Bow half angle of entrance

Discussion in 'Hydrodynamics and Aerodynamics' started by skaraborgcraft, Jan 11, 2023.

  1. skaraborgcraft
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    skaraborgcraft Senior Member

    Just read a bit about prismatic coefficients, and a study done on 4, 70 ton 65ft trawlers and the power required for 9 knots speed. The hull with a 26deg half angle required 123hp and the hull with a 9deg half angle required only 75hp for the same speed at the same displacement.

    The definition in the book was " This angle is the one the load waterline makes with the centreline at its forward ending"....... which leaves me with the question of how far aft of the stem is the measurement taken? Searches on the site did not lead to any clarity that i could find.

    Given the power reducing requirement for the same speed and load, and having the option to extend the forefoot of the stem on the boat i am modelling, i consider this a worthy line of enquiry.
     
  2. HJS
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    HJS Member

    Just read a bit about prismatic coefficients, and a study done on 4, 70 ton 65ft trawlers and the power required for 9 knots speed. The hull with a 26deg half angle required 123hp and the hull with a 9deg half angle required only 75hp for the same speed at the same displacement.

    Please provide the source.
    JS
     
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  3. baeckmo
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    baeckmo Hydrodynamics

    Generally it is understood as the wl tangent at the stem.
     
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  4. skaraborgcraft
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    skaraborgcraft Senior Member

    "How to design a boat" by John Teal. A simple diagram of the 4 boats over laid would have solved much head scratching.
     
  5. skaraborgcraft
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    skaraborgcraft Senior Member

    A tangent has to have an end point to be measured.

    I read somewhere that "the point" was considered to be a line 20% of the half breadth from the centreline, where it crosses the load water line.

    It seems there is more than one opinion.
     
  6. HJS
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    HJS Member

    4, 70 ton 65ft trawlers
    Show us what you are referring to with illustrations from the book.
    JS
     
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  7. skaraborgcraft
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    skaraborgcraft Senior Member

    There are no illustrations, that was my point.
     
  8. skaraborgcraft
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    skaraborgcraft Senior Member

    Even on this thread there are differences, but, even so : "Half angle of entrance : the angle, measured at the LWL, between the hull centerline and the actual waterline shape." .........still does not address the point of measurement.

    The waterline is continous around the hull, but without giving a reference spot to measure from/to, how is one supposed to figure the angle? A 17deg difference in angle is a lot.

    I have not found any diagrams that are the same, so, should i conclude that without knowing the "rule" Mr Teale was using, this is an almost meaningless if there is not one established method in use?
     
  9. jehardiman
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    jehardiman Senior Member

    You state that the paper claims that one hull of the same length, speed, and displacement has only 60% of the resistance of another hull based solely on the entrance angle? I doubt that; regardless of how creative you are with the entrance angle measurement.

    Personally, I would be more interested in the wetted surface curve and the sectional area curve. I think the answer will be found in those two curves and the entrance angle is just an artifact of the different volume distribution.
     
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  10. skaraborgcraft
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    skaraborgcraft Senior Member

    I didnt want to add what may be unneccesary, as my focus was on entry angle, but If it helps, these were the other coefficients between the 2 hulls.

    block coeff (123hp) 0.383 (75hp) 0.359
    prismatic coeff 0.645 0.537
    midship coeff 0.594 0.675
    half angle entrance 26 deg 9 deg

    same length, beam, waterline.
    The 75hp model had 1ft of deeper draft forward and displaced half a ton more. Speed/length ratio of 1.14

    Scan of table posted below for clarity.
     
  11. skaraborgcraft
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    skaraborgcraft Senior Member

  12. HJS
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    HJS Member

    It is impossible to judge the accuracy of the claim about the resistance of the boats if we do not know what the boats look like. Show us what J Teal refers to, the full investigation report including lines plan
    JS
     
  13. skaraborgcraft
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    skaraborgcraft Senior Member

    I already responded that there are no diagrams, if there were, i would not be here asking for an explanation. It does not appear there is a recognized formula for this or it would have been mentioned by now.

    I made an assumption it was a recognized value obtained via a single process, perhaps something used by Teale himself and via his own method, but very badly written about .
     
  14. jehardiman
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    jehardiman Senior Member

    There is your issue right there, the first hull is pathetically inefficient for its volume.
     

  15. skaraborgcraft
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    skaraborgcraft Senior Member

    As an example between full and not so full bow sections.
    [​IMG]


    [​IMG]


    I imagine the less full bow of "Romp" is going to be more easily driven than the fuller bodied bow of the Pinky........ but halving the power to achive the same speed on the same length and displacement +/- seems to good to be true, though the Cp of about 0.54 is correct for the speed requirement.
     
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