Is it stable enough for a powerboat?

Discussion in 'Powerboats' started by magwas, Aug 29, 2010.

  1. magwas
    Joined: Oct 2009
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    Location: Hungary

    magwas Senior Member

    I have drawn this boat. It is aimed to be "something similar to" Neptune 20 (see http://www.davisonsboats.com/norman20.php ). The hull was a Platt Monfort style long ago, but I have heavily modified the aft part.
    It is now in the "we have a closed hull" stage. The opening for outboard and drain is not yet drawn in the stern.

    The question is whether the stability is adequate for a powerboat.
    I am also interested in any comments regarding planning properties.

    I did some calculations with FreeShip.
    At 1 ton projected displacement, and 0.6m estimated CoG it says:
    1. Amplitude of roll: 21.6 deg
    2. Maximum dynamic heeling angle: 90.0 deg
    3. Maximum heeling moment: 2.5 kNm
    4. Maximum wind moment: 2.1 kNm
    5. Weather criteria: K=1.185
    I am especially at loss with this weather criteria. I tried to google it, but found nothing.

    Looking at the SSD and DSD it seems that the righting lever is always positive, except at some 128 degrees in the static case. I cannot correlate it with the maximum dynamic heeling angle of 90 degree.

    The original request for the design is here: http://www.boatdesign.net/forums/se...to-software-program-few-33901.html#post388372 (no $ in the game, I am just a hobbyist interested in whether I am able to design such a boat with the help of this forum)

    There are two Freeship files, because I have desinged the cabin door out of the centerline -> that part is not symmetric -> there are leak points. So there is one file with no door but hydrostatically OK modell, and one with actual details.

    Hydrostatic data:

    Design length : 6.048 m
    Length over all : 6.048 m
    Design beam : 2.498 m
    Beam over all : 2.488 m
    Design draft : 0.300 m
    Midship location : 3.024 m
    Water density : 1.000 t/m^3
    Appendage coefficient : 1.0000
    Volume properties:
    Displaced volume : 1.006 m^3
    Displacement : 1.006 tonnes
    Total length of submerged body : 5.876 m
    Total beam of submerged body : 1.699 m
    Block coefficient : 0.3358
    Prismatic coefficient : 0.6547
    Vert. prismatic coefficient : 0.4678
    Wetted surface area : 8.060 m^2
    Longitudinal center of buoyancy : 2.466 m
    Longitudinal center of buoyancy : -8.034 %
    Tranverse center of buoyancy : 0.000 m
    Vertical center of buoyancy : 0.209 m
    Midship properties:
    Midship section area : 0.261 m^2
    Midship coefficient : 0.5128
    Waterplane properties:
    Length on waterline : 5.876 m
    Beam on waterline : 1.699 m
    Waterplane area : 7.165 m^2
    Waterplane coefficient : 0.7178
    Waterplane center of floatation : 2.341 m
    Y coordinate of DWL area CoG : 0.000 m
    Half entrance angle of DWL : 13.950 degr
    Transverse moment of inertia : 1.308 m^4
    Longitudinal moment of inertia : 13.664 m^4
    Initial stability:
    Vertical of transverse metacenter : 1.510 m
    Tranverse metacentric radius : 1.301 m
    Longitudinal transverse metacenter : 13.798 m
    Longitudinal metacentric radius : 13.589 m
    Lateral plane:
    Lateral area : 1.474 m^2
    Longitudinal center of effort : 3.026 m
    Vertical center of effort : 0.168 m
    Hull characteristics above waterline:
    Lateral wind area : 4.860 m^2
    Z coordinate of wind area CoG above DWL : 0.658 m
    Distance from bow to wind area CoG : 2.073 m

    All plans and drawings are freely useable under CC-BY-SA.
     

