""It is far from clear how this symbol is operating as a trademark," Daniel Nazer, an attorney with the Electronic Frontier Foundation, emailed Ars.
Simply taking a well-known symbol and registering it as a mark for clothing doesn't automatically give you rights to it. To be a legitimate trademark, it must have "secondary meaning"—this means that consumers in the relevant industry associate the mark with the company. In this case, it would mean someone seeing a plimsoll on a T-shirt thinks "that's Plimsoll Gear clothing' not just 'that's a maritime-themed shirt." I doubt there is any secondary meaning here. It seems to me that Plimsoll Gear is just using the trademark system to claim ownership over a public symbol."
"Similarly, Annemarie Bridy, a law professor at the University of Idaho, in an email, called this a "classic case of trademark overreach."
Bridy went on:
Unless consumers who see the Plimsoll icon on gCaptain merch would be likely to mistakenly think that merch comes from Plimsoll Gear, there's no viable trademark claim. The Plimsoll symbol has meaning to the public and within the shipping industry that predates and exceeds any meaning it has in connection with Plimsoll Gear's brand. Plimsoll Gear's trademark registration doesn't give it the exclusive right to use the historic symbol. A trademark isn't a patent, even though a lot of trademark rights holders would like that to be the case."