Hello Eric,
That is the difficulty, vocabulary about older fishing methods. Boomkorvisserij is beam trawling in English (thank you Google), and in Dutch, the name for side trawling is "Enkelvoudige trawlvisserij", and there is a good synthetic explanation on the same site, with a picture of Z41.
It was the most common method until beam trawling was introduced, so as to destroy the seabottom.
Side trawling was a pelagic or a bottom fishing method, depending on the season, place, fish and of course the gear used. Boards kept the net open, like minesweepers do, and the "mouth" was kept open by small spherical "floats" at the top side, and weights beneath, made of a mix of chains and rubber (cut out from tires..) so that they could (more or less) roll on the bottom.
But North Star has "galgen" (gallows?) only on one side, although it is a North Sea trawler, so that it could be a seine trawler, with an encircling net.
We look for a Fisherman's friend...
Regards,
Luc
Edit: I found a good site (http://www.mi.mun.ca/mi-net/fishdeve/methods.htm) in English