believe it or not, baldrick, the waterlines on trawlers were not usually straight.
always painted with a roller brush on the slip way, they were usually painted, unless new from the builders yard, and the "water line" followed usually the underside of the second row of plates so ran basically parallel to the bulwark tops, and then at the bow and stern would taper down towards the sea giving a very odd shape waterline indeed.
it can be seen on these three shots of trawlers, and this fine model of a friend of mine.
so don't worry if you don't get it straight, lol.