33 C.F.R. § 183.3
Beam means the transverse distance between the outer sides of the boat excluding handles, and other similar fittings, attachments, and extensions.
Boat means any vessel-
Full transom means a transom with a maximum width which exceeds one-half the maximum beam of the boat.
Length means the straight line horizontal measurement of the overall length from the foremost part of the boat to the aftermost part of the boat, measured from end to end over the deck excluding sheer, and measured parallel to the centerline. Bow sprits, bumpkins, rudders, outboard motor brackets, handles, and other similar fittings, attachments, and extensions are not included in the measurement.
Monohull boat means a boat on which the line of intersection of the water surface and the boat at any operating draft forms a single closed curve. For example, a catamaran, trimaran, or a pontoon boat is not a monohull boat.
Motorwell means any arrangement of bulkheads or structures that prevents water from entering the passenger carrying area of the boat through any cutout area in the transom for mounting an outboard motor.
Motorwell height means the vertical distance from the lowest point of water ingress along the top of the motorwell to a line representing a longitudinal extension of the centerline of the boat's bottom surface, excluding keels. This distance is measured as a projection on the centerline plane of the boat. See Figure 183.3.
Permanent appurtenances means equipment that is mounted or fastened, so that it is not removable without the use of tools. Seats, inboard engines, windshields, helm stations, or hardtops are permanent appurtenances. Outboard motors, controls, batteries, and portable fuel tanks are not permanent appurtenances.
Remote steering means any mechanical assist device which is rigidly attached to the boat and used in steering the vessel, including but not limited to mechanical, hydraulic, or electrical control systems.
Sailboat means a boat designed or intended to use sails as the primary means of propulsion.
Sheer means the topmost line in a boat's side. The sheer intersects the vertical centerline plane of the boat at the forward end and intersects the transom (stern) at the aft end. For the purposes of this definition, the topmost line in a boat's side is the line defined by a series of points of contact with the boat structure, by straight lines at 45 degree angles to the horizontal and contained in a vertical plane normal to the outside edge of the boat as seen from above and which are brought into contact with the outside of the horizontal boat. A boat is horizontal when it is transversely level and when the lowest points at 40 percent and 75 percent of the boat's length behind the most forward point of the boat are level.
Transom means the surface at the stern of a boat projecting or facing aft. The upper boundary of the transom is the line defined by a series of points of contact, with the boat structure, by straight lines at 45 degree angles to the horizontal and contained in a vertical longitudinal plane and which are brought into contact with the stern of the horizontal boat. A boat is horizontal when it is transversely level and when the lowest points at 40 percent and 75 percent of the boat's length behind the most forward point of the boat are level.
Transom height means the vertical distance from the lowest point of water ingress along the top of the transom to a line representing a longitudinal extension of the centerline of the boat's bottom surface, excluding keels. This distance is measured as a projection on the centerline plane of the boat. See Figure 183.3.
Vessel includes every description of watercraft, other than a seaplane on the water, used or capable of being used as a means of transportation on the water.
Figure 183.3-Transom and Motorwell Height
33 C.F.R. §183.3