladed, low-aspect ratio, model airplane props in the 18"x 18" range.
    It also depends on the crank size, rider position, duration, sea-conditions, boat weight and design, advance-rate, drivetrain losses, etc, etc, etc.
    Let us know what ends up working for you.
    (By the way, there is no ideal ratio as some variables are ever changing.)
     
  5. Astrocat505
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    Astrocat505 Junior Member

    Thanks for your reply. You are correct, as conditions and outside factors are always dynamic. The static variables will be craft weight(unloaded), hull design, and for the most part power plant output. I've taken great efforts to try and reduce losses across the drive train as much as possible. However, efficiency and noise are not the be all end all to my design as there is no escaping the two unless extensively and meticulously engineered. While I intend to refine my product over time, I also intend to enjoy the research and development of a working(hopefully) prototype.
     
  6. Rumars
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    Rumars Senior Member

    You designed that prop to be efficient at a certain rpm, so that's your target. You pedal at a certain preferred cadence, that's your input. The ideal gear ratio is the one matching the two numbers.
     
  7. Astrocat505
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    Astrocat505 Junior Member

    Thanks for your reply. I will update my findings as I hash out the results.
     
  8. gonzo
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    gonzo Senior Member

    You are the engine, so the horsepower you can deliver at a certain RPMs will determine the gear ratio. Say you are an average cyclist and can maintain 60 RPM, producing 70 Watts for 3 hours. You need to make a curve of power input/speed for your boat. Look at where the curve shows 70 Watts. Now you can calculate the RPMs of the propeller. The ratio of 60 to propeller RPM is ideal.
     
  9. fredrosse
    Joined: Jan 2005
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    fredrosse USACE Steam

    What 90 degree gearbox are you using? I am building a small (1/10 horsepower) boat that needs two right angle drives, and consider using either the typical electric drill 90 degree drives, or Dewalt 1/4 hex right angle drives. I had previously used the lower end of a 3HP gasoline outboard, for a similar project, but that gearbox is way too big for this project.

    Thanks in advance for your information.
     
  10. Astrocat505
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    Astrocat505 Junior Member

    I sacrificed two cheap angle grinders from Harbor Freight and used those for the direction change. The gears were metal which was a plus.
     
  11. philSweet
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    philSweet Senior Member

    But angle grinders have no torque. A person has about the same torque as a school bus diesel. Try school bus gears or a roto-tiller gearbox running backwards. The lightest off-the-shelf gearbox I found that I thought would be suitable for a pedalboat weighed 13 kilograms. I did find a spectacular motorized deep-sea fishing reel for about $3000 that I though might work pretty good if you were careful with it. It had 6:1 gearing, as did the roto-tiller. Even if you go with a 52:13 chain drive to a pair of 3:2 gears, they still need to be several times heavier than a cheap angle grinder head.

    For comparison, this 54 pound bush hog gearbox would be rated for about 3 hp at 75 rpm input.

    upload_2021-2-23_23-38-5.png
    This 51 pound 5:1 gearbox would accept about 1.15 hp at crank speed, which is getting a bit sketchy. You'd want to bump the rpms up some with a chain primary. I can exceed the overhung load limit rather easily with standard chainrings, so you would want to go to really big sprockets, like 80:63 or something.
    upload_2021-2-23_23-53-1.png

    upload_2021-2-23_23-53-43.png

    Which is why it's a good idea to stick with chain drive as much as possible.
     
    BlueBell likes this.

  12. Tiny Turnip
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    Tiny Turnip Senior Member

    As I recall, Rick W and others always reckoned the Mitrpak boxes were the best bet for getting the 90 degree change of direction. They improved efficiency over twisted chain solutions by maybe a couple of percent, iirc
     
    BlueBell likes this.
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