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

    If the energy never got transmitted to the crank you must be saying that the energy was wasted in inefficient muscle movement - so that is where it would have gone. Otherwise there are no gains to be made. It sounds like you believe your "solution", through geometry, is allowing more muscle energy to be transmitted through the system.

    In a bicycle crank, most of the muscle energy is expended during the downstroke when the cranks are near horizontal - this is when the greatest torque is applied. Two pulse per revolution. The muscle efficiency here is extremely high, there is very little room for improvement which you seem to acknowledge since your device applies force in the same way - except for the equivalent of a full rotation. If you apply the same force continuously the human will be required to expend a proportionally greater amount of muscle energy. It does not come for free.

    You have represented the "flawed" crank as if the muscles are expending energy continuously, thinking that the "flawed" crank only makes good use of that energy when the crank is horizontal. That is where you have gotten the 67% efficient number. It does not matter that the mechanical advantage of a crank is only greatest when it is horizontal, because that is when the bulk of the muscle energy is applied. For the remainder of the revolution the muscles are not working hard at keeping the crank spinning. I tried to explain this in the very beginning; a cyclist does not expend energy pushing on the pedals when the pedals are not in a position to do anything useful. Muscle energy is not wasted. Your "solution" is not going to waste less energy than a bicycle crank, well maybe if it is exceeding clever it can improve by a fraction of a percent but you are fooling yourself if you think that there are large gains to be made, the gains simply are not available.

    One way to figure out what your power output is is to time yourself climbing stairs. Knowing your weight, the altitude gained and the time it took you can calculate your personal power output. This establishes what your max continuous output is. Then that power output (let's say it is 100 watts) can be used to configure the rest of the propulsion system. There is no device that you can put between the human and the wheel (or whatever engages with the environment to move the vehicle) that will increase that 100 watts of power, it will only decrease. In the case of the bicycle it decreases by about 5%. In the case of a pedal powered boat it decreases at least 20%, mostly lost in the propeller.
     
  2. W9GFO
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    W9GFO Senior Member

    I understand what you are getting at but I do not agree with you. Any energy that is spent getting the leg back to the start is also contributing some torque to the crank. If you are isolating the forces required to lift the weight of the leg back to the starting position then you must also consider that the weight of the other leg cancels that out.

    The time that has been "wasted" between power pulses is not actually wasted. If more energy was exerted during that time then less energy would be available to push down on the pedal during the power portion of the stroke. In other words, since you are spending your energy in short bursts you can spend more of it during that burst than if you were to spend it continuously.

    Imagine a merry-go-round next to a light post. You are on the merry-go-round. To spin the merry-go-round you push against the light post, once per revolution. You will be able to get the merry-go-round up to a certain rpm, then you will be able to maintain that rpm based upon the power output your body is capable of sustaining.

    Now instead of one light post there are dozens encircling the merry-go-round. You can be pushing against these light posts continuously. Still, you will not be able to reach a greater rpm than if there were only one light post to push against. The reason is that you won't have enough energy to push against the many light posts with the same force that you use to push against the single light post once per revolution, your body can only produce so much power.

    In either case you are putting all the power you have available into spinning the merry-go-round. Power isn't being wasted in either method.
     
  3. Deering
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    Deering Senior Member

    I’ve climbed a lot of stairs and I have a good idea of what my max output is on them. But I also know that if I climb stairs that have railings on each side that I can grasp and pull, I can climb faster (for awhile anyway). So what is my max power output? Just stairs or stairs plus railings? Point being that there’s always another opportunity to capture more potential human power output. Even bicycles, as good as they are, continue to get tweaks to improve that - shoes, seats, handlebars, frame geometry... None of those make the bicycle more efficient or the human more fit, they just allow the human to transfer more power into the bicycle. Keep in mind that a bicycle must also be designed to balance and turn and adapt to varying grades and wind conditions- that leads to trade offs.

    Again, I’m not suggesting that DHaggsway’s magical system does that, but there will always be ways to improve on the transfer of human power.
     
  4. W9GFO
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    W9GFO Senior Member

    Doesn't matter, as long as it is sustainable. You need to climb long enough to find out what is sustainable, not what you can do for a short time. Although figuring out what you can do for a short time is also useful to know.
     
  5. kerosene
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    kerosene Senior Member

    If you can indeed get more effort from a cycle it will tire you down more.
    Efficiency is the key. Not the ultimate maximum work done per stroke.
     
