electric propulsion thrust realy mesured

Discussion in 'Boat Design' started by surfango, May 7, 2015.

  1. surfango
    Joined: Dec 2009
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    surfango Junior Member

    Did anybody made real measurement of electric motors?
    I did with small one which claim 18 lbs . I use rope which I attached on rear side of the boat and on shore with weight scale in the middle. Scale shows 5 lbs.
    How producers make measurements?
     
  2. gonzo
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    gonzo Senior Member

    It is probably made by the sales team
     
  3. sharpii2
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    sharpii2 Senior Member

    You may be confusing what is known as "bollard pull" with maximum thrust.

    The two are different.

    With bollard pull you are measuring how much thrust your electric motor can generate against a fixed, non moving object (your line led ashore).

    With maximum thrust, you are measuring how much thrust the motor can deliver under ideal or near ideal conditions.

    The propeller held stationary is not what anyone would consider an ideal condition. A propeller designed to be efficient in that situation, like say a tug boat propeller, would not be as efficient, once the boat starts moving.

    There may be other problems with your test.

    Was it done on a windless day, with no contrary current?

    If not, the force of the current or the contrary wind needs to be taken into account.

    Do you know how accurate the scale is?

    A lot of bathroom scales are not all that accurate at their lower weight ends. And they may not work well at all, if not used in their customary vertical orientation.

    Were there a lot of pulleys and/or other objects of possible friction?

    If so, that too needs to be taken into account.

    Finally, was the battery powering this motor fully charged?

    If it wasn't, or even if it was used on a cold day, it would not produce the wattage necessary to produce the advertized thrust.
     
  4. Jim Caldwell
    Joined: Aug 2013
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    Jim Caldwell Senior Member

    Load cell between the transom and drive is the only accurate way I have seen.
     
  5. BMcF
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    BMcF Senior Member

    That's how we measure thrust in many of our model tests....even with outboard engines to 250HP.
     
  6. Mr Efficiency
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    Mr Efficiency Senior Member

    Can you expand on how these "load cells" work ?
     
  7. surfango
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    surfango Junior Member

    Sharpii2 . you are right. I made "bollard pull" with maximum thrust.
    All other conditions are almost ideal, deep water, fully charged battery, no obstacles, no current or wind. I made measure with digital hang scale (luggage scale ) which is quite accurate. But still difference is huge. Is there way to check and better understand factory specifications?
     
  8. Mr Efficiency
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    Mr Efficiency Senior Member

    What speed does it drive the boat to, at maximum output ? Get an accurate fix on that, then tow the boat (with the electric motor out of the water !) with another boat at the same speed, and check what your strain gauge is showing. That would be the effective thrust you are getting.
     
  9. BMcF
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    BMcF Senior Member

    The outboard engines are mounted on hinged plates. The torque around the hinge that is produced by prop thrust is reacted with a link low on the plate that is connected to a standard/typical pancake load cell. Easily calibrated, we simply attach a cable to the the lower unit in line with the prop shaft and pull on a tension guage.
     

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  10. baeckmo
    Joined: Jun 2009
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    baeckmo Hydrodynamics

    When talking outboards and in/outdrives having hydraulic trim, there is practically always the possibility to connect a manometer to the hydraulic circuit.

    I've used this for validation of real hulls versus calc's; you calibrate pressure versus bollard pull with a tension scale. The readings are of course net thrust, ie propeller thrust minus drive drag.

    The setup has to be calibrated separately for different trim angles, but is simple, cheap and reasonably precise when checked against considerably more expensive load cells.

    For smaller Engines, I made an "A-frame" that hung from the transom with the top of the A facing rearwards. That made it possible to connect a scale from the "A" tip to the rig of the outboard. That should work fine on the electric outboard.

    The thrust from inboards is a little trickier, but you can get engineering precision by measuring the axial movement of the engine if it is supported on flexible mounts. Best is to have a "zero mark" and pull the engine back to the zero mark with a puller connected to the engine block with a scale. This "backpulling" should be applied as cose to the propeller shaft thrust line as possible, but in this setup there are more quirks and the fault margin is certainly greater. But its better than no measurement at all......

    Good luck!
     

  11. Mr Efficiency
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    Mr Efficiency Senior Member

    Thanks for that.
     
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