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

    So, what you're saying is that it is entirely propulsive efficiency.

    The goal is to generate, say 30 lbs of force. It takes X amount of energy to pull the tow rope with 30 lbs of force, and 2X to make 30 lbs of force on the front of the oarlocks or propeller thrust bearing, because the oars or propeller are about 50% efficient.
     
  2. philSweet
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    philSweet Senior Member

    Yes it takes less energy and power to tow a vessel upstream than it takes to self-propel a vessel upstream. The reason is the difference in the advance speed of the propulsor. Both the towed and the self-propelled vessel require the same force, but the towing propulser has to work at three miles per hour, whereas the self-propelled propulsor has to work at three miles per hour plus whatever the current speed is. Energy and power go as force time distance and force times speed. If the distance (energy) or speed (power) is greater, the energy and power will be greater. Let's say you wanted to go downstream at three miles per hour and the current was running two miles per hour. It that case, the relationship is reversed and the self-propelled vessel requires less propulsion power than the towed one. Again, the force is the same, but now the tow power is three times the self propelled power because the tow propulsor is advancing three miles per hour but the self propelled one is advancing at one mile per hour.
     
  3. misanthropicexplore
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    misanthropicexplore Junior Member

    It would seem to me that a paddle, oar of propeller is just a very low head pump, and that by moving the boat by pumping water, you have to do the work of pumping the water, as well as moving the boat, and that when towing, you just move the boat. Ad Hoc says that's not the case though.
     
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  4. DogCavalry
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    DogCavalry Soy Soylent Green: I can't believe it's not people

    Sort of. Consider classic example of a guy in a boat, throwing a heavy rock backwards, or pushing against something.
    He has long arms, and he's strong, so he can shove the rock backwards 1m, with 100 kg of force.(don't be pedantic, I know it's newtons, but if you insist I'll switch to pounds) By coincidence, he and his boat mass 100kg together. His boat moves forward 0.5m and the rock goes the other way 0.5m. Then he does the same thing against a dock. Pushes 100kg worth, for 1m. He goes twice as far, for identical work. Rock flinging is 50% efficient.
     
  5. Ad Hoc
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    Ad Hoc Naval Architect

    You need to reread PSweet’s post again, it is all about relative velocities.

    To put this into context for you.

    Power = velocity x thrust, P=v.T

    If planning, say Fn=0.5.
    Therefore the velocity = Fn.(L.g)^0.5 = 0.5 x (3.05x9.814)^0.5 = 2.73m/s

    So, we have the speed, and if you’re saying it is using a 2hp engine = 1.49kW.

    PC = EHP/SHP, we have SHP =outboard of 2hp, and taking an assumption of a prop boat PC=0.5, we arrive at
    EHP = SHP.PC = 1.49kW x 0.5 = 745W = P or the EHP

    Therefore the thrust = P/v = 745/2.73 = 274.7N or 28 kg.

    But that is just instantaneous or an impulse force, not continuous. Thus you more force to maintain this “equivalence”, as such, you’ll need about twice that owing to the cycle of applied force and retardation = 2 x 28kg = 56kg.

    So, can you row at a constant “thrust” (you rowing) of 56 kg?...or 28/30kg per arm for a continuous amount of time and maintain this?
    That is in a situation with no current/tide with or against you, as you row.

    Then if you add in the fact you are now rowing against the current, which is at 2.7 m/s (6 miles/h), to plan you need even more force/thrust to get the required the power to plan than with no current.
    Just substitute 2.73m/s no current for for 5.4 m/s (total relative velocities). with current. A simple pro-rata suggests more than twice the thrust.
     
  6. tlouth7
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    tlouth7 Senior Member

    This is somewhat incorrect. I agree that pulling upwards on a bow line affects the trim which is likely to increase drag. Pulling upwards on a towline with the line of force going through the centre of flotation (so secured near amidships) will lift the boat evenly, decreasing displacement without negatively affecting trim. This should reduce drag.

