Back in the days of steam ships and later the coal and diesel these were use to calculate the speed of a boat. They hung from the taff rail at the back of the boat
Taff rail logs may be old tech, but did not pass out of use after steam and coal. I do not date to the days of steam and coal (well, perhaps to the death rattles of the coal era) and I remember off shore cruising on friends' parents' sailboats in the 1960s and early 1970s when taff rail logs, along with RDFs, sextants (for at a minimum the taking of noon sights), proper chartplotting and DR, were the tools (along with a compass) a recreational boater had and the tools one used. Actually, I know of a couple of people who still use taff rail logs on long passages, as well as sextants.
I only vaguely remember how to use a sextant, sight reduction tables, RDF, etc., and will probably never use them again in my lifetime (unless, perhaps, if the satellites go down). I love my chartplotter and radar for the type of boating I do now. Although I still use paper charts, I am a bad boy and only occasionally keep up my plot.