My posts from here on in are going to be mini-posts or amalgams of many short sessions. There is a new baby in the household and it doesn't allow for much more than a one-hour boat building session at a time!
I found a one-hour chunk to mark a waterline on the boat. This is something I could easily have done back during the hull planking stage, but I forgot. Now what??? Lou has the answer!
Using his method, you start by finding where the waterline sits at the bow and stern. On an old boat you can literally float the boat with an occupant and mark it in situ. For a new boat (like mine), just use the plans and mark the DWL front and back.
Next level the hull by placing a spirit level across the gunwales with the hull inverted. Check front and back for level and shim accordingly. Boards are put boards across the stem and stern at the DWL marks and are leveled too. Here I screwed a board to the transom and clamped the other one to my stepladder
Figure 156: Neato method of marking a waterline!
Then take a string and run it from one board to the other. Move one end of the string toward the keel of the boat until it sits about a millimetre above the hull . This is the DWL for that point on the hull.
Continue adjusting the string to get a series of DWL marks along the hull. Connect them with tape and paint a waterline stripe (or just mark it and leave it for now). For the few areas where I couldn't get the string close enough, like under the stern where the hull tucks up sharply, I used a short spirit level to find a point on the hull at the same elevation as the string. This part is slow, but seems as accurate as anything.
If you had a level shop floor you could easily level the stern and stem DWL marks to the floor and make a gauge that references the floor. My shop floor is decidedly UN-level, so the only option was to reference the hull itself. Works either way.