Hydrostatic
Coefficients
Stability
Displacement:
The original 'Hard Chine' design weigh was 20,397 pounds. The 'True Round' design weigh was 20,285 pounds.
Length of the Design Waterline:
The original 'Hard Chine' had a 'Design Waterline' length of 31.83 feet, while the 'True Rounds' 'Design Waterline' was 31.52 feet. The decrease was triggered by raising the 'Design Waterline' 0.750" to accommodate the corrected displacement.
Waterline Beam:
A most noticeable change would be the 'Beam at the Waterline'. The 'Hard Chine' version was 10.04 feet and the converted true round version is 11.14 feet. Again, looking at the above drawing it is easy to see why the beam increased.
Waterplane Area:
With an increase of 'Waterline Beam', the area of the 'Waterplane' naturally increases. The 'Hard Chine' version is 223.10 square feet, while the 'True Round version increase to 246.35 square feet.
Wetted Surface Area:
Changing the middle surface of the hull from flat to a free-formed curve increases the Wetted Surface. The original 'Hard Chine' design had a 'Wetted surface' area of 335.13 square feet. The 'True Round' hull has a 'Wetted surface' of 349.20 square feet.
Lateral Plane:
The 'Hard Chine' design has a lateral plane of 98.18 square feet, while the 'True Round' design was reduced to 93.54 square feet. Raising the hull 0.750" to correct the displacement triggered the reduction.
Righting Moment per degree:
The 'Righting Moment per degree for the 'Hard Chine' design was 1,130 pounds. The 'True Round' Design was larger at 1,697 pounds. The 'True Round' hull is initially more stable than the 'Hard Chine' design. The increase from 1130 to 1697 is due to the increased width at the design waterline.
Draft:
The draft went from 5.75 feet in the 'Hard Chine' design to 5.69' in the 'True Round' design. We lowered the waterline thereby reducing the draft and increasing the freeboard.
Vertical Center of Gravity:
The 'Vertical Center of Gravity' for the 'Hard Chine' was located 1.25 inches above the 'Design Waterline'. In raising the hull to correct for displacement, the 'Vertical Center of Gravity' moved with the hull placing it at 2.000 inches above the 'Design Waterline'.
Coefficients:
All the coefficients are not changed to any meaningful degree, however the 'Displacement to Length' ration increased from 282 to 294. This is a result of shorting the "Design Waterline'.
Typical Stability Chart
On the Generic ‘Stability Chart’ below I have drawn an 'S' curve that illustrates a typical stability curve applicable to the two designs used in our comparison. Plotted here are three angles of heel that are commonly used to compare hull designs.
· ‘Righting Moment’ at 30-degrees - This angle of heel is considered the high end of the normal sailing range. The 'Righting Moment' at 30-degrees is also used to calculate the physical size of the mast.
· Maximum ‘Righting Moment’ Angle - The 'Angle of Heel' at which the hull produces the greatest 'Righting Moment' to return to an upright position.
· AVS - 'Angle of Vanishing Stability' - At this angle of heel, the boat will no longer return to an upright position, but will capsize.
The left vertical side of the chart plots the 'Righting
Moments' in pounds,
while the bottom of the chart plots the 'Angle of Heel'.
The Base line of the chart is the Zero (0) line indication a stable static position. Any line drawn vertical from an 'Angle of Heel' that crosses the 'S' curve above the 'Zero' Base line is positive Righting Moment. Any crossing below the Zero ’Base line’ is negative 'Righting Moment'.
Stability - Double Hard Chine Hull:
- Thirty-degree (30) angle of Heel : - 32,305 Pounds
- Maximum Righting Moment Angle: - 70-degrees at 40,435 Pound.
- AVS – Angle of Vanishing Stability: - 127-degrees.
Stability - True Round Hull:
- Thirty-degree (30) angle of Heel : - 35,515 Pounds.
- Maximum Righting Moment Angle: - 70-degrees at 44,638 Pounds.
- AVS – Angle of Vanishing Stability: - 122-degrees.
Conclusion:
I see no adverse results due to the conversion!
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