    Attached Files:

  2. vazer

    vazer Previous Member

    magwas


    thank you for your effort but that "design" is more for professional builder, to much complicated for me ,curved,I would like if it is more square,more simple with "hard chine"

    the plywood is 8. 10, 12 mm and wood (planks) for battens and frames is white oak

    and transom would be for stern drive, i got cheep mercruiser 3.0 with alpha one leg from burn out boat,(some German tourist was filing petrol, smoking and running engine at the same time)
     
  3. magwas
    Joined: Oct 2009
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    Location: Hungary

    magwas Senior Member

    This design is a hard chined one optimized for stitch&glue. As the hullform chines are relatively narrow, I believe the hull part is doable out of even 20mm plywood. I don't yet have a number for hull thickness, but if it will be thicker than what you have or you can bend, you can use multiple layers of thin plywood. I will redesign the cabin sides to be more square. I can also convert the hull to use only flat panels, but I won't recommend it.
    Of course I will provide the developments. The developments for the current design is attached as an example. Can you read dfx files?
     

    Attached Files:

    Last edited: Aug 30, 2010
  4. vazer

    vazer Previous Member

    thank you magwas


    i would like if you can do everyrhing as much flat as possible, something like this....



    [​IMG]

    [​IMG]

    [​IMG]
     
  5. magwas
    Joined: Oct 2009
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    magwas Senior Member

    Okay, but it will cost you twice in design price, as I don;t like it. Let's see: 2*0$ = 0$.
    I will came back with
    - the same but two chines eliminated
    - a flat bottomed one chine version
    - the above, but no bending at all
    And you will choose.
     
  6. apex1

    apex1 Guest

    Magwas,

    should´nt you tell Vazer that he gets what he pays for? That your sketches are by no means a "design" in terms of Naval Architecture, but a novices trial drawings, and data produced by Freeship?

    Just to be fair, I think you should.
     
  7. magwas
    Joined: Oct 2009
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    magwas Senior Member

    I did tell him that I am a hobbtist and cannot reliably tell anything about the stability or mechanical strength of the vessel. You should have read it, as you never miss anything in this forum.
    Do you have anything to add to the topic itself?
     
  8. Alik
    Joined: Jul 2003
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    Alik Senior Member

    Take ISO12217 and check it.

    'Weather criteria' is not critical for boats of category C and D (I assume Yours is cat.C). More important is offset load test (i.e. initial stability) that is critical.

    I suppose this 'weather criteria' is calculated in FreeShip using IMO formulas for commercial ships? Nothing to do with small craft...

    You need to get real weight/CG calculations, and increase VCG fo 5% of D as safety margin for further calculations.

    The problem You having is very common for amateur software users - they can press the keys, but have now clew what the result is.
     
  9. magwas
    Joined: Oct 2009
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    magwas Senior Member

    Thank you for the clarification.

    (But at least I know I don't have a clue, willing to learn, and don't ask without at least some research.)
     
  10. Alik
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    Alik Senior Member

    That is really good attitude.

    I have seen one guy 'designed' a sailboat in FreeShip, and calculated the resistance by Savitsky method :)
     
  11. magwas
    Joined: Oct 2009
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    magwas Senior Member

    There are the three variations of hull form.
    Choose one.
     

    Attached Files:

  12. daiquiri
    Joined: May 2004
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    Location: Italy (Garda Lake) and Croatia (Istria)

    daiquiri Engineering and Design

    Sounds logical. Both Sailboat and Savitsky start with "S".
     
  13. magwas
    Joined: Oct 2009
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    magwas Senior Member

    BTW I am also at loss with resistance calculation methods. Is there any description on which one is for what type of craft?
    I am usually try all of them, and see which gives any result.
    Unfortunately some of them in FreeShip does not state plainly that they are not for that job, just simply fail to produce any result.

    I am getting pretty off-topic...
     
  14. Alik
    Joined: Jul 2003
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    Alik Senior Member

    I do not use FreeShip (once installed and tried - nothing interesting) :)

    You have to study applicability of methods using Principles of Naval Architecture or other reference books. There are excellent papers with review of methods from Savitsky/Mercier (for semi-planning boats) and Almeter (planning craft).

    Using methods outside their range of applicability will cause completely wrong results, even if pressing 'magic key' can produce some graph on 'magic screen' :)
     

  15. vazer

    vazer Previous Member

    thats it , Alik, reminded me, I could not remember word, i am going for "semi-planning" hull

    Thank you Alik


    and, for the boat of this size (20 ft), is it ok and safe to put 82 galon fuel tank
     
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