Doctor's Diagnosis

Late July 2005. There stood Wendy Ann, nearly 80 tons of her waiting for us to figure out what the hell to do next. John had a final trick up his sleeve for us and introduced us to the brilliant Di Roberts- marine surveyor. I wish I had a photo of her to show you. Maybe I should give her a call. So after all these months and thinking along the lines of closing doors after horses bolt we embarked on the process of obtaining a real, out of the water hull thickness survey.
Di met us at the boatyard, and had brought along a cool bit of kit- an ultrasonic thickness meter. These work by firing three beams of ultrasound into the steel at your chosen point, listening for echoes and then agreeing with itself (or not, as we were about to find out) about how thick the metal is (or isn’t). You use jelly on the probe, pressed against the hull to enable good conduction, and read the results from a little LCD screen, so Wendy Ann was about to undergo a medical.
Stethoscope ready, Doctor Di showed me what to do, and the first few readings came nice and clear 7, 8, even 9mm. We marked the point and thickness in chalk and moved along, grinding little spots where scale or thick paint was in the way. My heart lifted prematurely. Not as bad as I’d thought, But as Di worked along and under the hull it became apparent that there was a huge area directly below the forward compartment where the meter just couldn’t find a reading to settle on. She explained that if the steel was uneven (outside, inside, or both) this would cause those echoes to go bouncing off all over the place and produce a distorted or non-existent reading. Hmmm. Lots of lumpy hull then. What causes lumpy steel? Corrosion. Eek. The job was to build a picture of the state of the hull overall, and after lunch Di left me in charge of reading to complete the survey. I found myself trying to mirror what she had done, and baffling away at the results. Wendy’s bulk looked incomprehensibly huge with my nose about three inches from her and I kept thinking ‘how do I decipher this?’ But the next day Di came to collect the ultrasound device and helped me to step back and see the wider picture for what it was.
There were two main areas of concern and impossible readings (never mind the myriad other little eyesores, or the gaping hole in the keel). A large area of the hull bottom forward, and most of the stern around the propellor. Readings suggested the following:
Forward seemed to have rusted away inside more than outside, we knew there was concrete in there, poured in as a quick fix by some previous owner of miss Ann. Concrete is cheap, moulds itself to complex shapes and physically supports dying steelwork. It has it’s own drawbacks though. And for the very first time the horrifying suggestion was bandied about that we might have to remove the concrete before repairing the hull.
Aft it seemed to have gone mainly because of cavitation around the prop, but inside the final stern compartment was once a trim tank, into which raw seawater was pumped to adjust Wendy’s attitude whilst working. More internal corrosion. Anyone brave enough to clamber into the trim tank could see a litany of rust and heavy scale, it really didn’t look good.
We spent a while weighing up the pros and cons of patching (overplating, or doubling) versus replacement (proper job, and expensive). Then asked Di if she knew any good welders who might be up for it, “well let me see...” she said.