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 TB 55-1900-201-45/1
This should be demonstrated by comparing ultrasonic gaging measurements with measurements of plates or pipe similar
to the metal walls to be measured. Test pieces may also be used to check the calibration of the instrument.
e. Gaging is used to provide a quantitative basis for evaluating a questionable local condition. Even though this
involves only limited gaging, it should not be undertaken unless there is sufficient cause. Deep pitting over an area,
holes, fractures, excessively thin edges on structural shapes, bands or belts of corrosion across bottom plating which
may indicate heavy working, are all justifiable bases for requiring gaging in the affected area. However, care must be
exercised not to extend the gaging so as to have it become a "fishing expedition".
f. Belt gaging goes far beyond the investigating of a local condition as described in the previous paragraph. It in-
volves taking readings around several complete transverse sections of the hull including deck, sides and bottom. It is
intended to furnish information which may be used to assess the average wastage of the hull envelope and its conse-
quent effect on the longitudinal strength of the vessel. Belt gaging is a major undertaking and should be required only for
good and sufficient cause; as for example:
(1)
A vessel undergoing a major conversion.
(2)
A vessel which has had a structural failure which could be attributable to age or general deterioration.
(3) A vessel in which the general condition, as evident by the parameters described in paragraph e above, is
such that there is serious question concerning the vessel's seaworthiness.
g. For belt gaging to be of value in determining the average condition of the vessel, the location of the belts on
the vessel and the points to be gaged within the belts should be selected by the surveyor. Experience is required to
make sound judgments in this regard. The following are some of the important factors which must be considered:
(1)
The belts should be located in sections of the hull wherein the seaworthiness is most in question.
(2) The locations of the individual readings within the belts should be chosen with a view towards determin-
ing the average remaining plate thickness. However, the worst single spot should be sought out first to determine if more
extensive gaging is justified. In many instances, it may be necessary to obtain more than one reading to determine the
average remaining plate thickness.
(3) After the gagings have been recorded, comparison with the original scantlings can be made only by re-
ferring to the vessel's construction plans. (Reference nomograph Table 2-1, "Percentage of Wastage", to facilitate
evaluation of the results.)
2-2
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