There appear to be no less than six commonly used ways to measure the length of a boat. Most of the discrepancies have to do with whether stern platforms and bow pulpits are "integral parts of the hull" or are bolted on separately.

**Units:** Linear measure (usually metres/centimetres or feet/inches).

The length from the forward tip of the stem to the aftmost point on the hull. Usually includes any bow or stern appendages that are a structurally integral part of the hull (eg. a swim platform or anchor pulpit moulded as part of the hull), but excludes bowsprits, boomkins, bolted-on pulpits and stern platforms, or other appendages that aren't an integral part of the hull itself.

The length of the design waterline, i.e. the length of the part of the boat that's in the water when she's loaded exactly to the waterline shown on the plans.

The length from the forward point of the design waterline (same as for L_{WL}) to the centre of the rudder stock. Used mainly for large ships, and rarely mentioned aboard anything under a hundred feet or so.

The length from the foremost point of the deck to the aftmost point of the deck; it leaves out reverse-sloping transoms and most types of integral pulpits.

The overall length of the hull itself. Defined similarly to L_{OA}, but usually excluding bow pulpits.

Depends on which rating rule your boat is subject to. This is usually some combination of LOA, LWL and various parameters describing the shape of the bow and stern overhangs.

Marina administrators often like to use the total length of the boat, including bowsprits, boomkins, pulpits, protruding sterndrives, the trailer tongue, and other length-extenders that would be considered "cheating" if you used them to describe your boat to a friend. That 18-foot runabout? 22'6" to the boatyard, once you add the tilted-up outboard and the trailer tongue.

If you're being billed by length, you'll try to low-ball the measurement. For a powerboat, L_{PP} is usually the shortest, so you might start there. That might seem suspicious, though, so L_{WL} and L_{H} are the next to be presented. Eventually, you and the marina manager will grudgingly agree to use L_{OA}, at which point a long and drawn-out argument will ensue over whether that anchor pulpit really is "integral to the hull".

It could be any of a number of quantities of questionable origin. Popular ones include "Take L_{OA} in feet, round up, append two random digits". Or "Take L_{OA} in feet, add swim platform but not anchor pulpit, round to nearest inch, state as FtIn". Or, "Take L_{OD} in dekametres, add pulpit but not swim platform, round to nearest multiple of five".

At Marsh Design, the model number is the L_{OA} in centimetres, using the standard definition given above.

That would be L_{WL}. For speed and seakeeping, you care about how much boat is actually in the water. That is, after all, what the waves see, and it's the length used to compare Froude number or speed/length ratio.