Hi Row,
what appears symmetrical it is not, it is sufficient to see where the LCB Dot is positioned. The most close to symmetry is the 43-900 curve with only LCB of -2.0%.
Each boat has is own COA and for me it is the “identification card” and telling all what it is needed to know. This is why my design are always including the COA curve. COA curve help me a lot when drawing the shadows and where corrections are required (fairing).
Here below some COA (Curve of Areas) examples :
…
Observing each graph, there is the LCB position marked by a dotted vertical line. In each COA the LCB is different.
Some curve look more “fat” indicating that the volumes are sprayed toward the Ends, some instead have the volume concentr-900 Curveated to the central part and this is revealed also by the Prismatic Coefficient calculated at the bottom of each graphic.
Close to 0.50 will indicate that the volume is centralized and toward 0.60 will indicate the opposite. A shoe box have a PC = 1.0.
In general as “Rule of Thumb”, Low PC range, 0.50 - 0.54, are dedicated to models navigating is calm waters low winds condition, 0.55 - 058, to medium conditions (all round) and over 0.58 for higher winds and multi hulls.
A graph also provide the relation between PC and Vr (relative speed).
This point is very interesting since our models will never go to surf /planning conditions and therefore the maximum achievable speed is around a coefficient of 1.2, in the scale from 0.2 to 3.5.
Coefficient of 1.0 is the equivalent to the critical speed of each boat. Coefficient of 2.0 is the begin of planning condition.
I will present some graphics another time, but are the ones that dictate all the times my design approaches.
Books reveal the LCB vs PC shall ‘obey’ to a certain curve. The “ideal” would be to keep the LCB close to -3.5% of the LWL.
The sign (-) indicate that the LCB is positioned behind the Shadows n° 5 when 10 shadows are used or referred to the Central Shadow in any case.
As conclusion the COA curve of the J’Class look rather symmetric and sharp and this is due to the effect of the long keel where most of the volumes are concentrated. The PC is also very low because of the COA curve shape, where the Central shadows have a very large surface due to the presence of keel. One should remove the keel and recalculate the volume and we will find out that the COA could become more similar to rounded hull equipped with Fin/Bulb and the PC raising around 0.55 or more.
The COA curve reveal also the water lines profiles.
You may notice the entrance of the water lines of the American Eagle that are rather fine compared to others and this is observable by deeper bent of the curve from shadows 2 to 0.
Another times we could discuss more about. It happen to write a book some years ago on the subject, unfortunately is in French and Italian since nothing was written on the Hull Drawing subject when compared to the reach literature available in English.
Actually I like first to finish the “job”, one boat is still missing and some shadows too.
Cheers
ClaudioD
PS:
Forgot, when observing the American Eagle curve it is also noted the LCB of -5.6%. This is due to the particular long keel toward the stern that induce a volume shift in the overall calculation and therefore also the LCB shift. Another peculiarity, for the educated observers, is the almost strait slope of the curve from shadow 7 to 10 when compared with the soft curve of the others.
