Racing Area A - Summary of Tidal Current Effects
By Stuart Walker
Published times of change and strength are altered by the moon (stronger currents at full and new moon and at perigee - included in the Current Tables), by atmospheric pressure (higher tides and stronger currents with low pressure), and in the Chesapeake, most of all, by wind so that the time of Ebb Begins (EB) and Flood Begins (FB) may differ by 2 or more hours and strength may be diminished to near zero or increased (from typical maximum of 0.7 kn) to more than 1.5 kn. The predicted time difference between the two phases of the cycle (between EB and FB) at Annapolis is 6 hrs 12 min, so that (theoretically) each day the cycle is 48 minutes later than it was on the day before.
To predict the current in a race to come requires recognition of the time of the race relative to the tidal cycle - to the time that Ebb or Flood Begins. Anchored ships in the Bay are gross indicators: they turn to head south at 1 1/2 -2 hours after FB and turn to head north at EB or (very shortly thereafter). The Current Tables (and the published predictions) are usually not more than 2 hours off so that combining them with observations made with a current stick before the start permits a better estimate of the flow to come. A test with the current stick referrable to one of the crab pot buoys a few hundred yards southeast of the Greenbury Spider is helpful: the flow is to the east from 1-3 hours after FB, to the north from 3-6 hours after FB, to the west 1-2 hours after EB, to the southwest from 2-4 hours after EB, and back to the west from 4-6 hours after EB (when it shifts to the east again).
The generalizations made below derive from studies made by Bob Putnam and me. We spent several days testing (with anchored buoys and current stick) 12 different sites within and around Area A at every hour of the complete 12 hr 24 minute tidal cycle.
Generalizations about the EBB
- Ebb begins inshore and offshore simultaneously.
- Maximum ebb inshore is 2 hours after EB and offshore is 3-5 hours after EB and is strongest just offshore of a line from Hackett’s PT. to Tolley Pt. and inshore of the main channel buoys. The current is one-half to one-third as strong over the center of the racing area as it is along and offshore of the Hackett’s/Tolley line throughout the ebb.
- The strongest ebb (stronger than in mid-Bay) and flowing SW is always over the shoal to the northeast of the course area - 1/4 - 3/8 mile inshore of Can 35 - as the ebb from the northern Bay sweeps over this protruding shoal off Hackett’s Pt - and is strongest at this site 3 hours after EB.
- Maximum ebb in the outer Annapolis channel is SSE at 2 hours EB and swings SW to NW 5 hours after EB.
Maximum ebb along the 30 ft depth line offshore is SSW at 2 hours after EB and swings abruptly W to N at 6 hours after EB.
Maximum ebb inshore and mid-course is SSW (SW near Hackett’s) and swings gradually W to NW (W to N and NE near Hackett’s) at 6 hours after EB.
Management - Beating against the ebb in a northeaster:
(In a northeaster, current usually trumps wind shifts)
- From EB to 4 hrs after EB - go left (inshore) on stbd - then out on or above the port tack layline
- From 4 hrs after EB to 6 hrs after EB - go left but tack out under the port tack layline
Generalizations about the FLOOD:
- Flood begins first inshore (flowing east) at 5 hrs after EB and is flooding throughout the course at 6 hours after EB - but may not be evident in mid-Bay (at the anchored ships) until 1 1/2 - 2 hrs after FB.
- Flood begins offshore flowing strongly west along the 30 ft. line and northwest in the Annapolis Channel. By 3 hours after FB it turns north offshore and becomes much weaker, but continues to flow NW in the Annapolis Channel and to strengthen until 5 hrs after FB.
- Maximum flood offshore to W is at 1 hr after FB; flow turns NW at 2-3 hrs after FB and N at 4 hrs after FB.
- Flow is stronger - to N in the approaches to the Whitehall Channel and to NE near Hackett’s Pt - over the center of the course area and inshore of the center than it is offshore of the center of the course area (over the Hackett’s/Tolley line and farther out) from 3 to 6 hrs after FB.
Management - Beating against the flood in a southerly:
(In a southerly, wind shifts often negate the effect of the current.)
- From FB to 3 hrs after FB - Go right (on port) to the stbd layline
- From 3 hrs after FB to EB (and into EBB) - Go left (on stbd) to close to the port layline
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