The widest part of a boat's width is its beam. Measuring the beam is a two-step process, because you must find your boat's centerline and make the measurement perpendicular to the centerline. All you need is a ball of twine, a carpenter's square, some duct tape and a measuring tape. For federal documentation, rather than state registration, have a marine surveyor make all measurements, including your boat's beam.
Define the Centerline
Measure the width across the top of the boat's stern, its back end. Mark the center of the stern. Tape the end of the twine to the center of the stern. Stretch the twine forward and tape it to the point of the bow.
Measure the Beam
Visually estimate the widest part of the boat. Pull the tape measure across the gunwales -- the tops of the boat's two sides -- and move it ahead and to the rear of that point, to refine your estimate. Hold the carpenter's square to the twine that marks the centerline and square the tape, so that it's at a right angle to the centerline. Read the tape; the measurement is the boat's beam.
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