Inclining test

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Inclining Test

The inclining test or inclining experiment is a method used by naval architects to verify the stability characteristics of a finished vessel. As a vessel is being constructed the naval architect/designer can keep track of weights and centres, but as outfit is finished it is not possible to know the precise centre of gravity of every single component. The inclining test is a measure of the righting arm produced as the vessel is heeled through a small angle. With an accurate VCG, the LCG, TCG and displacement can be determined by a lightship survey (which will be conducted at the same time as the incline).

Contents

Theory

The true position of the vessels overall vertical centre of gravity (VCG) can be determined by moving a known weight a predetermined distance aboard the vessel. Moving this weight will cause the vessel to heel, this is in turn measured and used to calculate the VCG. A similar, but less rigorous, way of determining a vessels VCG is by performing a roll-period test.

Freeboard measurements will be simultaneously taken, allowing for the LCG and TCG of the vessel to be determined (see lightship survey). The results of the inclining test become the basis of a vessels stability book.

Reconducting

Inclining tests should be re-conducted on vessels periodically, particularly if they have been modified or have large passenger loads (or significant crew turnover). Over time, the vessels displacement will change, which may push the vessels stability outside a safe range.

Sister Ships

If a series of vessels can maintain their lightship weights, TCG, & LCG position within ~1% of each other (based upon lightship surveys), then it is common to incline only one of the vessels and use the same stability book, or stability book data, for the entire series. The acceptance of sister ship inclining test results is at the discretion of both the flag state and classification.

Incomplete Ships

Vessels may be inclined before they are entirely finished. In this case, the inclined displacement of the vessel (exclusive of inclining weights) generally must be within ~1% of the final displacement.

Conditions

The inclining test should be performed in calm water, favorable wind conditions and unrestrained by mooring lines or gangways.

Procedure

ASTM F 1321 is one standard for performing inclining tests. The general methodology, however, is fairly straight-forward;

  • Weights are placed aboard the vessel. A freeboard survey is conducted, and the draft marks noted.
  • All weights aboard are counted. Any weighs which are to be in the final vessel, but are not currently on board, are counted. This includes their LCG, TCG and VCG.
  • Soundings are taken for all tanks. The bilge levels are checked, but should be dry.
  • A number of shifts are made of the weights a known distance, producing a measurable (but small) heeling force.
  • The heel angle is measured at a number of inclining pendulum locations. The heel angle can afterwards be compared with the heeling moment and the slope measured & used to produce the vessels GM.
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