Suspect Local Knowledge
Suspect Local Knowledge
We discovered the ebb begins inshore and offshore simultaneously, but that inshore it increases in strength rapidly to near maximum 1 to 2 hours after EB and that offshore it increases slowly to maximum 3 to 5 hours after EB. Not surprisingly we found that the ebb was but one-half to one-third as strong over the near-shore center of the racing area as it was between the 25-foot and 30-foot depth lines offshore. But we were amazed to find that the flow was stronger along that underwater bank at the outer edge of our racing area than it was in mid-Bay, and that the strongest flow of all was over the shoal on the northeast edge of the course area! Apparently the shoals protruding offshore from Hackett's Point diminish the width and depth of the Bay, constrict the mass of water ebbing south from the northern Bay, and (acting like the shallows of a river) accelerate rather than retard the surface flow.
We discovered the flood begins inshore flowing east an hour before official FB, but is not evident on the ships anchored in mid-Bay until 1.5 to 2 hours after FB. Flood begins in the racing area at FB, flowing strongly west along the 30-foot depth line and northwest in the Annapolis Channel. Not until 3 hours after FB does the flow turn north along the 30-foot depth line and thereafter it weakens progressively. To my great surprise, from 3 to 6 hours after FB, we found that the flow was stronger inshore of the center of the course area (flowing north, in the approaches to the Whitehall Channel and northeast near Hackett's Point) than it was offshore of the center of the course area (over the 25-foot line and farther offshore).
The flood, like the ebb, was behaving as if it were flowing in a river. As the flood traversed the progressively lessening depths to fill Whitehall Bay and its many tributaries, it accelerated and from 3 to 6 hours after FB, was moving to the north on the inshore half of the racing area twice as fast as the water in the deep, wide Bay. Dave Curtis had been right, from 3 to 6 hours after Flood Begins one should go offshore, not inshore, to escape the worst of the adverse current!
One wonders how much other local knowledge in other tidal venues is based on similar false presumptions and begs to be corroborated or refuted by days of testing with a current stick-and whether I'll have time to test them all!
Ask the right questions
When a racing sailor seeks knowledge about local current he should ask:
1. Does it vary across the course?
2. How strong does it get?
3. When and where in each tidal cycle is it