Reasonable efficiency estimate for a pedal drive

Discussion in 'Boat Design' started by kerosene, Nov 19, 2021.

  1. kerosene
    Joined: Jul 2006
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    kerosene Senior Member

    Hi folks. I too keep dreaming of pedal boat*
    I am trying to do some calculations on expected performance. Just ballpark stuff. So my question is:
    is 65% efficiency for the mechanism a reasonable or overly optimistic assumption. Setup would be using commercial drive units like one below with a custom prop matched to boat and speed as best possible. unit has two gear angle drives. The ratio is in the 10:1 range so 600-700 rpm is roughly the prop speed.

    Boat speed in my dreams, 5 knots for 3+ hours (9.3 kph), 4.5 knots for 5-6 hours (8.3kph). Basically 40km+ in a day without it requiring athlete level ability or taking all day.
    In practice in non flat water consistent ~ 4 knots/7.5kph would be great.

    Also what would be best way to estimate the unit's drag and the rudder's drag?
    Using this and in my view conservative values for C (Cd) I get still very low numbers. 2N resistance at worst case which is easily lost in margin of error for my other calcs.

    upload_2021-11-19_12-0-18.png

    Drag Force Calculation and Equations. Drag Coefficients https://www.lmnoeng.com/Drag/index.php


    * or a rowing boat - better exercise and better for you in generally if done hours on end but being backwards is bit annoying, as is inability to do anything else while at it


    edit:
    Smaller unit picture

    [​IMG]
     
    Last edited: Nov 19, 2021
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  2. portacruise
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    portacruise Senior Member

    Depends on your power output capability and sustainability, as to whether your goals can be reached, but I don't think the typical 100 watts would be enough. Appendage drag is an important consideration and the less of it the better. The 24 hour speed record was set with a super efficient Hull boat designed by Rick Willoughby and it used a flexible long tail shaft drive, which had considerably less drag than your pictured pedal drive, and the flex drive weight could be considerably less, depending on how it's built.

    AdventuresofGreg - Pedal vs. Paddle Challenge - The 24 hour human powered boat distance record attempt http://www.adventuresofgreg.com/PredictContest.html

    A search should turn up pictures of the pedal drive arrangement, and possibly even some efficiency numbers? But a flex shaft drive is not available commercially, as far as I know, so you would have to build a copy of Rick's.
     
  3. kerosene
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    kerosene Senior Member

    yes I am aware of Rick's free shaft drive and Greg's records.

    I am willing to accept trade off towards convenience and robustness of the commercial unit over the ultimate efficiency.
    From Adventures of Greg: "I was able to average 10.5 kph over 6 hours on exactly 120 watts in variable conditions from 8″ waves and 15 kph winds to reflective ripples and calm."

    Those are pretty fantastic numbers. My not so scientific process uses resistance estimates from freeship "KAPER resistance for canoes and kayaks, according to John Winters."
    At 5 knots/9.3 kph my calcs require 135 Watts from pedals with 65% drivetrain+prop efficiency. Big difference to 15kph with less power.

    So quite a bit slower than Greg's boat but would still satisfy me. But I am not sure if the 65% is too optimistic. Basically it could come down to 93% efficiency at both angle drives and about 75% prop efficiency. Seems a bit optimistic on the prop.




    upload_2021-11-19_21-45-0.png
     
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  4. kerosene
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    kerosene Senior Member

  5. portacruise
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    portacruise Senior Member

    Is there some way to take Greg's numbers and calculate the drivetrain plus prop efficiency, might be interesting to see how that compares to 65%, without involving boat drag? The relatively High one to 10 ratio is probably done in two up converter stages (or planetary?), and may be a little Lossy, but allows for a small propeller, compared to most h p b props?
     
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  6. kerosene
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    kerosene Senior Member

    I tried java prop with Old Town PDL drive dimensions and it suggests around 72% efficiency for a skinny optimized prop.
    With that and the gear efficiencies (conservative side at 93%) I get that the total efficiency would fall in 62-63% range. So not _that_ far off. Though not sure if making such a prop is really realistic.

    So that would mean at 250kg (550lb) displacement 4 knots is more realistic than 4.5 for all day speed. Bottom row is power at pedals.

    Playing with javaprop: the ratio on these pedal drives is too fast at above 10. If the RPM was in 300 range instead of 600-700 you would get 5% efficiency gain "for free".

    upload_2021-11-20_0-35-45.png
     
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  7. kerosene
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    kerosene Senior Member

    Helix unit, gets decent amount of praise and has 1:6 ratio. But there is zero attempt at making it streamlined.



    [​IMG]
     
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  8. jehardiman
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    jehardiman Senior Member

    Ok, lots of apples, oranges, and flying saucers here.
    1) Most reasonable mechanical gearing efficiencies for marine units are on the order of 0.95+. Worm gears and spiroids are specifically selected to prevent back driving. They are windlass gears, not propulsion gearing.
    2) The most efficient propeller is large and slow turning. Always fit the largest propeller you can. Open water efficiencies are on the order of 0.85+.
    3) Air propeller shapes are not what is needed for marine propellers. The root and hub are too small.
    4) Over half the drag of a marine HPV is in wetted appendage interference drag. The unit shown in post #1 will have much more interference drag than the combined wetted surface and form drag.
    5) Leg propulsion drag is far, far, greater than the increase in open water efficiency of the prop. It is better to put the propeller in the wake to allow for the recovery of wake energy. I.e....Lower effective J for more thrust at speed.
    6) N0n-professional athlete sustained power output (non-anaerobic) is less than 150 watts. Burst is about 400 watts. Pro cyclist can maintain ~350 watts for an hour.
     
