The photo shows a camera tripod with a piece of plywood fixed to the top. This was moved around the boat to four different positions and the ply wood levelled by eye by moving the tripods legs around. On top of the plywood is an aluminium disk from the scrap box that has been drilled and tapped for 3 jacking screws. On top of the aluminium plate is a Bosch "pen" laser level (these are relatively inexpensive, $55 from Bunings). The laser level projects a flat red laser line that can be seen just above the blue tape (click the photo for an enlargedment). In use the laser level is moved around on the plate and the line marked in pencil on the hull ever 100 mm or so. Obviously the plate has to be level and at the right height; this was achieved by adjustment of the jacking screws and the tripod's rise and fall mechanism. Several iterations are needed before the adjustment is complete.
Due to space constraints in my shed I had to mark the water line in four sections working around the hull. Each time the tripod was moved the whole adjustment process had to be redone. I was amazed, when I returned to the starting point, to see that the line I had carried all round the hull fell exactly on the point I had started from.
Having marked the line the tape was placed to create a "fair" curve through the points. The apparent breaks in the tape line are where it zig-zags across the plank laps.
Once the bottom of the boat has its white paint I will remove the tape in the photo and retape over the finish coat and apply the blue paint to the topsides. That's the plan anyway!
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