Steel Hull Plate Gauging

From a recent survey -

According to available builder’s data the steel hull is welded steel-alloy plating on 5/32" X 1" X 1" – 18" OC frames with the following plate scantlings (nominal thickness):
Keel Shoe – 25/32" (0.78")
Keel Plate – 13/64" (0.203")
Hull Plate – 5/32" (0.197") or 5.0mm
Deck Plate – 9/64" (0.140")
The vessel was carefully lifted out of the water and blocked for inspection. The hull was remarkably clean and unfouled. The exterior of the hull was not pressure washed prior to visual and ultrasonic gauging. The topsides are substantially filled and faired which precluded a visual and mechanical examination. Similarly, weather decks are such that precluded examination as well. Therefore – the vessel was not belt gauged – only the underbody and keel was subject to steel gauging.
No corrosion or damage was observed on the topsides, weather decks and or superstructure. Dissimilar metal corrosion at the mast deck step was observed to be slight, properly controlled and monitored by ownership. No visual evidence of grounding or other damage was observed above or below the waterline. The hull underbody and keel plating is coated with anti-fouling paint generally smooth and uniform in thickness with only minor flaking in localized areas. The keel is rather full – with integral lead ballast down low. Tankage for liquid stores is incorporated into the keel. The water tank was recently renewed as evidenced a replaced (welded) section of plating on the portside. The bronze alloy propeller is well protected in its aperture. The rudder is of steel construction with negligible play in the hinges.
Upon visual examination – the surface of the hull and keel showed no rust blooms, pitting, corrosion, fatigue, structural failure, and or wastage and was fair to the eye with no hard spots or proud plates. Minor inward hull plating buckling was observed below the water line - portside to at the lower underbody and keel at the frame bays. Minor buckling was also observed sporadically starboard side to. Welded plate seams were observed to be in serviceable condition with no visual cracks - fractures or corrosion. Welded in place through-hull penetrations were thoroughly examined – again with no visual evidence of wastage or fatigue. Sacrificial zinc anodes are properly attached to the steel hull plating and to the propeller tail-shaft. The keel shoe was found to be smooth and unabraded. The leading edge of the keel and stem showed no evidence of damage or abrasion.
The steel hull plating was systematically ultrasonically gauged to provide a quantitative basis for evaluating a questionable local and overall condition. In preparation – a TM-8811 portable 5mhz ultrasonic gauging instrument was carefully calibrated just prior to inspection with a known 5mm or 0.197" steel plate (medical grade glycerin was used as an interface agent for all readings).
The underbody of each hull side was prepared for mapping by identifying and referencing a (welded) deck cleat (on each beam) forward of amidships (eventually known as station number 7). An accurately marked fiberglass measuring tape was carefully positioned (slightly down from the bottom of the black boot stripe) longitudinally along each hull side on the unfilled and faired water line section of the hull. The tape was extended forward to near the cut-water aspect of the stem (tape set at the ‘zero’ foot mark) and then aft to the stern (ending at approximately the ‘32’ foot mark). On both beams, the tape’s approximate ten (10) foot mark was visually observed to be vertically aligned with the above mentioned deck cleat and identified as station number 7. Reference stations 1 through 16 were set along the tape at every twenty-four (24) inches fore and aft. From this tape-line - a series of parallel reference lines running down to the so-called garboard seam at the upper most portion of the keel plate were established on approximately twelve (12) inch centers.
All steel plate measurements were taken with a eight (8) millimeter probe on clean, smooth steel plate hull surfaces at the intersection of the above described horizontal and vertical reference lines (12" X 24"). In the event a reading coincided with an interior transverse framing member – the probe was slightly repositioned. All readings were hand recorded then compared with the original scantlings. If wastage - corrosion in the steel plate surface was visually observed outside the pre-determined ultrasonic steel gauging points - it would be considered a justifiable basis for requiring gauging in that affected area. The criteria for such would be deep pitting, holes, fractures, excessively thin edges on structural shapes, bands and or belts of corrosion across hull and keel plating).
Reference to NVIC 8-68 nomograph Table 2-1 "Percentage of Wastage" was made to set the parameters to determine the scantling’s maximum wastage limits and facilitate evaluation of the results based on a recommended standard of no more than 25% wastage of original thickness. The limits of wastage were determined to be as follows:
Hull plating – approximately 0.148"
Keel plating – approximately 0.152"