Power to tow vs. power to propel

Discussion in 'Boat Design' started by misanthropicexplore, Jan 10, 2022.

  1. misanthropicexplore
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    misanthropicexplore Junior Member

    Does tow path towing take significantly less energy than on-boat propulsion (oars, propeller, etc.) for the same speed? If so, why?

    In my personal experience, walking along the bank towing a canoe takes a lot less energy than rowing it. This seems to remain true even if the boat is swamped and carrying more than my weight in water, so it's not just the removal of my weight from the boat. The effect is so strong it seems like it can't be merely propulsive efficiency: with the sort of equipment I can afford and my level of skill and aerobic ability, rowing, paddling, and trolling motors are all about 50%. But towing doesn't feel twice as easy, it feels incredibly easier.

    I've also hand towed a 10' jonboat up a 6 mph current. Being that the waterline of 10' jonboat is around 8', the hull speed is around 4.3 mph. I found it totally impossible to row the boat upstream, but towing it was no problem. It takes like 2 hp to get a 10' jonboat to plane...how was I able to walk it up a current faster than hull speed with 1/10hp? (One manpower is usually considered to be 1/10 hp.)

    Searching the internet for an answer, I found a single person claiming that that in the 1900's, it was found that pusher or tug needed 4x more power to move the same barge train at the same speed as a locomotive pulling from a railroad on a tow path. From the estimation of how much of a workout out I'm getting rowing vs walking upstream, that sounds about right...but why? It seems to violate physics somehow. What's happening here?
     
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  2. bajansailor
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    bajansailor Marine Surveyor

    If you have a propeller driving the boat, then you are going to have a certain amount of 'slip'.
    That is, if the pitch of the propeller is say 24", then the boat is not going to move forwards 24" in one revolution of the propeller - it will be much less.

    Similarly, the propulsive efficiency of even a good pair of oars when rowing your jon boat is going to be a lot less than 1.0.

    On the other hand, if you are towing a boat using one man power on the tow path you do not have any 'slip' - every step you take is positive forward motion.

    Does this sound plausible?
     
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  3. gonzo
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    gonzo Senior Member

    The force necessary to move an object does not change because of the location of the applied power. Pushing or pulling requires the same force. You would have to find a way to measure the applied force to be able to calculate the energy input. A more efficient method will use less power simply because the ratio of power used for propulsion vs power wasted is larger.
     
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  4. DogCavalry
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    DogCavalry I aim to misbehave.

    Your perception is entirely correct. The important aspect to consider is how thrust is generated.
    Thrust in a boat is made by momentum transfer: push something one way, and the reaction pushes in the opposite direction. Momentum is equal to mv, mass times velocity. But pushing water the other way accelerates it, gives it kinetic energy, does work on it. Kinetic energy is mv². To move a boat one way, by moving a much smaller amount of water in the other direction takes a lot of work.
    m(boat)×v(small)=m(water)×v(large) that's momentum transfer, how the boat gets moved. But you pay m(water)×v(water)²
    So a relatively huge amount of work to move the boat. It's why bigger propellers are more efficient than smaller propellers: they move more water. And why jets are much less efficient than props. If you're standing on the ground the m you're pushing one way, to move your boat the other, is essentially infinite. Then the work you put into the system is only on the boat, not the boat going one way slowly, and a small amount of water going the other way quickly.

    Got it?
     
    Last edited: Jan 10, 2022
  5. portacruise
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    portacruise Senior Member

    When powering or towing Upstream, isn't there additional energy needed because you are climbing? I think the fish ladders may help by allowing fish to rest on long runs upstream, in addition to diminishing physical barriers.

    https://coloradooutdoorsmag.com/2020/08/19/ahra-chutes-and-ladders/

    Maybe a better walking tow path at the fish ladder would help boaters wanting to go Upstream, haha?
     
  6. Heimfried
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    Heimfried Senior Member

    One obvious point isn't mentioned yet: the force in towing line has not only a forward component but an upward one also. That means the bow trims up so the drag will be lower.
     
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  7. portacruise
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    portacruise Senior Member

    Would polling be easier than rowing, if the energy input is exactly the same?
     
  8. DogCavalry
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    DogCavalry I aim to misbehave.

    Much easier. Only working to move the boat one way, rather than the boat one way, and water the other.
     
