Depth Perception: It''s More Than What's Under The Keel

Depth Perception: It''s More Than What's Under The Keel

- Capt. Daniel S. Parrott

In days of yore, navigators expended great energy and effort attempting to establish their position. When they finally did, it was often with great doubt.

Whether by bearings converted from compass to true, or by celestial methods, navigating was extremely time consuming and highly dependent upon good visibility. Such techniques were prone to human error. Even when sailors executed them perfectly, they knew only where they had been, never where they actually were.

An exception was the positional information obtained by soundings. Depth information is in real time and does not rely upon visibility. Monitoring depth goes far beyond determining whether there is risk of grounding. The depth sounder is like a blind storyteller reciting the narrative of the seafloor. Comparing that tale to the one written into your chart can provide a basis for assessing whether you are where you believe you are.

The significance of soundings, as well as the consistency of the bottom, found voice in the sailing directions of the British Isles as early as the mid-1400s:

When you come out of Spain and are at Cape Finisterre, set your course northeast. When you reckon you are two thirds of the way across … go north by east till you come into soundings. If you then find 100 fathoms deep or 90, then ye shall go north until ye sound again in 72 fathoms in fair grey sand. That is the ridge that lieth between Cape Clear (Ireland) and Scilly. Then go north till ye come into a sounding of ooze and set your course east northeast.