Wednesday, May 14, 2008

Shipboard - Inclining Experiment

The largest weight that is entered into a ship's stability calculation is that of the light ship itself. The determination of the light ship weight is not a trivial task. The vertical center of gravity of the light ship is determined by performing a full-scale test on a newly constructed or recently modified vessel. The full-scale test that is performed using the vessel is known as an inclining experiment.

While you the ship's Master or Mate is not required to perform the experiment, you may be present on behalf of the owners of the vessel to assure them that it was done properly and to assist with any particular information unique to the vessel. There are also certain legal requirements regarding the inclining experiment of which the ship's officer should be aware. The practical application of the inclining experiment for­mula in solving list problems due to an off the centerline weight or position of G is essential for the ship's officer to master and use frequently.

The inclining experiment addresses the problem of finding the KG for a vessel in a light condition. While ships of identical hull form have the same physical dimensions (length, draft, beam, etc.) they do not necessar­ily have the same KG even at the same light ship displacement. This is due to the fact that perfect standardization is impossible. Any variation in the weight of structural members, equipment, welds, and riveting in vessels of identical design will produce a different light ship KG. There is only one accurate method of finding KG for any particular vessel and that is the inclining experiment. Between experiments, an accurate record must be made of any additional weights added to or removed from the ship . Added weights such as a new radar scanner with a tripod to support it atop the wheelhouse will raise the original light ship center of gravity. The removal of king posts and booms during a conversion of a general cargo ship to a container ship will lower the original light ship center of gravity. Significant modifications to a vessel such as these will necessitate an inclining experiment.

The inclining experiment is a method of determining the vertical position of a vessel's center of gravity, usually for the light condition. It is performed by moving a known weight a known distance across the de determining the angle of heel the vessel assumes, and with these values solving for GM, the metacentric height. Then, looking up the KM for vessel at the correct draft, subtract GM, and the result will be KG. (KM - GM = KG).
Required Gear and Data
1. A weight large enough to list the vessel 2 to 3 degrees when moved across the deck.
2. A method to move the weight across the deck. Note this can be a track or skids built across the deck on which the weight is moved, or the weight can be simply lifted with a crane ashore and moved to the required location.
3. A plumb line suspended from the hatch coaming to the lower hold. Note, the plumb line is used to determine the angle of the vessel assumes. Preferably the larger the plumb line the better as long as it is protected from the elements.
4. A wooden batten placed horizontally across the ship's lower hold. The batten is used to record the movement of the plumb line. It should be approximately 3 feet above the deck.
5. The displacement of the vessel must be accurately known.
6. The value of KM for the displacement at the time of the inclining experiment as determined from the hydrostatic data for the ship.

Performing the Experiment
Assume that skids have been built up horizontally across No.3 hatch, and that the weight is placed on the centerline. A pendulum has been suspended from No.3 hatch coaming to a foot above the inner bottom, and a batten has been built up horizontally about three feet above the inner bottom.
Measure the length of the plumb line from the point of support to the point where it cuts the batten (the plumb line should swing clear of the batten about 1/16 inch). The vessel should now be in an erect position.
The weight is now shifted across the deck to the deck edge, the vessel will take a small list. The pendulum will move over to the low side and come to rest. Mark where it cuts the batten. Measure the distance between the first and second marks on the batten. This distance will be referred to as the deflection. Measure the distance the weight has been moved across the deck.
Using the above data you will be able to solve for the tangent of the angle of inclination (tan 0). In a right triangle the length of the pendulum is the adjacent side, and the deflection the opposide side. To get the tangent of the angle of inclination (tan 0), divide the opposite side (deflec­tion) by the adjacent side (length of the pendulum). You know the weight moved, the distance the weight is moved, and the displacement. With these values plus the tan o, you can find the vessel's GM by formula. The formula is:

GM = w x d / Displacement x tan 0

w - is the weight in tons moved on deck.

d - is the distance the weight moved in feet.

Tan 0 is the tangent of angle of inclination.

Displacement is the vessel's displacement in tons.