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  9. bajansailor
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    bajansailor Marine Surveyor

    For maximum efficiency (ie minimum towing force required) the towing rope should be horizontal, such that there is no upward component at all.
    If the towing rope is above horizontal, then you are trying to lift the bow out of the water - this does not decrease the drag.
    Rather, if you resolve the vectors, the force required to tow the boat will then be the hypotenuse of the triangle, where the other two sides are the vertical and horizontal components.
    If you have no upward component, then everything goes into the horizontal component.
     
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  10. Heimfried
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    Heimfried Senior Member

    How would you establish this if you are towing a canoe walking along the bank? There will be some difference in the altitude of the point where the line is tightened at the canoe and the other end of the line in your hand or over your shoulder, won't it?

     
  11. bajansailor
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    bajansailor Marine Surveyor

    Do you mean establish if you are towing the boat horizontally?
    Maybe just approximate it by eye, roughly?
    A canoe is not going to take much effort to tow, especially if it is light.
    And if it is 'heavy' (relatively) then it will take extra effort to get it going from a standstill, but once you get it going, then you can effectively 'throttle back' a bit on your pulling in order to maintain the momentum.
    (we now have to go back to DC's momentum and kinetic energy post again) :)
     
  12. Heimfried
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    Heimfried Senior Member

    What I meant is very simple. In normal circumstances the highest point (equals end of line) of a canoe in a river will be about at your feet level if you are walking on the bank. So there will be a slope in the line if you are not down on all fours just because the geometry. I may be wrong.
     
  13. DogCavalry
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    DogCavalry I aim to misbehave.

    Gotcha @Heimfried , but don't forget: a displacement boat's lowest drag is when she's on her design lines. Unless the designer is ... not good.
     
  14. Ad Hoc
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    Ad Hoc Naval Architect

    No.

    Personal experiences of "feeling" a force (for a given power) is highly subjective.
    Also, please define "a lot less"... in terms of percentage of actual values with and without...

    First of all, no such thing as "hull speed". For more, see HERE and HERE.
    To paraphrase, it is a misnomer term used by non naval architects before they knew or understood the basics of high speed hydrodynamics.

    Of course YOU will find it impossible to row faster, because you are unable to provide enough power, the power being equal to what you are providing to tow it.
    Since if YOU are towing it, you can (and are) provide more "power" to it, since you are going faster in THAT boat than the towed boat on its own anyway, ergo you are providing more power to it that you can row.

    The forces in thrust and towing and basically the same, how the thrust is converted into forward motion via a means of propulsion and all the losses in the system, such as: appendage, hull, propeller, RRE, shaft efficiencies etc, is another matter.
    And for another thread.

    Very simply you ca make a brick shape, a box, and exceed its hydrodynamic speed of power v speed if you are towing it from a means that provides more power that is required to reach its hydrodynamic maximum via an onboard means of propulsion. It is why hull shapes change as you approach the prismatic hump (at Fn =0.45-0.5) , to "promote" the flow of water away and reduce the negative pressure field aft that is "holding" it back.
     
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  15. portacruise
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    portacruise Senior Member

    Yes, to get the best efficiency, you would have to get into the water and pull, and sometimes it is done that way- if the water is shallow and walkable. Usually the tow rope is placed as low as possible on the Hull to reduce tilt on a kayak, which might cause water to pour inside from the front.

    From a Google search : ["Anyway, the general idea is that you get two ropes and attach one to the front and one to the back of the canoe. It's a little like flying a kite. You want to keep the front of the boat pointed out into the middle of the flow so that the water keeps it there and not crashing into the bank beside you, but also move upstream. It's a lot easier with two people but can be done with one. And well, sometimes what you're doing will just amount to dragging.

    Back rope: Main pulling rope, pulls the boat upstream, must ensure that it's not let out further than the front.

    Front rope: Controls angle of the boat to the flow. The front needs to be further away from the bank you're on than the rear but not so far that the flow exerts more force than you can handle. It needs to be further out so that you can control it with the tension of the rope, if it were nearer you would need to push it back and a rope wouldn't be much use."]

    All of this to say that there will be some additional losses (besides the drop angle) associated with the process of keeping the kayak from dragging on the bank. Seems an easy pull for me when I've done it, but I don't know if it uses more or less energy than paddling ( no measurements or accounting for leverage and leg strength of the walking position). It has worked for me, when paddling or electric power will no longer work going Upstream.

    Trailing Upstream can also be done by one person controlling the two ropes, or by using a special Bridle built for the particular boat, which is the way I do it, on my up River Runs, haha! If there is no walking path along the shore line, polling can be used, but I haven't ever tried that one.
     
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