Understand that water lines (or load lines or Plimsoll lines) weren't especially common to sailing ships for many years. It would be more a static line for when the ship was sitting in harbor and needed protection from marine growth. Overall it made little or no difference in calculating sailing. Ships would worry about storing cargo and supplies so that the ship remained in its best position by making it bow heavy, neutral or stern heavy, depending upon the sailing qualities of the boat, but not worrying about the water line.
To establish a static waterline, put the boat into the water on a calm day, with very minimal wave action. Seat two people in the middle of the boat of the minimum boat weight, both port to starboard, and bow to stern.. In other words, your people will be in a position that they'll never sit in while the boat is sailing, but will be lowering the boat in an even fashion. The further off from the center lines your people are, the more of an error you will have.
Walk around the boat with a marker, and every foot or so, place a mark where the water rises to on the hull. That will be your static water line.
To calculate a load or Plimsoll line, add more people to bring the boat up to its maximum weight capacity and sit as close to the center line of the boat as possible.
The math calculations for this involve displacement, Archimedes' principle, weight, and isn't worth the time you'd spend calculating it. I doubt that even Hobie has the exact displacement of its boats. The Coast Guard publishes how to calculate this for its purposes at
https://www.uscgboating.org/regulations ... OADING.pdfUnderstand that after all this work, the line is meaningless as soon as your shift weight, add sails, or move. Even motorboats don't sail on their waterline. About the only thing a waterline is useful for beyond decoration is to determine where to put bottom paint.
Jim Clark-Dawe
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