Stability

Overview

Stability 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 shape defined in the offset filet. Freeboards are measured at freeboard points identified in the hull offset file together with specific gravity of the water that equalize displacement calculation for boats measured in fresh water with those measured at the sea.

Inclining test

An inclining test is performed on a boat in 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 (approximately at the longitudinal centre of flotation)

-     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 to the shrouds.

-     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

-     Longitudinal position of the weight and distance from the boat’s centerline is measured on both sides    

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.