Savitsky Power Prediction

Discussion in 'Software' started by guest12020101217, Sep 23, 2003.

  1. Chris Krumm
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    Chris Krumm Junior Member

    I have a copy of the Savitsky method applet published by Arneson (somewhere) . If I remember correctly, it is a very "short form" and simplified Savitsky power prediction method - assumes thrust line passes through CG, etc. Dingos' work in Exel exposes the calculations and formulas, which is really cool for gaining an underdstanding of what is behind "the answer."

    Chris Krumm
     
  2. rxcomposite
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    rxcomposite Senior Member

    Savitsky

    Yes, the program was very simple. Keep up the good work chris. We are all in search of "what happens if?". Dingos work is great. I have to find my copy of SNAME's publication on planing craft to decipher the formulas.
     
  3. DBDMarine
    Joined: Jul 2003
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    Location: USA, Asia

    DBDMarine New Member

    Same modeller for a Stern drive application

    Hi all
    We make a new stern drive leg and are looking for a application that we can use to not only predict vessel speeds, gearing prop diameter and pitch.
    Since the leg doesnt contain a gearbox, the drive uses a ZF 8 degree downangle box, which attaches to the drive.
    Now what way would I do this is there no strut, rudder, shaft etc but planing hull?
    Im trying to figure this out, looks like this is very close
    Can you give me any hints please
    Regards
    Darren Finch
    DBD Marine
    www.dbdmarine.com
     
  4. atahawaii
    Joined: Jun 2005
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    Location: Kaneohe, Hawaii

    atahawaii Naval Architect, P. E.

    There is a Savitsky spreadsheet available on the web at www.hawaii-marine.com/templates. It does not use Excel's Solver or macros or Visual Basic. It does the iterations automatically and evaluates several speeds. It does both the Short and Long Forms of Savitsky's methods.
     
  5. micheler
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    Location: Venice, Italy

    micheler New Member

    savitsky/CFD combined procedure

    Hi all,
    I am a new member of the forum. I found the spreadsheet of Dingo really interesting. I'm interested in making a slight modification on it in order to input manually the resistance forces that I obtain from a CFD planing computation.
    The procedure may be the following:
    1. determine the position of the boat with the Dingo's spreadsheet
    2. compute the drag and lift forces acting on the hull via a CFD free-surface computation (with sink&trim from step 1.)
    3. insert the CFD-computed drag estimates at the various speeds in the spreadsheet and re-run the computation uptating sink&trim
    4. return to step 2. until convergence...

    I have to say that I want to make CFD computations only of the hydrodynamic part computing viscous and wave resistance, but without simulating spray formation (almost impossible to get reliable spray-induced-drag up to date). So the results I get are only a part of the total drag/lift.
    Dingo could you suggest me where I should input my CFD drag/lift forces? I think the "Berekenings" sheet, but where? How the spray formation and aerodynamic forces are taken into account?

    Thanks
    Michele.
     
  6. Dingo
    Joined: Dec 2003
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    Dingo Junior Member

    Input

    Micheler,

    I will look into where putting the CFD results should go.

    Lines 78 to 83 is where the input for the air drag due to superstructure is placed. This can be modified for lift also but then the total moments must then be recalculated (add lift due to s/str in line 13).

    Please note that my e-mail address has changed.

    It has also come to my attention that the calculations when using a trim-tab might be slightly off. I will look into it and re-post the spreadsheet if incorrect and as soon as I have time. Sorry for any problems.
     
  7. Tim B
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    Tim B Senior Member

    Be careful using CFD, there may be a lot of instances with multi-phase flow. You also have a trim question to get right.

    Dingo, you don't have a copy of the SNAME Savitsky paper do you?

    Tim B.
     
  8. Dingo
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    Dingo Junior Member

    Savitsky paper

    Tim B,

    I do have a copy of Savitsky's 1964 paper at SNAME, but it is not the only paper that I used for my spreadsheet. All references are quoted in the spreadsheet in the 'Berekenings' section.
     
  9. Tim B
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    Tim B Senior Member

    Ok, I'll have a look at it sometime, I'd really like to add a Savitsky method to LMH at some point, but it will need to be written in C/C++. It also infers that LMH should be finished... yeah, Christmas perhaps.

    Cheers,

    Tim B.
     
  10. micheler
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    micheler New Member

    hydrodynamic forces computations

    Thanks for your replies.
    I'm trying to understand the "Berekenings" section. But I haven't the Savitsky paper so I have some questions.
    I'm especially interested in the first part of the section, where the Savitsky method is applied for computing the planing hull resistance (up to row #43).
    For computing the resistance, it seems that the Schoenherr '32 + the ATTC Ca of 0.0004 are applied on the Sf area.
    What does the Sf (=lambda * B^2 / cos (beta_e) ) area exactly represents?
    Why the Df force is computed with the 0.97 factor instead of the more usual 0.5? What it accounts for? Is it a correction for spray formation? Or is it merely a units conversion factor?

    After computing all force components, it is applied an M-factor. What does this correction stands for?

    Thanks
    Michele.
     
  11. subbu
    Joined: Nov 2005
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    Location: INDIA

    subbu New Member

    Hi

    Hi,

    Its great to hear from u that "Dingos' work in Exel exposes the calculations and formulas, which is really cool for gaining an underdstanding of what is behind "the answer." ".

