Ultrasound gives precise measure of hull-plate thickness


In the days of wooden tugs a wise buyer would hire a knowledgeable marine surveyor to come aboard with his pocket knife to probe into the mysterious depths of deadwoods, frames and planking. Since the techniques were often thought of as more art than science, the buyer had little choice but to accept the diagnosis of the surveyor.

In the early days of steel hulls, short of cutting a hole in the hull plate, it was still difficult to gauge a vessel’s soundness. Today, thanks to computerized hand-held ultrasonic wall-measuring instruments, science has taken the place of art and guess work. This remarkable device has a small transducer attached by wire to a hand-held instrument with a digital readout showing the thickness of the metal. It is similar to the ultrasound machine that doctors use to examine pregnant woman; but rather than viewing the contents of the hull, it measures the thickness of the hull’s skin.

Recently marine surveyor Marc McAllister was employed by the potential buyer of a tug to check the thickness of the vessel’s hull plates. The boat had been lifted out of the water by the Travelift at Shelter Island Marina just south of Vancouver, British Columbia. The tug Jacques Cartier B.C. was built by Victoria Machinery Depot in 1962. McAllister took his General Electric Inspection Technologies ultrasonic instrument from its rugged foam-lined case and applied a touch of coupling fluid from a plastic bottle.

The readings recorded on the bow show the steel is close to the original specs.

He placed a small steel test block on the tailgate of his pickup truck and touched the tip of the transducer to the thickest part of the block. The digital display on the machine read out .500, showing that the instrument was reading the half-inch thickness of the block with an accuracy of one one-thousandths of an inch. He then checked it against the thinner .400, .300, .200 and .100 thick steps on the test block.

Moving to the tug’s bow, McAllister used a tool to scrape off some of the bottom paint to get to a small patch of bare metal. Applying the transducer to the spot, he saw a .316 on the digital readout.

“This boat would have been plated with .3125 inch steel,” McAllister explained, “but in those days, no supplier wanted to be accused of delivering undersize steel, so they usually went a bit over. With the good care that the owner Lafarge Cement has given this boat, there are still points where the thickness is greater than the original specs.”

Moving a little aft just above the keel, McAllister got a reading of .291, still excellent for a nearly 50-year-old hull. The next reading was forward and half way up to the waterline. It showed .286 inch. This proved to be the lowest reading on the hull. McAllister explained it was in the chain locker, where interior corrosion is most pronounced. Just aft of the watertight bulkhead that separates the chain locker, he got a reading of .325, once again higher than the original specs.

Similar readings were obtained over the length of the hull: some a little under the original specs and others a little over. In all cases, they were more than satisfactory and the potential buyer moved to the tug’s interior for further inspection.

McAllister returned to his truck, where he had left the steel test block in the sun on the tailgate. Touching the transducer to the block he showed the digital read out at .504. “The heat has expanded the block by four one-thousandths,” he said. An amazing instrument to have in your tool kit.

The gauging of the hull plates was one of the final checks by a conscientious buyer. In this case, the results were positive and the sale was closed within the week.

By Professional Mariner Staff