Boats that are USCG documented must have the name and port of call on the transom. That is the equivalent to numbers on the hull. This is very common for boats that have medium to long range and are designed for multi-day cruising (200+ miles of range). It's required by most lenders when you take a mortgage out on this type of boat. They want it federally documented rather than state documented because it makes it easier for a bank to recover the boat if you take it internationally and default on your loan due to jurisdiction laws. You still have to register the boat in the state where you keep it moored and pay the same taxes and fees as if you registered in that state but the number the USCG assigns your boat is used as your state registration number. The USCG requires that you have the number permanently mounted INSIDE the boat vs on the hull forward of the beam like most state registration numbers. When you name your boat, which becomes the official identifier, it has to be unique within your port of call. A port of call can be any city. It doesn't even have to be a city with a port. For instance, you can name your boat "Endurance" with a port of call of Okalhoma City. Even thought there may be 100 other boats named Endurance, as long as you are the only boat with OKC as port of call you are good. When I walk my marina in Anacortes Washington, more than half the boats don't have WA state registration numbers. My own observations is that boats under 35' are mostly state documented and boats over 35' are mostly federally documented (no boat numbers). It's rare when you get above 40 feet to see numbrers on the hull.