Suspect Local Knowledge
Suspect Local Knowledge
"Local knowledge" is a wonderful thing, but its value depends on whether someone is willing to provide it and whether that someone knows what they're talking about. I was the first American to win the International 14 Prince of Wales Cup because I asked the local fishermen about the strong, parallel-to-shore tidal currents. They knew the best way to avoid the strong southerly tidal current in a northerly was to tack up the beach, in and out of the protruding "groins."
When, in the '50s, '60s, and '70s, I was asked by visitors to Annapolis how to handle the north-going flood in a southerly, I told them to go right and stay inshore until they reached the starboard layline. I was willing to talk, but I didn't know what I was talking about at the time. I realized this about 15 years ago when Dave Curtis beat me to the weather mark.
I had tacked to port immediately after starting and led most of the fleet inshore, while Dave, leading the rest, held starboard at least halfway to the port layline before tacking. I was confident that inshore, in shallower water and less adverse current, I would pull out on his leebow and beat him easily. Instead, in the dying southerly, he gradually pulled abeam and rounded the mark 200 yards ahead. Afterwards he told me that he was convinced the adverse current had been weaker farther offshore. I scoffed at the idea, and assured him that, due to increasing friction, the Chesapeake Bay's north-south current always becomes progressively weaker as one moves inshore from the deep Bay into shallower water.
However, after a few others beat me on other similar occasions, I realized I needed to check my supposed local knowledge. I recognized that the Chesapeake's complicated underwater contours and multiple and sizeable indentations could deflect the tidal currents into odd patterns of flow. I remembered my experience in San Francisco Bay before the Olympic Trials of 1972. I'd distrusted the presumptions about the currents on the Berkeley Circle and spent several days with a current stick determining current strength and direction at every hour at each of the fixed marks of the circle. I was surprised to find that in the shallow water of the racing area the flow developed a circular eddy different from the implications of the local knowledge and the touted hydrological model.
The most important consequence of current is that, when it differs in strength across the course, it is possible for boats to sail in areas of differing strength. Because adverse current prolongs the duration of exposure, the most important strategic requirement when racing in adverse current is to discover and sail in an area of lessened strength. Current produces three other important effects: it moves the boat referrable to the fixed marks of the course (which effects starting tactics, laylines, sailing angles, and mark roundings); it alters the duration of legs (thereby exaggerating or diminishing the effects of performance differences); and, if it is not recognized and properly managed, it may alter apparent-wind strength and boatspeed on runs. But recognizing differences in its strength and utilizing that knowledge is by far the most important.
In tidal waters, current is often the most important aspect of local knowledge as it operates every day and is more predictable than local winds, and is less effected by variations in the weather. When a racing sailor seeks knowledge about the local current, his first question should be: Does it vary across the course? His second question should be: How strong does it get? The third: When and where in each tidal cycle is it strongest and weakest? Unless you find someone who has personally checked the racing area, the answers you receive may not be correct. And, of course, in order to utilize the information you receive and to apply it to the time you are racing, you need to determine (from the published Current Tables or by applying offsets to the Tide Tables) when Ebb Begins (EB) and when Flood Begins (FB).
When I decided to unravel the current behavior in our Annapolis racing area, I asked for help from my friend Bob Putnam-who owned the necessary tool, a powerboat. Bob and I anchored a number of buoys which, together with several government marks and crab pot buoys, established a 12-mark grid within and around the racing area. Over a three-day period we circled the grid, dropped our current stick, and measured its speed and direction of displacement from each of those marks during every hour of the tidal cycle.
I recognized the study would only reflect the current's timing and strength on the days of testing and that on other days these factors would vary with the differing positions of the moon and the sun, with barometric pressure, and with wind flow in the Chesapeake Bay where the tide is higher and the current stronger in southerlies that push ocean water into the Bay and lower and weaker in northerlies that push water out of the Bay. But a few days of testing throughout an entire tidal cycle (12.5 hours at Annapolis) uncovered relative differences in the strength and direction of flow at particular sites within the racing area-the information most important to the racer.
We discovered a number of significant, but previously unrecognized, manifestations: the pattern of flow in the ebb was quite different from the pattern in the flood-not a mere reversal of flow; the strength of surface flow was not proportional to the depth (as, due to friction affecting the bottom layer, it usually is in tidal waters); the change from ebb to flood (and vice versa) was neither, as is usual, an abrupt reversal of direction nor a gradual increase in strength.