Wednesday, April 16, 2014

Charleston Race Week 2014 Debrief - Currents and Tides

Sailing in tidal currents is something foreign to most lake sailors. However, when sailing in a harbor such as Charleston, understanding it is key.
Charleston is complicated in that two rivers flow directly into the harbor at a right angle to each other and there is an island in the center of the northern part of the harbor.
Before racing in a tidal harbor, checking tide tables is vital. Knowing the timing of the tides allows you to figure out when currents will strengthen and weaken as well as what to expect on different parts of the course.

Current lines can be observed in active harbors such as Charleston and San Francisco and are denoted by a foam line running along the axis of the tidal current. If you’re on a boat equipped with both boatspeed and speed over ground instrumentation, it is very easy to tell when the boat is in a new current pattern.
First, a primer on sailing in current. Generally, current is stronger in deeper water and lessened in shallow water, so current relief can be found over shoals, and generally near shore on un-dredged waterways.

Tides also generally switch first along the shore where the differing water level has a more immediate effect.

This means that in the middle of an ebb or flood cycle, the shore can be a relief.
At the change between an ebb or flood, the shore can produce a back-eddy or reverse current, and just after that can create a stronger current as the flow strengthens.
Current lines are important to understand when calling shifts up the course. On Saturday of Charleston Race Week, we sailed in a SSE breeze with a Westerly Current (Wind direction is the direction the wind is coming FROM, current direction is the direction the current is flowing TOWARD). The result of this was that boats near the Southern shore seemed to show a right shift in the breeze. However, this angle difference amongst the boats was not due to a change in true wind direction, it was a local phenomenon caused by shifts in apparent wind caused by the current and it affecting boats in the stronger Westerly flow near shore.
When sailing close hauled on Starboard Tack, pointing SSE, the current swept the boat sideways to the West along the shore, increasing the apparent wind speed and also moving it aft, allowing the boats to point their bows higher, thereby creating the illusion of a right shift. Similarly, boats on Port Tack, pointing WSW, were being swept mostly forward by the Westerly currents, creating an increase in apparent wind but also moving it forward, causing them to fall off and point lower, also indicating a right hand shift.
Recognizing that boats displaying this shift pattern were on the other side of a foam line on the water was of great importance, so that we could recognize that the right hand shift was not a mobile puff that would reach us if given more time, but something occurring on that particular patch of water and this knowledge kept us from getting tricked into wandering too far Right hoping for leverage.
For another critical lesson in sailing in current, watch the following videos from Sunday, where the fleet, sailing in light SE breeze with a SE current attempted to round the leeward mark. The only successful boats came in with speed and carried spinnakers past the mark. The rest of us stopped, drifted backwards while other boats behind piled up and stole the breeze, and it turned into a nice, old-fashioned raft-up. Albeit with a lot more yelling.

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