Mirror & Hydrostatics

DELFTship forum General Questions Mirror & Hydrostatics

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    • #44964
      John949
      Participant

      I’m trying to design a model yacht and want to get Hydrostatics at fairly large heel angles.  The suggested method is to turn hydro and symmetry off, mirror the hull and then turn hydro back on.  You can then rotate the hull and hydro will calculate correctly.

      This seems to work fine for a hull, but my model has a fin keel, bulb and rudder.  When I try the above with these it all goes pear shaped.  The fin and rudder were created with the wizard and when I try to mirror these, the shape I get is not symmetrical about the vertical axis and hydro fails.  When I try it with the bulb (which I drew manually – a simple torpedo shape), I seem to get a symmetric shape when I mirror it but Hydro still fails.  Note Hydro fails before I try and rotate anything it seems to be the mirror command that is not working as I would expect.  I have no leak points even with the rather bizarre fin / rudder shapes.

      Anyone know what I’m doing wrong?  (Buying the full version with the Heel & Trim package is a bit OTT for a model yacht.)

    • #44966
      Marven
      Keymaster

      If you make sure the entire model is mirrored (including keel and rudder) then this should work fine

    • #44967
      John949
      Participant

      Tried that, no good.  Hydro still fails if I enable it for the any of the fin, rudder and bulb.  More worryingly the calculation for CLR doesn’t seem to work properly once the model is heeled.  If I make a significant change to the profile (e.g. increase the under water area at either end) the CLR calculation returns an unchanged value.  It does what I expect with a symmetrical model but not once it has been rotated.  LCB seems to work okay thought.

    • #44968
      Marven
      Keymaster

      If the hydrostatics fail then you have one or more leak points in your model.

    • #44969
      John949
      Participant

      Yes the bulb had leak points (doesn’t have them when set to symmetrical but they appear when I mirror it!)  The mirror command is definitely not working properly for a fin keel, see attached.  The LHS is what I drew, the RHS is what mirror produces.  The bottom is worse as the mirrored section doesn’t even reach the bottom.  BTW leak points are on and there aren’t any but still no hydro.

      I have also hit a major snag trying to calculate heeled CLR.  The program calculates the CLR for the unheeled profile (i.e. the CLR for the centre line of my model)  Whereas it should use the the vertical projection of the heeled model.  You can demonstrate this by moving points off the centre line – they have no effect on the calculated CLR, even though the side view is visibly changed below the waterline.  If you change a point on the centre line, CLR changes – even if it does not affect the visible profile.  I guess I’m going to have to resort to Simpsons Rule!

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