Wednesday, December 17, 2008

Stability Thoughts.....

Here's a short stability ditty for the folks in the Philippines.....

Stability Testing - A simplified stability test may not be as conservative as is commonly assumed. The simplified test is done on a pass/fail basis moving weight and comparing the angle of heel with a reference mark. The required weight shift is based on passengers remaining in one general area and may not adequately account for large passenger movements.

As an example, consider the possibility where 150 passengers line the rail on one side of a vessel with an 18 foot beam to observe a manatee. If a large glowing UFO suddenly surfaces on the opposite side of the vessel and passengers shift 10 feet, a 240,000 ft-lbs heeling moment is generated (150 passengers x 160 lbs. x 10 ft.). The required stability test heeling moment is only 64,000 ft-lbs (16 ft. beam accessible to passengers x 150 passengers x 160 lbs. divided by 6). In this example actual passenger weight shift exceeded the stability test requirements by almost 400%!

Passenger Movement and Stability - Weight shift by passengers on the upper deck may have a significant effect on vessel stability and all operators are reminded to review their Certificate of Inspection and Stability Letter for upper deck passenger limitations.

Vessel operators should implement stringent control procedures to strictly enforce upper deck passenger limits. Passengers naturally tend to go up and in any circumstances may actually crowd the rails to view a special occurrence such as a whale sighting. Adding extra passengers on the upper deck shifts the vessel’s center of gravity up which adversely affects stability. Passenger movement from side to side on the vessel also presents a significant heeling moment, and may affect the vessel’s stability.

Wind Heeling and Stability - Another important consideration is the effect that wind plays on a vessel’s stability. High winds or wind gusts can adversely affect the vessel’s stability; which could cause it to capsize in severe conditions. The effect that wind has varies with the wind’s strength and direction, the type of vessel and its speed, and has a greater effect on slow moving vessels with large superstructures. Wind heeling effect has less effect on vessels that have a low profile or that sit lower in the water. Vessels that sit higher in the water have a higher freeboard, or larger sail area are much more effected by wind. Regardless of vessel type, caution should be exercised when operating in high winds. The vessel’s configuration gets taken into consideration during all inclining experiments whether full or simplified. Vessel operators should know the characteristics of the vessel and how wind may affect its stability. All operators are reminded to review their Certificate of Inspection and Stability Letter for operating limitations.

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