Thrust / HP equivalence

Discussion in 'Boat Design' started by Guest1578132542, Jul 12, 2010.

  1. daiquiri
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    daiquiri Engineering and Design

    No, Dung's mistakes started with the initial idea.
    And continued with increasing pace.

    Cheers to all, I'm out.
     
  2. FAST FRED
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    FAST FRED Senior Member

    Sure everything is different , but if all you want is a simple rule of thumb a boat prop will usually make 20 lbs of thrust from each HP.

    25lbs is common but requires better parts.
    Over that is specialized for one load and usually one speed.

    For an air boat or hover craft , pushing on air5lbs per hp is common over 7lbs is rare.

    Is that the info you need?

    FF
     
  3. Jeremy Harris
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    Jeremy Harris Senior Member

    I'm not at all sure that rule holds up at all well, as it seems way out and makes no allowance for speed.

    For example, at 4kts my electric propulsion unit and prop delivers about 30N (around 7lbsf) of thrust from 80 watts, which equates to around 65lbsf thrust per hp, more than three times what your 'rule' says it should.

    To add to the problem, a statement like '1hp = 20lbsf thrust' would only hold good for one particular speed, anyway; as we keep saying, you cannot ignore speed in this calculation.

    Jeremy
     
  4. Motivator-1
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    Motivator-1 Junior Member

    Here we go again

    Hi guys,
    I am going to re-direct this thread a bit, as I felt that some here were trying to close it, and I have some things that I would like addressed.

    I have been trying to sort through some of these same issues for some time in reference to the actual output force of a Trolling motor. Specifcally, in my thread about an Electric Catamaran, which has seemed to die out.

    I believe, that like many here, that we would simply like to know what a
    trolling motor can produce given the specs that the manufacturers provide, albeit very little informtion.

    I have watched all of these HP/Thrust threads go by without any kind of simple explanation for us simple thinkers. So, what I am asking is what kind of performance can one expect from a trolling motor, given the specs that are offered from the manufacturer?

    The following is from Minn Kota:

    Minn Kota did state the following numbers, though:
    70 lb Thrust @ 24 volts X 42 Amps
    80 lb Thrust @ 24 volts X 56 Amps
    101 lb Thrust @ 36 volts X 46 Amps

    As has been said before, you cannot explain force without speed, but how can these numbers of Thrust be turned into True Watts, or some other understandable measure of force for us so that we can reasonably predict some sort of realistic output from these Trolling motors?

    Changing props and other things will certainly change results, but we have no base to start from. Attached is a power calculation from our Dear Friend Rick that indicates the power requirements for my 500kg Cat Hulls. How does a trolling motor, and it's potential effecincy work into this design?


    Mental, from the Poorqueedo Thread had a good bit of input for me explaining some of the calculations for me to understand about what I was trying to do, but I am concerned about where you begin to start to apply all of the in-effiencies he mentions. Do the Minn Kota specs explain the starting point of what "Thrust" is, based on their Wattage requirements, and do these numbers include the in-effiency that all here consider in their calculations about the Low-Efficiency Trolling Motors.

    What I hope to inspire here is to help those like me to have a reasonable expectation of what a Trolling motor can do, given the specs above, or any data presently available.

    So....If, for the reason of example, you have a Trolling motor that has 100 lbs of Thrust, can anybody translate this into Watts of energy or something, so that we can apply this to a design?

    There is some energy there....How can we calculate it?
    Regards,
    John
     

    Attached Files:

  5. terhohalme
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    terhohalme BEng Boat Technology

    Input power (in Watts) is Voltage (in Volts) times Current (in Amps). Output power in DC motor is typically 80 - 90 % of input.

    So at 70 lb motor:
    input 24 V * 42 A = 1008 W ~1 kW
    output 0.8 * 1 kW = 0.8 kW ~1.1 hp

    To fix it to your curves the efficiency of propeller is typically 50 %, so 0.5 * 800 W = 400 W is the power to define your speed.
     
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  6. Jeremy Harris
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    Jeremy Harris Senior Member


    AFAIK, the trolling motor thrust figures are all static thrust, so although power is being absorbed by the motor, none is being delivered towards useful work (i.e. the efficiency is 0%). Without knowing the prop diameter, pitch, blade chord and section it's hard to to be definitive about thrust at any particular speed and power. Generally, thrust required at any particular speed is determined not by the motor and prop, but by the hull shape and displacement.

