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 TB 55-1900-201-45/1
of that strake. The reason for the anomalous reading should be noted or, preferably, the belt should be shifted to provide
a reading which is representative of the strake. Similarly, if a reading is so low that renewal in the area will be required
for local strength, the extent of the renewal should be indicated. Credit towards longitudinal strength could be given to
extensive renewal of the strake, but not to a localized replacement.
(4) In the case of longitudinally framed vessels, the belt gagings should include readings on all deck,
side and bottom longitudinals and on the plating of the longitudinal bulkheads. Readings on internals should be taken on
both the webs and the flanges and should be so identified. These gagings may be taken with a caliper.
h. Ultrasonic thickness measurement may be used in lieu of drilling and gaging when authorized by the local
Officer in Charge, Marine Inspection. When this method is used to comply with a specific requirement for gaging, the
inspector should approve the choice of locations and witness the readings. Also, he should satisfy himself as to the ac-
curacy and adjustment of the instrument by comparing results with the actual gaging of test holes. Test pieces may be
used to check the calibration of the instrument; however, care must be taken that the relative ease of taking accurate
readings on a prepared test piece does not result in unwarranted confidence in the accuracy of readings taken on the
vessel. Surface preparation to receive the probe, operator technique and competency, and condition of the "opposite"
side are among the many variables which can affect readings. Therefore, comparing the ultrasonic readings with actual
gagings of test holes is preferable to complete reliance on test pieces for instrument calibration.
i. After the gagings have been recorded, comparison with the original scantlings can be made only by referring
to the vessel's construction plans. The "Shell Expansion" is the most useful plan for this purpose; however, the "Midship
Section" is also useful. To facilitate evaluation of results, a "Percentage of Wastage" nomograph is appended to these
notes.
E-10. Corrosion Limits. Service experience of the classification society, which forms the basis for their rules on the con-
struction of ships, indicates that for most portions of a vessel, without other weakening factors, a local thickness deterio-
ration of up to about 25 percent may be accepted before replacement is necessary. This is based in part on the condition
usually found aboard ship that all structural members do not deteriorate uniformly. This means, in the application of this
percentage, considerable judgment is called for depending upon the location and extent of wasted material. Localized
wastage of some portions of plates or structural members in excess of 25 percent may be accepted in many cases, if the
condition of the adjacent material is sufficiently good to maintain an adequate margin of strength. In these instances,
careful attention should be given
E-6
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