  6. Deering
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    Deering Senior Member

    Define ‘sustainable’. 5 minutes? 5 hours?

    I can run a hundred yard dash or a marathon. My power output needs to only be sustainable for that activity.

    “In either case you are putting all the power you have available into spinning the merry-go-round. Power isn't being wasted in either method.”

    Only in some ideal universe where you can impart all of your available power in one push. I guarantee that I can get that merry-go-round spinning faster if I have a picket fence surrounding it rather than a single light pole. Power isn’t being wasted, it’s just not being utilized.
     
  7. W9GFO
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    W9GFO Senior Member

    Right, so you put in your best effort for climbing stairs for that time frame then you know what your power output potential is. I am not sure how many minutes you need to do it for to be able to assess what your output is over long durations. Probably you would need to stair climb long enough for your heart rate to stabilize to a "good" bpm. A little bit of research would answer that, but I don't know off the top of my head. I am not well versed in this area.

    The main point of the stair climbing example was that it is a real world way of accurately estimating your expected power output, without any fancy equipment. It is a straight forward calculation. Your weight * height gained in one minute/33,000 will tell you the power output in horsepower. If you use a ten minute time frame then you would divide by 333,000. I think I have the math right. One horsepower is equal to 33,000 lbs raised one foot in one minute. Then take that number and multiply by 746 to convert to watts.
     
  8. DCockey
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    DCockey Senior Member

    The answer to that question depends in on the magnitude of the variation in input compared to it's average value, the rotational inertia of the propulsion system, the mass of the vessel compared to it's resistance, the propeller characteristics, and other factors I haven't thought of.
     
  9. Deering
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    Deering Senior Member

    That’s a pretty straightforward way to measure output under those specific circumstances. I like it. But that doesn’t necessarily measure ‘potential’ output. For instance, would that output change if the air temperature was 20F or 90F? Or if the rise on each step was 4 inches or 12 inches? Or the climber was wearing sneakers or hiking boots? Point being there are a lot of factors that will enhance or degrade the human output associated in raising that body ‘x’ feet up those stairs. The same will apply to every other human power application.

    I could be wrong, but I believe that the most accurate way to measure human energy output is to measure O2 going in and CO2 going out.

    It’s not a given that spinning a shaft using a crank and pedals is the optimal way to extract power from a human. It just happens to be the best way we know to turn human output into mechanical energy. Theoretically I could see some future system that was able to capture and convert some of our waste heat into mechanical energy at the same time we were pedaling. Our energy output wouldn’t have changed, but the power we transferred to spinning the shaft would have.

    Maybe DHaggsway’s mysterious breakthrough accomplishes that through some sort of magic. I’m not holding my breath (pun intended).
     
  10. DCockey
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    DCockey Senior Member

    O2 in and CO2 out could indicate the total amount of energy produced by the conversion of blood sugar. It would include the energy used by for blood circulation, respiration, other internal functions, transfered as heat to the atmosphere, etc as well as used by the leg muscles.
     
  11. DCockey
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    DCockey Senior Member

    Who said that? What was it based on? What assumptions were made?
     
  12. DCockey
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    DCockey Senior Member

    I'll also add the frequency of the variations in input compared to the average rotational speed of the propeller.
     
  13. Deering
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    Deering Senior Member

    Yes. Other than some internal cellular functions, such as new cell growth, digestion, and cellular waste production, almost all of the energy produced by oxidizing sugars will come out either as motion or heat. Put them on a cycle or treadmill while monitoring respiration and you get a pretty accurate measure of fuel burn/power out, in other words, efficiency. Quick googling, for what that’s worth, indicates that human bodies are about 25% efficient. Do swimmers or rowers or XC skiers produce more power per fuel burn than cyclists or stair climbers? Don’t know. If you define efficiency as fuel burn/distance covered, the bicycle wins hands down.
     
  14. upchurchmr
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    upchurchmr Senior Member

    Why do you assume I should be Christian?
    What a narrow minded attitude.

    A Pity you can't read something that came from my professional life lessons.

    Value in engineering related subjects is not subject to your "faith".

    Actually I pity you that you cannot read and understand input from those who have a concern with wild claims and attacks on those who try to help.
     
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  15. W9GFO
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    W9GFO Senior Member

    The Bell X1 did not have tapered wings and was the first vehicle to break the sound barrier in level flight. I very much doubt that the engineers were stumped until observing a diving bird. Certainly, wildlife has been an inspiration to many designs - creatures have quite a head start on filtering out what works and what doesn't.
     
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