I believe regulation V/19 states 'all small craft should fit a radar reflector if practicable'. So if you re under 15 meters you are not legally required to have one. Think it is a good idea to have one though. My son has a Fairline 40 and we tried out radar reflections at various aspects and mostly diddnt show any returns at over 1/2 mile. A simple octahederal reflector increased the detection range to over 2 miles.