How could i calculate the pulling power of my Outboard

Discussion in 'Outboards' started by Anders Skole Overgaard, Oct 2, 2018.

  1. Anders Skole Overgaard
    Joined: Sep 2018
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    Anders Skole Overgaard Junior Member

    Sorry to repeat my question in this group - but maybe there is a specialist in this group, who can help.

    I made at test of 4 different anchors – and would like to know the pulling power my boat makes in these tests.

    I have not found anything similar / usable when googling for a several days so now I go to the experts here. [​IMG]

    I have made tests of a handful of anchors to learn how these different anchors behave – and I do NOT have any instrument to telle the actual pull force.
    I have just made a comparison of the revolutions of my engine pulling statically on my anchor.
    So for each anchor I have holding power in relation to revolutions/ minute as bollard pull.
    OK - but it would be nice to know the actual holding power.

    How could I calculate in kilograms how big the forces are?

    Boat:
    Our boat is 19 feet planing Flipper 570 with 60 HP Yamaha and standard alu propeller.
    Idle 700 revs/min – 5.4 knots speed = 1.800 revs/min
    Planing 12 knots = 3600revs/min – full = 4.100revs/min

    My testing: Delta 6kg, Rocna 6kg, Bulldog 8 and SeaGrip 10kg.
    All tested same place and in same conditions, AND All did hold rather good.
    Some did hold until 1000 revs/min
    Others did hold until 1.800 revs/min or little more.
    Strangely they do actually behave a little different – interesting
    Some dragging when "overloaded", others "popping out" from the bottom and needing resetting.

    So far I´m very satisfied to learn about the behavior of these anchors – but it would be nice to know the actual kg holding power.
    Can anyone help.

    My best guessing is that bollard pull / holding power at 1800 revs = 150 kg
    and at 700 revs/ min bollard pull / holding power = 30 kg
     
  2. rwatson
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    rwatson Senior Member

  3. Anders Skole Overgaard
    Joined: Sep 2018
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    Anders Skole Overgaard Junior Member

    Thank You R Watson
    I do really study these anchor-tests. I also am going to study your suggestion once more.
    I have also read this very interesting webpage very carefully : Independent Anchor Performance Testing https://www.petersmith.net.nz/boat-anchors/independent-performance-testing.php
    I also have a pretty good idea of how to test methodical, so that i CAN compare different tests i make.

    I would just like to do a little testing myself - and at the same time know more exact the pulling force - WITHOUT needing to buy expensive equipment.
    Especially I would like to test difference of my anchors in more or less soft bottom.
    I have been surprised to realise, that in sand wich is just a little muddy, the holding power seems very much lower - i would like to know HOW much lower it actually is.

    I do hope that some technician can give me a hint about the calculations :)
     
  4. baeckmo
    Joined: Jun 2009
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    baeckmo Hydrodynamics

    Sorry Anders, but there is no reliable way to calculate the propeller force in your test. First of all, you have a prop operating at zero forward speed with exhaust blow-out through the propeller center, and probably also leaking to the suction side of the propeller blades. In this situation, the prop is ventilating and there are no "standard" values of thrust coefficient applicable. Instead, an intelligent arranging of mom's bathroom scale would easily do the trick.

    Just make a run with the scales sandwiched between boat and bollard, noting the push at the relevant series of rpms. Then you have your calibration diagram, just read your anchor test rpms off the chart. But don't loose the scales........ or you will be in trouble!
     
    Barry likes this.
  5. Blueknarr
    Joined: Aug 2017
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    Blueknarr Senior Member

    Have you asked Yamaha about the pulling power of that model of 60 HP?
    Checked the range of published pull tests for other similar engines?
    Does the prop manufacturer publish thrust to rpm data?

    Other wise a long line to a block hanging from a stout tree branch. How many sandbags can you pull up!
     
  6. PPRINT
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    PPRINT RobT

  7. Mr Efficiency
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    Mr Efficiency Senior Member

    500 lbs might be cutting things a bit fine.
     
  8. Anders Skole Overgaard
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    Anders Skole Overgaard Junior Member

  9. rwatson
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    rwatson Senior Member

    If you were doing a serious study, the equipment would be cheap.
    As it is, I don't understand why you are bothering.

    There are plenty of indicative comparisons done on anchors, and all of them will only ever be approximate, because in real life you cant design your sea bottom.

    Likewise, pulling with a motor is not a good test. It's not the same as testing the effect of the boat being anchored, unless you are planning on being in a 200 mph constant cyclone . The anchor rode and chain used are as important as the fluke holding power, and that doesn't get tested in a straight bollard pull.
     
  10. Anders Skole Overgaard
    Joined: Sep 2018
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    Anders Skole Overgaard Junior Member

    I´m doing this as serious as possible.
    To get to know our own anchors behaviour.
    But I dont have plenty of money - and you should only respect, that i try to do my testing cheaply - if that is possible.

    When I have anchored with scope 1:5 - and pull with boat, then i also test how the anchor rode AND the anchor is working together.
    When the boat is anchored and I pull, then it IS "bollard pull". The boat does not sail anywhere :)
     
  11. philSweet
    Joined: May 2008
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    philSweet Senior Member

    For 168 USD, I'm tempted to buy one just to have around. I don't think I could cobble even a rough estimate gauge this cheap. You could use a series of monofilament fishing line fuses to calibrate bollard pull at the dock, but the boat's attitude would be different when backing down on an anchor unless you fed the line under the dock.

    Force Gauge PCE-CS 1000N | PCE Instruments https://www.pce-instruments.com/us/measuring-instruments/test-meters/force-gauge-digital-force-gauge-pce-instruments-force-gauge-pce-cs-1000n-det_2132926.htm?_list=qr.art&_listpos=4
     

  12. Anders Skole Overgaard
    Joined: Sep 2018
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    Anders Skole Overgaard Junior Member

    Yes thank you P Sweet - and everybody else.

    I have been Googling and reading and telephoning engineers for many days now - and it seems NOT possible to find the numbers I´m looking for.

    Yesterday i then made some tests with the instrument i had at hand - a small scale / for suitcases - and made a simple weigth exchange with a stake.
    Some of the first pull-tests were directly - the stronger pulling was over my balanced stake + a bit of calculating.

    I did actually measure smallest pull- force at idle = 700 rpm - only 14 kilograms
    Increasing gradually to 1900 RPM and measuring 78 kilograms

    My measurements were not very precise but shows clearly that the pulling power (bollard pull) is far less than I did expect.
    The bollard pull was also far less than calculations and thumbrules from calculating on ships bollard pull.

    My primitive testing shows clearly, that if I buy a cheap scale going to 200 kilograms, i could make these bollard pull measurements quite presise and easy another day.

    In short:
    I realise that it is not possible to calculate these things realistically.
    I now know the pulling force goes from 14 kg to APPROXIMATELY 100-120 kg at the most.
    I realise it could be measured vith a scale going to 200 kg

    In general - im very surprised that the forces are so modest.

    The pict shows force left in Kg - and RPM at bottom.
    [​IMG]
     
    philSweet likes this.
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