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© 2009 Mark Kennett

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Boat design analysis:
Intact statical stability curves for the Independence 48

Click here for an introduction to intact statical stability, or find this article in the technical articles section. 

The Independence 48, an offshore displacement motor boat, shows a useful curve of static stability, or GZ curve.

As could be expected with a design based on fishing boat lines, the relatively low freeboard results in the deck edge becoming immersed at 16.5 degrees. Due to the bold sheer of this craft this is in fact a ‘point’ immersion and there is plenty of freeboard fore and aft of this point.

The GZ arm continues to lengthen right up to 90 degrees. At 45 degrees  the wheelhouse and cabin margins start to get wet and at 90 degrees (on its side) the hull is partially floating on its superstructure as do all boats at this angle of inclination.

At 135 degrees the hull is on its way to being upside down. It still shows a distinct positive righting arm, the superstructure playing an important role in this.

The hull shows a modest positive stability at 175 degrees and so can be described as self-righting, if watertight integrity is maintained at all times.

See the actual graphs of statical stability and righting moment below. Note that they represent the hull in a light condition. Additional weights added low in the hull will increase both the righting arm and the righting moment.

GZ curve: Independence 48

boat design: Independence 48 - gz curve

Righting moment Independence 48

boat design: Independence 48 - righting moments

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