When did naval radar become effective?

At what point was radar used most effectively?

From about 1941 onward, after early installations beginning in 1938-40 had been matched by enough fleet experience to use them well.

Early sets were already entering service in 1938, and many new ships completed with radar by 1940 because their designs allowed the equipment to be fitted without major delay. The stronger operational payoff came from 1941 onward. The gap reflects training and accumulated experience rather than a simple switch from no capability to full capability. Like someone learning a first smartphone, crews could operate a new system before they understood its rhythms, limits, and best uses.