What dangers did Skerryvore Rock pose?

 title: 'Technical illustration of a scientific device.'

The Skerryvore Rock and its surrounding cluster of rocks have been a cause of terror to mariners because of the difficulty of access and perpetual surf[1]. The smaller class of coasting vessels used the sheltered sounds to avoid the difficulties of the outward passage, as these rocks lie in the track of larger vessels[1].

The danger posed by the reef was insidious because there was no warning of its position in the dark until a vessel was unexpectedly among the breakers[1]. Even the knowledge of the reef's existence often caused seamen to give it too wide a berth, which made the ship liable to being carried off course by winds or tides[1].