    I would like to have that copy, can u send it to me

    Subbu
     
  12. patrik111
    Joined: Sep 2003
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    patrik111 Junior Member

    Hello All,

    Is the planing equations applicable to rounded bilged hulls? (i.e. modern round bottomed beach-cats).

    The question could also be read:
    Are the equations valid for long, skinny and rounded hulls?
    If not, at what are the input limits for the equations?

    Best Regards

    Patrik

    Best Regards
     
  13. patrik111
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    Location: Sweden

    patrik111 Junior Member

    Could deadrise for instance be approximated as some sort of average over the curved surfaces?

    Anyone have some sort of hint??
     
  14. rxcomposite
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    rxcomposite Senior Member

    deadrise

    Yes. The rulebook says that if it is a "S" curve deadrise should be measured from the keel intersection to the centerline to the outer edge of the chine.

    If the keel has a fairly rounded section, it should be measured from the point tangent to the radius to the edge of the chine.

    If you have a reverse chine, the edge of the chine pointing downward is still the edge.
     

  15. Willallison
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    Willallison Senior Member

    I've run Dingo's program on a number of occaisions since he 1st (very kindly) made it available to us all.
    I don't doubt the validity of the calculations, but perhaps someone might be able to give me a definitive means of interpereting them....
    For instance, I ran the numbers thru for a 40 footer powered by a single duoprop sterndive (hence the zero entries for rudder, strut etc...) with the following results:

    Inputs

    Hull Beam B 10.53 feet
    VCG VCG 3.31 feet = 1.009 metres
    Displacement D 14,265 lbf = 6,471 kg
    Deadrise @ Transom b T 9.3 °
    Deadrise @ Amidships b )0( 15 °
    Distance to Amidships L )0( 14.400 feet = 4.389 metres
    f 3.60 feet = 1.097 metres
    e 0.00 °
    q 2.182 °

    Minimum Speed Vmin 6.5 kn = 11.0 feet/s This is the minimum speed valid for this analysis
    Maximum Speed Vmax 141.8 kn = 239.3 feet/s This is the maximum speed valid for this analysis

    Number Number of Propellers N 1

    Trim Tab Chord cF 1 feet = 0.305 metres
    Span Ratio s 0.32 ( <= 1 )
    Deflection Angle d 0 °

    Rudder Chord crudder 0.00 feet = 0.000 metres
    Thickness t 0.00 feet = 0.000 metres
    Area Arudder 0.00 feet2 = 0.000 m2
    Centrepoint xc 0.00 feet from transom = 0.000 metres (+ve fwd)
    yc 0.00 feet from baseline = 0.000 metres (+ve up)

    Shaft Diameter of Shaft Fshaft 0.00 feet = 0.000 metres
    Length of Shaft & Hub l 0.00 feet = 0.000 metres
    Centrepoint xc 0.00 feet from transom = 0.000 metres (+ve fwd)
    yc 0.00 feet from baseline = 0.000 metres (+ve up)

    Strut Chord cstrut 0.00 feet = 0.000 metres
    Thickness t 0.00 feet = 0.000 metres
    Area Astrut 0.00 feet2 = 0.000 m2 ****
    Centrepoint xc 0.00 feet from transom = 0.000 metres (+ve fwd)
    yc 0.00 feet from baseline = 0.000 metres (+ve up)

    Output V LCG t D T Peffective h
    [kn] [ft] [metres] [°] [lbf] [kN] [lbf] [kN] [ehp] [ekW] [ft] [metres]
    6 10.5 3.2 4.9135 786 3.5 790 3.5 14 11 2.71 0.826
    7.5 10.5 3.2 5.4956 1,061 4.7 1,069 4.8 24 18 2.80 0.853
    10 10.5 3.2 6.7204 2,011 8.9 2,029 9.0 62 46 2.90 0.884
    12.5 10.5 3.2 7.4953 2,437 10.8 2,465 11.0 93 70 2.77 0.844
    13 10.5 3.2 7.518 2,465 11.0 2,493 11.1 98 73 2.72 0.829
    14 10.5 3.2 7.4427 2,478 11.0 2,505 11.1 106 79 2.59 0.789
    15 10.5 3.2 7.2444 2,451 10.9 2,477 11.0 113 84 2.45 0.747
    17.5 10.5 3.2 6.4822 2,313 10.3 2,333 10.4 124 93 2.11 0.643
    20 10.5 3.2 5.6434 2,175 9.7 2,190 9.7 133 100 1.82 0.555
    25 10.5 3.2 4.2328 2,034 9.1 2,043 9.1 156 116 1.40 0.427
    30 10.5 3.2 3.2351 2,067 9.2 2,073 9.2 190 142 1.13 0.344
    35 10.5 3.2 2.5363 2,227 9.9 2,232 9.9 239 179 0.94 0.287
    40 10.5 3.2 2.0349 2,483 11.0 2,487 11.1 305 227 0.82 0.250
    45 10.5 3.2 1.6647 2,815 12.5 2,818 12.5 389 290 0.72 0.219
    50 10.5 3.2 1.384 3,210 14.3 3,213 14.3 493 368 0.65 0.198

    ...hmmm I see that the table doesn't line up all that well, but the long and the short of it is that at my target max speed of 25 knots, the spreadsheet gives a required effective hp of 156hp.
    So, the question - how to relate this to SHP or even better, a published engine / propeller curve....?
     
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