The Conversion Process
(Hard Chine to True Round)
Hard chine hulls are only define the Sheer-line, Upper chine line, Lower chine line, Fairbody line, Stem profile, Transom profile, and Design waterline.
The crossing points of these lines with the station lines define exact 'XYZ' location as measured from a chosen point of reference, such as the Bottom of the Keel, centerline of the hull, design waterline or Fore Point.
Changing a Double Hard Chine hull to a True Round hull is straightforward because there is a commonality between the Upper and Lower chine lines of a ‘Hard Chine’ design, and the Upper and Lower theoretical chine lines of a ‘Bezier’ design.
The Commonality is simple, they are both ‘One and the Same’. This simple Truth is why the middle surface between the hard chines of a Double hard chine design can be replaced with a True Round surface that will flow smoothly (Tangent) into the Upper and Lower developable surface panels along the entire length of the hull.
The conversion process is beyond the intent of this writing, therefore, only the results of the Conversion are detailed here.
For those so inclined, this information can be found in another of my books ‘True Round Metal Boat Design – Bezier Design’.
The Renderings on the previous page illustrated the dramatic results of that conversion of 'Thunder Cloud', lets now look at the conversion from an Engineering point of view, beginning with 'Architectural Lines' were both versions of the design are overlayed onto each other.
Section View:
· The Sheer and Fairbody Lines remain unchanged.
· Upper and Lower Chine Lines remain unchanged.
· Stem and Transom Profiles remain unchanged.
· Keel and Rudder Configurations remain unchanged.
· The straight lines between the Upper and Lower Chine lines denotes the original Hard Chine Surface remain unchanged.
· The curved lines between the upper and Lower Chine lines represents the new ‘True Round’ surface.
Plan and Profile Views:
· The Profile view shows nothing that would indicate a change between the two versions of the design. Both versions appear the same, all lines overlay each other exactly in this view.
· The Plan view shows two (2) 'Design Waterlines'. One each for the original 'Hard Chine' design and one for the 'True Round' design. All other lines overlay each other exactly.
It may seem that I am indicating that nothing has changed in the conversion except the shape of the hull between the upper and lower chine lines of the 'Hard Chine' version and now the 'Theoretical' chine lines of the 'True Round' version of the ‘Thunder Cloud’, when in case there were Static Hydrostatic and Stability changes.
In the below section drawing, observe the darkened-in area at Station 8 betweenthe straight line that represents the Hard Chine design and the curved line of the True Round design. This is the area of increased volume of the hull. You will find some of the increased volume is above the 'Design Waterline' and some below.
The original 'Hard Chine' design weighed 20,015 pounds. After displacement calculations the converted true round version of the hull weighed 21,664 using (the Hard Chine waterline as a benchmark). There is adifference of 1,247 pounds.
No, the Conversion did not cause the boat to gain weight. The increased width of the 'Design Waterline' cause by softening the turn of the bilge added geometric hull volume to the new True Round hull form.
Displacement Adjustment: One way of looking at correcting the displacement, in the computer model, is by moving the Vertical position of the 'Design Waterline' down, on the True Round version of the hull. Moving the ‘Design Waterline’ down, within the computer, from its original position until it reflects the adjusted displacement of 20,264 pounds.
According to my calculations, the distance of the move is 0.750". In effect the draft of the hull decreased by 0.750" and the freeboard increased by the same amount.
Vertical Center of Gravity Adjustment: The second adjustment involves the ''Vertical Center of Gravity. The 'Weight Study' for the original Double Hard Chine design placed is 'Vertical Center of Gravity' at 1.25" above its 'Design Waterline'.
With the 'Vertical Center of Gravity' fixed to the physical boat, moving the 'Design Waterline' down by 0.750 now places the 'Vertical Center of Gravity for the true round version at 2.000" above its 'Design Waterline'.
The conversion process is so seamless that the builder could continue to use the original drawings to construct every aspect of the design, except the newly converted true round section between the freeboard and bottom of the hull.
Architectural drawings and full-size patterns for the transverse framing, longitudinal framing, and shell plating between the now theoretical chine lines will need to be updated.
The builder need not be concerned about Displacement and Center of Gravity changes as previously discussed for any construction purpose. At launch, your boat will simply float 0.750" higher in the water than shown on the original plans.
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