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  9. portacruise
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    portacruise Senior Member

    Some Marine propellers here including the orange one, or through your RC model shop usually for a lot less, and sometimes returnable

    https://www.apcprop.com/shop/page/3/?product_cat=pylon-racing,marine


    Then do some real-world side by side water testing (computer programs are not real life outputs) measuring power draw to see how efficient they are when isolated, using a cordless drill and flex shaft. For my particular situation, I have found RC props to be generally way more efficient than Troll Motor or IC props, and in some cases pretty close in efficiency to the HPB marine shape. Rick would sometimes hand-make propellers for a particular boat setup and a particular boat rider output, which would not be as efficient when some other boat or rider used it...
     
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  10. Tiny Turnip
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    Tiny Turnip Senior Member

    Without going through the whole PPB thread, IIRC Rick W was able to achieve overall efficiencies better than 80%; however, he was *extremely* diligent in tracking down and eliminating all possible inefficiencies, however small. Here is the thread on my build, which was my reason for joining the forum a little while ago now!

    Propellor for pedal powered catamaran https://www.boatdesign.net/threads/propellor-for-pedal-powered-catamaran.21911/

    Later I posted further material on my boat in the PPB thread.

    So, no actual figures, but anecdotally, I used dart 18 catamaran hulls (a good efficient shape but heavy) with recon commercial seacycle units. Two 'engines', both in their 50s, my friend who was a club level cyclist, and me who was of average fitness, max. Our longest trip, which was delightful, was a 15mile (13nm) circumnavigation of the Isle of Gigha off the West coast of Scotland. We did this in 4 hours, including a beach break of around half an hour, so we were probably achieving a scrape less than 4 knots over the water time. I was pleasantly tired at the end, but certainly not exhausted. Hope this gives you encouragement. Oh, and I'd strongly recommend cleated pedals and shoes.

    Here's a short video of the boat, well overloaded (in sheltered waters) with some underwater shots:

     
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  11. Andrew Kirk
    Joined: Jul 2021
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    Andrew Kirk Junior Member

    Since your boat will not be planing then to achieve 5 knots you'll need 14 feet at the waterline. That's a maximum speed so to reach 5 knots as a cruising speed you'll need it to be a long hull.
    My dinghy is pedal powered by a paddlewheel and can reach it's more modest hull speed of 4 mph or so in a sprint. I honestly think that the paddlewheel is efficient because the tip speed is little different to the speed I'm travelling, measured accurately. 93.4% of the paddlewheel tip speed at what I estimate from cycling experience to be about 100-150 watts of input. I can't explain why the figures are so good but I'm not complaining. I cruise on the canal at just under 3 mph but to go slightly quicker needs a big increase in effort.
    It would be interesting to build a long catamaran with paddlewheel propulsion just to see what I can achieve but for now I'm happy to cruise along for a few miles. Here's an account of a recent trip. https://mountainbiker.online/2021/10/21/returning-after-a-cold/
    I was videoed by someone I've never met and appear in this Youtube vid. at 13 minutes 23 seconds for a thrilling 16 seconds.


    Forget props. They're so 20th century!
     
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  12. kerosene
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    kerosene Senior Member

    I get Rick's record breaking boats and read that thread when Rick was still a member. I think he indeed has done a tremendous job but my goal is bit more universally usable "camp cruiser" where I can have a kid on board, or maybe he is pedaling in a few years while I make sandwiches on the fwd seat...

    And the biggest obstacle for me is not getting things done. Too many commitments outside of leisure. So commercial pedal unit, even if it means 10% efficiency loss, is much more appealing than a super efficient custom dream on a drawing table.

    the boat itself is TBD but current design I have been playing with is a "5 panel" hard chine plywood boat a little less than 20ft.
    We have 3 meter by 1.5 meter plywood sheets so bit under "two sheets" long.
    And outriggers to keep it upright as waterline beam should be narrow and there are no oars or paddles to balance with.

    Lakes and rivers and maybe small ventures on the semi sheltered coast/archipelago around here.
     
  13. kerosene
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    kerosene Senior Member

    IMG_20211120_181717.jpg

    Light bar fully optional
     
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  14. jehardiman
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    jehardiman Senior Member

    You are correct that in near DFC water paddle wheels at the apex of their development in the early 20th century could achieve efficiencies greater than propellers, and in much shallower water with side -wheelers having greater maneuvering capability. However, their limitations (size, weight, motion limits, torque requirements, etc.) quickly ruled them out of contention for most applications. Horses for courses.
     

  15. BlueBell
    Joined: May 2017
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    BlueBell "Whatever..."

    Kerosene,

    Nice sketch.
    You can get away with way smaller pontoons.
    Keep them long but leave room to paddle with a kayak paddle.
    The cross training versatility is optimal.
    The blade should be out of the water by the time it reaches your hip.

    6 or 7 to 1 ratio is preferred.
    Rick W has done all this before.
     
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