I have been working on human powered watercraft

Discussion in 'Projects & Proposals' started by DHaggsway, Aug 8, 2019.

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  1. W9GFO
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    W9GFO Senior Member

    No. When speaking of the efficiency of cycling being 95% efficient it is NOT talking about just the chain, it is about how 95% of the human energy put into the system is put to work moving the cyclist. This takes into account the pedals, crankset bearings, chain, wheel bearings and rolling friction.
     
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  2. W9GFO
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    W9GFO Senior Member

    The existence of those forces has little to do with what I wrote about how your mathematical crank model does not apply to real world pedaling.
    If the same force is applied over a greater arc length for a greater length of time then yes, you would get more energy out. More energy in = more energy out. There is no improvement in efficiency though.

    Efficiency is the ratio of power out divide by the power you put in. It seems like you are talking about how to get more force out of a given input, that is not what efficiency is.
     
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  3. upchurchmr
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    upchurchmr Senior Member

    Could we see your patent numbers?
    How about some math?
    Verbal descriptions of physical processes leave lot to be desired in communicating reality.
    Especially with people who don't understand the exact technical meaning of an engineering discipline - or those who insist on changing the meaning to suit themselves.
     
  4. W9GFO
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    W9GFO Senior Member

    If you could apply a constant force (the amount of force a cyclist applies to a pedal when the crank is horizontal) where that force was always at 90 degrees to the crank, then you would indeed get more power out of it - but here's the thing: it would also require the cyclist to input more energy. That is why there is no increase in efficiency. The energy input goes up because the cyclist now needs to be applying that force for the full rotation of the crank (continuously).

    Currently the amount of force a cyclist applies to the crank varies throughout the rotation. If you were to graph it, it would resemble a sine wave. You seem to be under the belief that a cyclist exerts a constant force throughout the entire revolution and is therefore wasting effort. That is not at all true.
     
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  5. W9GFO
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    W9GFO Senior Member

    If you restrict "crank" to mean a bicycle crank then we are in agreement that you would be hard pressed to find anything better.
     
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  6. Ad Hoc
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    Ad Hoc Naval Architect

    In mechanism engineering a "Crank" is defined as - a link that makes a complete revolution about a fixed grounded pivot.
    That's it.
     
  7. upchurchmr
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    upchurchmr Senior Member

    Sounds to me like he is talking about a "stepper" drive or "linear" drive bicycle.
    These have been around for a long time, always claimed to have better efficiency, since they eliminate the "dead spots" in a typical crank.
    Difficult to find a current illustration that looks usable.
    I probably don't have the current name to search under.
     
  8. W9GFO
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    W9GFO Senior Member

    I am not confident that all cranks are >99% efficient (maybe they are), but when it comes to bicycle cranks operated by human legs then that is different.
     
  9. W9GFO
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    W9GFO Senior Member

    Rick W. made one of his boats (V12) with such a drive system. He seemed pleased with its performance. Subsequent boats use cranks though.

    And yes, they do sometimes claim better biomechanical "efficiency". However, this is not the same as energy input/energy output. It is more along the lines of the geometry of the linear pedaling action is better suited to the way humans would like to move their legs, more comfortable motion - easier on joints. Maybe they should call it biomechanical effectiveness instead.
     
  10. W9GFO
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    W9GFO Senior Member

    If the math shows that it is 67% efficient, and an operator can manipulate it to perform to a higher level, then that should show you that the math is wrong.

    The only thing your math has calculated to show is that a constant downward force on a crank creates varying amounts of torque depending upon its position. We all agree with that. That is not a measurement of efficiency.

    The momentum absolutely is consumed, that is why it carries you through between power pulses. If it weren't consumed then you would no longer need to pedal.

    Fluctuating power is another thing, and that might be worthy of discussion. Any propeller has one ideal speed it would like to operate at for peak efficiency. Certainly the unevenness of the power applied to it through pedaling negatively impacts efficiency to some extent. If that can be eliminated then that would be a good thing.
     
  11. W9GFO
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    W9GFO Senior Member

    Your claim of the pedal/crank system being flawed is based upon the one configuration where force is applied to the pedals downwards only. That math calculation is showing mechanical advantage, not efficiency. Any claim that efficiency is lost must include an explanation as to where the lost energy went to.

    For example, a roller chain may be 99% efficient, the losses are due to the individual links sliding against each other and on the sprocket creating friction, which creates heat.

    What is happening to the 33% that you claim is being lost in the "flawed" bicycle crank, where has the energy gone to?

    There is no such thing as a "power amplifier" in this context. Torque and power are not the same thing. A longer lever arm will increase torque (and decrease speed), but it does not gain power.

    Total bicycle rating? You must be referring to the widely accepted fact that bicycles are 95% efficient (or better) at converting human energy into mechanical motion. What this means is that a human outputting 100 watts will transmit 95 or more watts of mechanical energy to the road. All parts of the system are included, from the pedals to the wheels. Very little energy is wasted. Certainly 33% is not wasted in the cranks.

    Easy, you determine how much of the energy that is put into the springs is lost as heat. Efficiency is all about not losing energy, to heat. It is not about mechanical advantage or some "performance multiplier".

    You can measure the energy expended by a human by measuring the content of CO2 and oxygen that they exhale. You can put this human on a bicycle with a dyno and measure how much energy they are putting into the system via their metabolic processes and how much mechanical energy is delivered to the dyno.

    In the case of a bicycle very little energy is lost in the mechanics of the bicycle - that includes the pedals and crank. The energy that is lost is accounted for in the frictional losses as heat. If one third of the energy was being lost due to the "flawed" bicycle crank as you assert then such a test would reveal that discrepancy, and it would be interesting to discover where exactly that 33% of input energy went to.
     
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  12. kerosene
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    kerosene Senior Member

    How do they cool the cranks on old steam engines putting through thousands of horsepowers (or kWs). 33% is a lot of heat... ;)

    Pretty silly thread. We are to believe that there is a major breakthrough yet the fundamental concepts of physics (force/torque vs efficiency) get happily mixed up.

    But to the vibration or undulating output of crank force. Instead of a flywheel that really just softens the changes howabout a set of two oval gears that would spin a flywheel. This unevenly geared flywheel would accelerate at peak torque and slow down at the low force part of the cycle. All the while keeping the output shaft speed even.

    Probably not worth the complexity.
     

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  13. kerosene
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    kerosene Senior Member

    I provided a solution for smoothing the oscillation.

    I was just illustrating how ridiculous the idea of 66% efficiency on crank setup is.
     
  14. kerosene
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    kerosene Senior Member

    I’ll get back on the oscillation solution. Will do a sketch.

    Meanwhile can you answer this: do you agree that a bicycle can turn muscle work of the body to propulsive force at the tire at 95% efficiency?
     

  15. kerosene
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    kerosene Senior Member

    Here pictured the moment when the muscles provide the peak torque to the crank. Elliptical gear accelerates the flywheel even if the crank (and power output) axle is rotating at steady rpm. This acceleration of the flywheel puts an additional resistance, lets call it negative torque to the crank.
    When the axles turn 90 degrees the force from pedals dips to it’s minimum. Yet the flywheel tries to maintain steady speed and provides positive, assistive, torque to the crank.

    Essentially you store peak torque and release it 90 degrees later.

    This should even out the bursts of force much better than a simple flywheel. And in case of pedal boat steady the prop rpm. Naturally the ratios would need be optimized.
     

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