From the book, How to Read Water:
Directly behind the ship there is an area where the water that has been whipped up by the propellers rises to the surface. It is easy to spot behind most boats as a different colour, usually with lots of bubbles rising to the surface, too. Soon the commotion in this line of water calms down, but the water does not return exactly to its previous state, it retains a peculiar glassiness. This has been observed for many years and has a few nicknames, like the ‘dead wake’. This dead wake remains noticeably different to the water around it for far longer than I would ever have suspected and depending on the size of boat, its speed and the natural water state, it is possible to spot the tracks of boats long departed.From Chapter 18 of How to Read Water by Tristan Gooley
The effect is compounded when an engine leaks even tiny amounts of oil, as in the case above.
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How to Read Water – The Book