Why fire ships worked against the Spanish Armada

What is the most crackpot naval scheme that actually worked?

The fire ships sent against the Spanish Armada, because a competent anchored fleet should have been able to intercept and tow them clear.

The Armada was large enough to deploy boats that met the unmanned fire ships and steered them away from the formation. Its failure to prepare that defense turned a simple and normally manageable threat into a disruptive success.