Bollard pull – testing anchor holding power

Discussion in 'Projects & Proposals' started by Anders Skole Overgaard, Sep 29, 2018.

  1. Anders Skole Overgaard
    Joined: Sep 2018
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    Location: Copenhagen

    Anders Skole Overgaard Junior Member

    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 couple of 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.

    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. Tiny Turnip
    Joined: Mar 2008
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    Location: Huddersfield, UK

    Tiny Turnip Senior Member

    Can't help you with the bollard pull, but here's a thought:

    If an anchor will hold your boat until the force reaches that applied by the motor at 1800 rpm,

    and the same force from the motor will move the boat at 5.4 knots,

    the anchor will hold until a different force (current, wind) is applied that would move the boat at 5.4 knots.

    In the case of the current, that is likely to be around 5.4 knots, or a scrape more.

    In case of wind speed, it will be notably more, depending on the boats windage. You would have to do some experiments to find out, I suspect use gps in open water to find out how fast your boat drifts in a known wind speed. I'm no expert, but this seems a helpful approach in a real world situation. Harder to estimate if you are subject to current and wind at the same time.
     
  3. Mr Efficiency
    Joined: Oct 2010
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    Mr Efficiency Senior Member

    Some sort of spring gauge should do the job, a jumbo version of a fish weight scales.
     
  4. BlueBell
    Joined: May 2017
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    Location: Victoria BC Canada

    BlueBell Ahhhhh...

    Agreed, a large fish scale should work.
     
  5. Anders Skole Overgaard
    Joined: Sep 2018
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    Location: Copenhagen

    Anders Skole Overgaard Junior Member

    Thank you Mr Efficiency and BlueBell - I have thougth about the forces that the anchor should hold.
    It is quite usefull to compare holding power of the anchor with the force from 1.800 RPM / sailing 5.4 knots - i realise that the holding power is quite sufficient for my own boat.

    So the whole question is a bit theoretical - but still interesting in comparison with more official anchor-tests.


    I also know that I could use a big scale for fish or similar - i just hoped that i could get some information from a boat engineer or some other specialist.
    Maybe that specialist pop up - thank you!
     
  6. Anders Skole Overgaard
    Joined: Sep 2018
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    Location: Copenhagen

    Anders Skole Overgaard Junior Member

    Yes thank you all.

    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 precise 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.
    [​IMG]
    The pict shows force left in Kg - and RPM at bottom.
     
  7. BlueBell
    Joined: May 2017
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    Location: Victoria BC Canada

    BlueBell Ahhhhh...

    Well, you wont pull the two plow anchors or Rocna out of mud,
    but you'll pull that ugly, black thing out easily.
     
  8. Anders Skole Overgaard
    Joined: Sep 2018
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    Location: Copenhagen

    Anders Skole Overgaard Junior Member

    Hello Blue Bell

    I can tell you, that in mud these anchors hold only 10-14 kg = the Delta and the Rocna. Both of these slip slow and steady in mud. Mud from rotten seaweed that was.
    Even when my boat is only pulling astern with 700 rpm, these two anchors slip in that mud.

    In mud the "Ugly black thing" ( I suppose that you think of the Sea Grip = a Swedish anchor design) is supposed to hold very good, as well as the Bulldog 8 should hold very good i mud, when the flukes are in the biggest angle ( 45 degrees) ------ but this I have not tested yet.

    Also I have made strongest possible pull with the Rocna and the Delta sitting in hard sand , and then they hold as much as my boat can pull.

    So it is very much dependent of the bottom material how these anchors behave.
     
    BlueBell likes this.
  9. BlueBell
    Joined: May 2017
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    Location: Victoria BC Canada

    BlueBell Ahhhhh...

    I'm surprised.
    My 1 kg Mantus (plow) anchor will hold up to 65 kg in thick mud.
    I would imagine your chosen four would have no problem exceeding 100 kg holding force.
    I guess that proves your point that bottom material is a large factor in holding power.

    I've had my 10 kg Danforth (like the wide fluked anchor of yours)
    holding my houseboat in 35 knot winds with an estimated 200 kg pulling force.
    It held very well in a hard mud bottom with 10m of chain and 7:1 scope.

    My apologies for the derogatory comment regarding the black anchor.
    Obviously, I am not familiar with it.
     
  10. BlueBell
    Joined: May 2017
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    Location: Victoria BC Canada

    BlueBell Ahhhhh...

    This makes intuitive sense TT, but I'm afraid your logic is flawed.
    The static pull at 0 knots is different than the pull at 5.4 knots.
    At 5.4 knots the propeller is advancing and the water flow over the blades is different than at 0 knots.
    Depending on many variables, mostly prop design, pull will be higher or lower at speed versus static.
     

  11. Anders Skole Overgaard
    Joined: Sep 2018
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    Location: Copenhagen

    Anders Skole Overgaard Junior Member

    To BlueBell
    I must repeat that the mud i did try has been very VERY soft mud - really mostly rotting organic material.

    Anyway - I want to make some more testing when possible - I think it is interesting to understand these anchor-things.

    In the real world - when we anchor ourselves, we have used our 6 kg Delta on sand and that is very safe.
    Now we try the Rocna 6 kg for some time.
    The only problems we have had, are when the anchor land on too much stones - then these (normally brilliant) anchors dont hold very well.
    Fair enough - they are not so very heavy - only 6 kg - and cannot grip in a layer of cobbles / stones.
     
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