Gerr Formula B vs. Robert Beebe graphs

Discussion in 'Hydrodynamics and Aerodynamics' started by makobuilders, Apr 24, 2016.

  1. makobuilders
    Joined: Feb 2009
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    makobuilders Member

    I've surmised that most shipyards that I've had discussions with and also the propellor manufacturers must all be using Gerr or a derivative formula, because the hp and speed predictions are all too conveniently similar.

    However these figures do not take into account the slenderness or fatness (is that a word?) of the vessel, if it has the correct prismatic, etc.

    Robert Beebe swore by the accuracy of his f1 and f2 charts. I have not been able to find any reference to how he created his graphs but I would assume that they are based on long, slender hulls because his powering calcs runs about 20% less than Gerr.

    Does anyone have some enlightenment?
     
  2. TANSL
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    TANSL Senior Member

    In my opinion, all these formulas were very useful at the time because there were no other methods of calculation. At present it is necessary and mandatory, perform other calculations, much more complete, to define the scantlings of a fiber helmet. I do not think any regulatory body today supports those calculations.
    Of course, imo, under current procedures you can get lighter structures.
     
  3. makobuilders
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    makobuilders Member

    Uhh, Tansl, I think you're referring to the other thread about hull scantlings? This one is about powering and speed prediction.

    Still, your feedback is appreciated :)
     
  4. TANSL
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    TANSL Senior Member

    Yes you're right. My mistake, I was thinking on scantlings since Gerr, if I'm not mistaken again, also has various formulas for scantlings.
    I have a software for predicting the power/speed for fishing vessels. If you are interested, I can indicate you the necessary data.
     
  5. makobuilders
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    makobuilders Member

    Yes that would be appreciated and would be interesting to see how it compares to Gerr B. What data do you need?
     
  6. TANSL
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    TANSL Senior Member

    Data :
    Draft, meters
    Displacement, m3
    wet surface, m2
    Block coefficient
    Waterline length, m
    Maximum beam at the waterline, m
    Propeller diameter, m
    Propeller r.p.m.
    Propeller directly coupled or geared
    Rudder area​
     

  7. FAST FRED
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    FAST FRED Senior Member

    I believe Bebee was using boat hulls that were very similar to sail hulls underwater and are very sweet to push.

    Also cruising speeds of his era were closer to the Sq Rt of the LWL,( SL1) rather than the faster speeds seen advertised today.

    Until high speeds and skinny boats 6-1 L/B ratio and above the simple Beede graph will do fine for ballpark estimates.
     
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