On a global scale passage makers are concerned with surface ocean currents.
These currents are mostly generated by the wind. The remaining contributing factors are water density differences caused by variation in levels of salinity, the topography of the ocean bottom and the Coriolis Effect.
Any cruiser who has sailed along the east coast of Florida quickly learns that currents act exactly like the moving sidewalks at an airport. Walking with the direction of a moving sidewalk speeds you up, and walking against the direction of a moving sidewalk slows you down. Sailing north in the Gulf Stream a few miles off Palm Beach yields a boost of about four knots to speed over ground, but sailing south at the same location slows a typical cruising monohull to a crawl.
Closer to shore, the effects of current require the coastal navigator to pay close attention to both the direction of the current and its speed.
Adverse currents can push a boat outside of a marked channel onto sandbars, reefs and rocks much to the dismay of an inattentive skipper.
Current is described by its set and drift.
Set is the direction it flows in degrees true, and drift is its speed in knots. This is the exact opposite of the way wind direction and leeway are defined. A strong northerly wind will cause southerly leeway on a vessel, while a northerly current will set a boat towards the north.
Along the coast and on estuaries tidal currents predominate.
These currents are generated by the rise and fall of the tide, but are described by the horizontal motion of the water. A tidal stream filling up a bay or harbor from the sea is said to flood, while a current heading back out to sea is called ebb. When current is neither ebbing nor flooding, it is at slack water. Tables that predict the ebb, flood and slack of currents are available in printed and digital formats. However, the prudent mariner will also use observational awareness to determine the actual set and drift by noting the motion of water as it flows past nearby buoys, day marks and dock pilings.
Inland rivers that are non-tidal have their gravity-induced stream of current that flows to the mouth of the river from its headwaters. When maneuvering on a river a vessel can go upstream, downstream or crosscurrent.
How to Dock a Boat in Current
The maneuver most feared by boat owners is docking, and docking in current can be a recipe for disaster unless the skipper remembers a cardinal rule of maneuvering in close quarters: don’t fight the natural forces, use them to your benefit. Given the choice, when docking in current, it is easiest to dock with the current off the bow. When coming bow first into a slip, the current then acts as a natural brake slowing the boat down.
Docking parallel to or alongside, the technique of ferry gliding serves as a natural bow thruster and gives the captain precise control when docking.
What is Ferry Gliding and how does that relate to a boat in current?
Any boat under power can accomplish ferry gliding by balancing the element of current off the bow with the speed and heading of the vessel. To dock successfully by ferry gliding, first maneuver the boat into the current parallel to the dock and a few boat widths off the dock. Keep the boat on station by using the engine and steering to keep the centerline of the vessel aligned with the flow of the current. Use an object on the dock, such as a shore power station, to determine when the boat is steady in a fixed position counteracting the drift of the current.
Next, turn the bow of the boat about thirty degrees toward the dock while staying on station. The pressure of the current on the bow will push the boat sideways toward the dock. The faster the current, the faster the sideways motion of the boat. To slow the momentum as the dock nears, just turn the bow of the boat away from the dock and more parallel to the set of the current. Just before the fenders kiss the dock bring the boat parallel to the current and pass the dock lines ashore. It is ‘no drama’ docking every time.
With practice, ferry gliding can also be done with the current off the stern by turning the stern towards the dock. Becoming familiar with how one’s boat handles in current can make the difference between an enjoyable day on the water and a frustrating and costly one.