What a WWII German U-boat Taught Me About Data Accuracy

By: Eric Parker

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Eric lives in Seattle and has been teaching Tableau and Alteryx for 5 years. He's helped thousands of students solve their most pressing problems. If you have a question, feel free to reach out to him directly via email.

I recently read the book Shadow Divers by Robert Kurson. Spoiler alert; I’m going to reveal a lot about the book’s plot.

The story takes place in the late 1980s and early 1990s. Some recreational, deep-wreck scuba divers in the New York area discover what they believe is a WWII era German U-boat submerged 200+ feet deep off the New Jersey coast.


They consult all the public records they can only to discover that no U-boat was supposedly sunk (or even operated) within 100 miles of their site. They wondered if it was possible the historical records were off by 100 miles, but that seemed like a pretty big stretch.

They started talking to experts many of whom knew little more than they did. Many of those experts, ranging from the U.S. Navy to military record-keepers in the UK to former U-boat crew in Germany thought they had solutions to the mystery. None of their stories fit.

The scuba divers were relentless in their search for the answer and kept diving the wreck looking for identifying artifacts. They found a knife with someone’s last name carved in the handle but that only muddied the waters further because that U-boat was reportedly sunk off the Spanish coast.

Time and again they thought they could identify the boat only to find a clue that debunked their theory.

After years of searching, they got their answer. An expert in Germany told them to further study the correspondence between the boat that “sank off the Spanish coast” and their commanders in Germany. They discovered that before that boat was ordered to Spain, it was told to go to New York. As they looked closer, they found the U-boat never officially acknowledged the command to divert from New York to Spain. They found their ship!

You might think that sounds obvious but there’s a reason they had a hard time identifying the boat; the data was murky. There were thousands of U-boats that sunk in WWII. The certainty of their sinking was graded A-F.

An “A” meant the sinking was a sure thing. Someone has dived the wreck or gotten an image on side-scan sonar or there was a massive explosion witnessed by multiple parties. An “F” meant the sinking report was suspect. Maybe one party saw an oil streak in the ocean and reported the U-boat as sunken.

The one that sank off the Spanish coast was an “A”. As the scuba divers dug further, they found out it was originally an “F”. What happened? Post-war assessors changed the grade as they were tidying up reports. They didn’t want a bunch of unconfirmed sinkings, and no one else had a plausible theory for the missing U-boat so they figured it sank in Spanish waters. In actuality, it likely hit itself with its own torpedo at night near New York. No one saw or heard the commotion as it sank and German command never heard from them again.


Data inaccuracy can plague any project. It’s important to have a data expert as part of your design sprint when you are embarking on a new data communication project. They can sniff when something seems wrong and have a 6th sense about the data and organization that few others do.

To learn more about how to ensure your data communication project is effective, check out the recording of our workshop; Tableau Dashboarding: From Mystery to Mastery or check out our old blog posts!