Stability

   
 

Freeboards and inclination measurements give important hydrostatic and stability data necessary for determining elements of both performance and safety. The results of a stability test gives a boat's righting moment, vertical centre of gravity (VCG) and limit of positive stability (LPS), which with the Stability Index (SI) can help determine a boat's eligibility to enter races categorized according to the WS Offshore special regulations.

Freeboard measurements give a boat's waterline in measurement trim from which displacement, wetted surface and overhangs are calculated from the hull lines available in an offset file from the designer or by hull measurement. Freeboards are measured on the port and starboard sides at freeboard points identified in the hull offset file at specific distances from stem (SFFP and SAFP). Measured freeboards (FFM and FAM) are then adjusted for the specific gravity of the water, and also for pre-2013 measured boats for any differences in empty (“lightship”) measurement trim.

   
   
 

Inclining test

   
  An inclining test is performed on a boat in lightship measurement trim while floating in calm water and not affected on any side by lying to a mooring, and with no one aboard.
 
  • Two poles are simultaneously positioned on port and starboard sides, at defined position from the stem (approximately at the longitudinal centre of flotation, or LCF). The poles are suspended outboard to provide arms for supporting weights, and arranged to be perpendicular to the boat’s centreline and as horizontal as possible but still allowing sufficient clearance to prevent the weights from touching the water.
  • Either a manometer or an ORC-approved electronic inclinometer is placed on the deck and positioned athwart the boat where it can be read by the measurer. 
  • Weights, depending on the size of the boat, are suspended on one the pole on port side and resulting heel angle is recorded.
  • Half of total weight is suspended at the same time on the port in starboard sides so that the total distance between weights can be measured.
  • All weights are then suspended on starboard side and resulting heel angle is recorded again.
  Alternatively to the procedure defined above and particularly on boats that would require heavier weights to be suspended, a boat’s boom may be used to suspend weights as follows:
 
  • The boom is placed outboard and fixed with its end at defined position from the stem (approximately at the longitudinal centre of flotation, or LCF)
  • Heel angle without weights is be recorded either with a manometer or an ORC-approved electronic inclinometer
  • Weights are suspended on the end of the boom and resulting angle recorded again either with a manometer or an ORC-approved electronic inclinometer.
  • Same procedure is repeated on port and starboard sides, averaging the results
 
 
Inclining test   Electronic inclinometer and result
shown on computer screen
 


From the resulting heel angle, the amount of weight and the distance between weights it is possible to calculate the position of the vertical centre of gravity and the complete set of hydrostatic and stability data for the boat. A stability curve can then be calculated showing the righting arm against heel angle. The important point on that curve is the angle when the righting arm is equal to 0, called LPS (limit of positive stability) or AVS (angle of vanishing stability). This means that if the boat is heeled at that angle from an upright position it will still return to being upright, while beyond this angle the boat will capsize.
 

   
 

   
 

For boats with a canting keel, an inclining test is performed with the keel on centerline. After the keel is fully-canted the resulting heel is recorded so that the effect of the canting keel on the boat's stability can be calculated.

A complete explanation of stability curve and inclining test results can be found in the Stability and Hydrostatics Datasheet Explanation.

   
 


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