Marking the Waterline
A simple tool to help you along
The waterline given on plans is the calculated level that the boat will sink in the water when ready to use with passengers aboard. A small boat, moored without passengers, will not float on her waterline. From a practical standpoint, smaller boats do not delineate waterlines as they spend most of their life on a trailer. If such a boat will be moored for a long time, the only practical way to determine the waterline is to put the boat in the water in ready to use condition and physically mark the waterline level.
As the boat becomes larger, the designer’s waterline should become more accurate. Passengers are not the major weight and, due to the larger bottom volume, a little more or less weight does not vary the waterline as much.
The bottom paint does not end at the designed waterline; it usually extends about 3” or more above. Above that point a painted stripe called a “boot top” is common practice. A boot top should appear to be of the same width from stem to stern. To gain this appearance the boot top will be wider at the stem and possibly stern depending on the boat shape. The greater the vee, the wider the boot top becomes.
The sketch illustrates a simple tool to properly mark the boot top line. It’s use requires that the boat be level about the waterline and the base of the tool held level.
Marking the waterline requires the boat be level. Usually the waterline is referenced to a base line, set up level or other horizontal reference line. Transferring the waterline to the exterior is readily done, measuring at the transom is a simple way; only one point is needed as the waterline is level. The tried and true method is to use a length of clear plastic pipe filled with water. A builder’s spirit level or laser can also be used. It isn’t necessary for the scribed waterline be a continuous line; a series of dashes are adequate to use as a guide to stretch the masking tape at the proper position. •SCA•
Excerpted from the Boatbuilder’s Notebook.
There is a super simple method one step above even the very simple water level method. A rolling cart also requires a smooth level floor.
The cheap dual axis laser level, $39 at most big box stores or on line for even less. Level the boat, establish the fore n aft waterline marks at the stem and transom, dim the lights and voila a perfect waterline. I usually mark it with a fine felt tip pen and/or a pizza cutter for a permanent mark.
That is a hard way to mark a waterline. One can use a simple water level and complete the task in much less time.