    Secondly, it is not true that pulling upwards increases work done. Remember that work is force multiplied by distance in the direction of the force. You can consider the horizontal and vertical components of tension in the towline. Clearly you move no distance vertically, so work associated with this component is zero. Work in the horizontal direction is exactly the same as if the line was horizontal (less any reduction in drag as above).

    Of course a far greater impact will result from the ergonomics of holding the line; having it over your shoulder will be far less tiring than trying to pull it along at knee height. This may also go some way to explaining OP's perception of effort; pulling along the bank uses your legs which are used to working harder than your arms and upper body.
     
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  7. Heimfried
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    Heimfried Senior Member

    That were my thoughts also.
     
  8. bajansailor
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    bajansailor Marine Surveyor

    I don't think that I made a claim to that effect? I only mentioned forces, not work done.

    Or perhaps even better, having the tow rope attached to a wide waist band, rather than having it run over your shoulder?
     
  9. sharpii2
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    sharpii2 Senior Member

     
  10. misanthropicexplore
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    misanthropicexplore Junior Member

    So, it's not an unreasonable rough estimate that in these human scale numbers: small boats, low total speeds, etc., due to the effect of both different velocities, and the fact that walking is an more efficient way method of imputing motion that a propeller or oars, that towing is 2>3x more efficient? That's kind of incredible, and something I think more small boat/canoe expedition type folks should know. Would poling be much like towing?
     
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  11. tlouth7
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    tlouth7 Senior Member

    I apologise, I thought you implied that this increased force resulted in increased effort given that you mentioned efficiency.
    I agree that towing with a waist band would be even better.
     
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  12. DogCavalry
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    DogCavalry Soy Soylent Green: I can't believe it's not people

    We've gone badly astray from the original question: why does it take dramatically less work to move a vessel when towing from land than by propelling by oars or paddles?

    I will finish my involvement by referring to cable ferries, because there is ample engineering work available to quantify. On replacing cable ferries with free ferries, engineers determined that roughly ten times more power was required for the same performance. @misanthropicexplore , I trust that ten times less work seems satisfactory.
     
  13. Skyak
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    Skyak Senior Member

    There are lots of true statements explaining this but I don't see the simple concise answer -so I will provide one.
    P=F*V
    F=Ma
    a=change in velocity over time or Vdot
    In the case of your boat being towed you are applying force solidly to the planet.
    In the case of paddle, prop, or other applied to the water, a much smaller mass (the chunk or stream of water) is accelerated, so the get the same force you accelerate that smaller mass to a higher velocity.
    The boat takes the same force to go the same distance at the same velocity in either case (baring differences in trim or other behavior) so the productive work done is the same. But if you propel to the fluid, the power will be greater because the force will be applied at a greater velocity. Pushing larger amounts of fluid to smaller velocities can gain efficiency, but the mechanism to do so gets bigger, more cumbersome and brings new modes for losses.
     
  14. portacruise
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    portacruise Senior Member

    I don't think Towing with a waistband will work, unless maybe if someone is walking in the water, at least I cannot do it that way (from the quite imperfect, create your own, walking path on the bank)! It's not like the bank water area is always deep Stillwater, lined with frictionless rollers, it gets shallow and jagged near the bank with intervening rocks, debris, and unpredictable currents, especially in natural unimproved settings. Hand Towing is an active process "like flying a kite". The goal is to keep line/boat away from the bank and away from intervening obstructions between boat and Bank, Plus keep boat in deep enough water. Sometimes you have to lift up on the line to clear obstructions, or continuously vary walking speed, or jerk line/ react quickly, even when there's no strong current coming from Upstream. It takes a little practice to get it to work, but the challenge is always unique and fun for me in the remote places I visit, haha!
     
    Last edited: Jan 11, 2022

  15. gonzo
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    gonzo Senior Member

    Do you have any references to the claim of ten times? It doesn't seem credible. One problem with cable or chain ferries is that they create an obstruction to navigation. Also, they have a fix route, like a train. Changing locations is difficult.
     
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