Ships Are Inspected Underwater Instead of Being Pulled Out
By Nadia Fournier, Marine surveyor and insurance adjuster. Reviewed by Dana Whitcombe, technical reviewer.
Ship Husbandry & In-Water Repair | Hull & Bottom Cleaning I was standing on the wing of a 157,000-dwt bulker in Kaohsiung, Taiwan, in March 2019, watching ballast water bubble out of the sea chest. The ship was due for drydocking in seven weeks. The owner wanted to skip it. The local agent kept saying the yard could find a slot, but the charterer was screaming about schedule. I had to decide whether a diver with a camera in murky water could stand in for a dock. The answer had nothing to do with how good the diver was. It had everything to do with whether the flag state and class society gave us permission. The Coast Guard's rule on underwater surveys in lieu of drydocking is 46 CFR 71.50-5, and it belongs to Subchapter H, which is passenger vessels. But the principle shows up across merchant shipping. As a marine surveyor and insurance adjuster, I've ordered dozens of these. Here's the part nobody writes in the brochure: you don't just call a dive company and point at a hull.
Get the regulatory blessing before you wet a camera
The owner's first question is always 'Can we do an underwater survey instead of a drydock?' The real question is 'Will the flag and class let you?' For a U.S.-flag passenger vessel, you need approval under 46 CFR 71.50-5, with the actual examination required under 71.50-3. For other ships, the logic runs through the classification society and what your flag administration has allowed. The general pattern you'll see is two credited underwater exams within a five-year period, with a maximum interval between them set in the specific approval. There is no blanket rule that says 'you get one free underwater survey every five years' — that would make things too easy. You need to write to your class, explain why you need it, and get it in writing. Ask for the approval before you spend money on a dive team. I've seen owners lose an entire charter because they did the survey first and the flag said no.
Hire a dive team with commercial credentials, not a dive shop
The dive team needs to be a commercial operation, accredited by the Association of Diving Contractors International (ADCI) or an equivalent standard. PADI cards are worthless here. OSHA sets the rules for commercial diving in 29 CFR 1910 Subpart T, and the dive contractor should be working to those standards or your local equivalent. For an in-water hull survey (UWILD), you need a diver who has done this before — someone who knows a proper bearing check from a blurry video of the rudder pintle. The team must have a topside supervisor, a standby diver, and an umbilical or communications line. They should bring their own underwater video and still camera, plus lights. Ask for sample footage before they mobilize. I had one team show up with a GoPro in a plastic housing and no topside monitor. I sent them home. The footage is your evidence; without it, the whole exercise is just a man in a wetsuit guessing.
Plan around the water and the ship's systems
Before the diver goes in, you need to know the water. Visibility will decide how much you're going to see — in Singapore it can be two meters; in Houston, sometimes thirty centimeters. Tides and current matter more than anyone admits. A slack-water window might be two hours, and that's all you get. On the ship's side, the biggest hazard is differential pressure. If the diver puts his hand, arm, or body over a sea chest or a discharge opening while the valve is open, the full static head pushes him onto the opening. You don't need flowing water at high velocity to get killed. A ten-foot head is about 4.3 pounds per square inch, and against a torso that's thousands of pounds of force. The only safe control is proper lockout/tagout of the sea valves, physically securing the values and blanking them if needed. Tell the chief engineer that the dive won't start until the valve is cracked open in your sight, tagged out, and the diver knows the exact location of every opening. I've seen a port engineer argue they could just 'keep the diver clear.' That man is no longer in the field.
Give the diver a checklist, not a vague directive
A good UWILD diver doesn't just swim around and film whatever looks nice. You need a structured scope: hull plating and welds, propeller (blade edges, pitch, any dings or cracks), rudder including the leading edge, trailing edge, and around the pintle, sea chests and gratings, anodes (amount left, location, how well attached), echo sounder and speed log covers, and any areas where you know there's spotty coating. Have the diver measure anodes with a gauge, not just say 'looks okay.' If class says you need ultrasonic thickness readings, the dive team should bring a calibrated A-scan gauge and show you the crystal beam. Make them document the pressure and temperature of the water at depth, because that affects the readings. And tell them to hold the camera still for ten seconds on each zone. The worst UWILD videos are the ones where the diver pans away after two seconds because his neck is tired. That's not evidence; that's a teaser trailer.
Decide what still needs a drydock
An underwater survey is a condition snapshot, not a substitute for a full structural exam. You can't measure rudder bearing clearance without drydocking. You can't check the tail shaft seal unless you can physically rotate the shaft, and even then it's not the same. If you have any reason to suspect a sea chest is partially blocked that a camera can't prove, or if the propeller has a fresh crack, the answer is no UWILD — the ship goes in the dock. The class rules often allow UWILD for a single intermediate survey, but the special survey will still require a drydock. When I signed off that bulk carrier in Kaohsiung, I wrote in my report that the underwater survey cleared the immediate hull and stern gear, but I recommended a drydock at the next planned period. The owner tried to argue the class would waive it entirely. They didn't. Two years later, that ship was in a yard in Shanghai, and the propeller shaft bearing had measurable wear. Not catastrophic, but enough to prove that the water doesn't show you everything.
Write the report like an underwriter will read it
Your report is going to go to the flag, the class, the owner, and possibly the underwriters. Make it clear what the dive covered, who did it, what their qualifications were, what the visibility was, what the water temperature was, and what you actually saw. Include the lockout verification on the sea valves, the calibration certificate for the ultrasonic gauge, and the diver's name on every page. State plainly that the survey was approved by the class society and cite the approval reference. Then list the limitations: what you could not inspect, how dirty the hull was, any areas of suspect coating. I always add a line that says 'No underwater examination can replace the direct visual and physical inspection in a drydock for critical bearing clearances.' That one sentence has saved more arguments than any other part of the survey.
Frequently Asked Questions
Does an underwater survey mean I'll never have to drydock again? No. Class rules generally allow an underwater survey as a substitute for a drydocking for one intermediate survey, but you'll still need a drydock for the special survey or periodic extension. The approval letter will say exactly what you're getting and for how long. How dangerous is a UWILD dive? It can be very dangerous if the ship's systems aren't secured. The main hazard is differential pressure from sea chests and discharge openings. A diver can be held against an opening by static head alone, with thousands of pounds of force over their torso. That's why lockout/tagout of all sea valves is mandatory before the dive starts. Is a UWILD cheaper than a drydock? Usually, yes, because you skip the cost of the docker berth and the lost time in a yard. But you're paying for a certified commercial dive team, video documentation, possibly ultrasonic gauging, and your time for planning and review. If the survey finds anything questionable, you'll have to drydock anyway, so the savings can vanish fast.