Registration numbers must be affixed on the forward portion of both the port and starboard side and must be displayed:

• on both sides of the vessel, or on the transom if it’s a sailing boat
• in solid and clear text, in a contrasting colour to the hull
• in numerals and uppercase letters
• well above the normal waterline so it can be seen at all times

Check your local legislation, but in general, most boat registration decals must be over 75mm high and some jurisdictions stipulate 150mm high decals for planing vessels.