    I can't give examples for trolling motors, but their efficiency/power/thrust curves would follow the same form as my electric system, albeit with very different values. The attached PDF shows the power and thrust vs speed for my boat and its propulsion system; note that thrust is read from the right hand red Y axis - thrust and speed are not on the same scale.

    There is some useful data from Denny on this thread: http://www.boatdesign.net/forums/boat-design/efficient-electric-boat-27996-15.html#post302305, where he's measured real-world trolling motor performance propelling his boat. This can't be read across easily to another boat hull though, as motor thrust has to equal hull resistance at any given speed and this will vary widely between different hull shapes and displacements. It might help get a feel for things though.

    Jeremy

    Edited to add another file showing efficiency curves. The PDF 'Motor Formulae - TowerPro 2 to 1 drive.pdf' shows system efficiency ( the top three curves, read against the right hand Y axis) and power (the bottom three curves, read against the left hand Y axis) for each component of the propulsion system.
     

    Attached Files:

  7. Motivator-1
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    Motivator-1 Junior Member

    Trying to Compile

    Thanks for the follow-ups guys. I'm starting to see things clearer, and your notes, terhohalme, on effiency losses are explained much like what Mental has calculated for me.

    So, from what I can pull from here and McDenny's work on the Electric Boat thread:

    A standard Trolling Motor with Standard Prop is only 40% effecient.

    Switching to the High Aspect Props brings the effiency to at least 60%.

    With careful design towards a single speed goal, the effeciency can go to maybe 70%? using a Trolling Motor and an optimum Prop.

    After this, can one interpolate the effect of the Fairing and Prop Spinner, and how much it is worth at the higher 9 knot speeds that I'm looking for?

    I feel that if it were possible in McDenny's tests, that Shaft RPM might have also been helpful, though hard to get. Now, from these latest observations, can a Trolling Motor HP be identified for those still looking for that equivalent. Personally, just using the Wattage numbers in the calculations against the Wattage figures of the Hull Drag is the simplest way to understand the force for mine and others trying to evaluate using the Trolling motors.

    Now, going back to the Over-Volting of the Trolling Motors, will this simply allow the use of the Standard Prop of 4" + - Pitch at twice the RPM to give double the speed, and if so, will prop loss still be 50%? Would the 12 Volt motor be more efficient at 24 Volts? This information might just wrap it up enough for me in regards to the direction that I'm going.

    Your inputs are welcome.
    John
     
  8. terhohalme
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    terhohalme BEng Boat Technology

    In general, I would be suprised if you can find a real propeller with efficiency over 60 %.
     
  9. daiquiri
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    daiquiri Engineering and Design

    Commercial or off-the-shelf would be a better word, imho. I'm pretty sure that's what you intended. ;)
    Cheers
     
  10. Jeremy Harris
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    Jeremy Harris Senior Member

    It's pretty easy to get efficiency up to over 70%, but takes a bit of work to get over 80%. Mine should be around 80% efficient, and Rick Willoughby has amassed a lot of evidence to prove that he's been getting over 80% fairly easily with his props.

    Aircraft props fairly easily get 70% efficiency, in fact if one of my aircraft had a prop that was only working at 60% then I'd change it pretty quickly. There's no good reason why a boat prop shouldn't be every bit as efficient as an aircraft prop; they are both working in incompressible fluids and, when you correct for viscosity and density, they aren't too far apart in terms of operating conditions, at least at low loadings. There are significant differences once you get into the onset of cavitation, or when dealing with aeration, but for a low speed displacement boat with a lightly loaded prop the analogy works fine.



    Jeremy
     

  11. jehardiman
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    jehardiman Senior Member

    I'd tend to agree that an off-the-shelf high speed outboard prop slaped on the back of any production hull should be about 60%. But for a wake adapted ship's main screw I'd sue if it wasn't in the mid 70's. Large and thin lightly loaded HPV props should be mid to high 80's.

    See the graph in this old post, it shows the state of the art for well designed propulsion systems.

    http://www.boatdesign.net/forums/jet-drives/jet-vs-prop-better-choice-17003.html#post135535
     
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