How much speed helped merchants against submarines?

How fast did a Second World War merchant ship need to be to reduce submarine risk?

Around 18 knots was usually enough, although a slower ship could benefit substantially from consistent zigzagging.

Speed made it harder for a submarine to establish an attack position, but it was not the only defense. A merchant ship maintaining roughly 18 knots was usually in a favorable category, while slower ships could improve their prospects through effective zigzagging. The answer treats route discipline and speed as complementary precautions. It is a practical rule of thumb rather than a guarantee against a submarine already in position.