The idea of a 3 knot push might sound like a dream come true! People spend thousands of dollars to have feathering propellers and faired underbodies, all in the interest of gaining a few tenths of a knot, and here you can magically gain 3+ knots, for FREE!
Currents are caused by the movement of water from one place to another, as the water flows, it will carry everything in it along for a ride. If you happen to be sailing in the same direction as the current, then you will get a nice push in the right direction! Your speed over ground will become your speed through the water + the speed of the current. If you are sailing against the current, your speed over ground will become your speed through the water - the speed of the current.
It is important to know what direction the current is flowing that way you can use it to your advantage!
Offshore, currents tend to move in a single direction at a fairly constant rate. They also tend to be well known, such as the Gulf Stream and its famed "counter currents" that run parallel to it on its bounderies. In shore, currents are typically caused by draining rivers or tidal waters ebbing and flooding. While the currents offshore all flow in the same direction, the currents inland tend to correspond with the tides and will alter direction throughout the day.
Knowing when the current will be flowing and how strong can really help a vessel cover serious ground in a single day. We sailed across the Bogue Sound in about 3.5 hours, covering 24 miles at an average speed of 6.8 knots! We are by no means a fast boat, and we usually average less than 2 knots on a passage since we are dependent on the wind being in the right direction. This day was perfect, we were on a beam reach and had a very strong current pushing us along at over 3 knots! While our speed over ground may seem impressive, our speed through the water was our typical sailing speed of around 3-4 knots. By timing the tides and its current, we were swept into the Bogue Sound and reached its middle before slack tide. When slack water approached, we slowed to 3-4 knots since we no longer had the current helping us. As the tide began to ebb, we were swept out with it on the other side and sped across the remainder of the sound, reaching the other side by lunch. When we crossed the inlet, we were then faced with an opposite current as it was also draining into the inlet and our speed plummeted from 6 knots to 2 knots as we fought a 4 knot current on the nose.
Using these speed boosts can seem fun and will help you cover more ground with less effort, but it does take some precautions on the part of the captain.
Bridges and other obstacles will approach at a much faster rate than you are used to. This means that you need to line up with the opening in the bridge perfectly well in advance, as the current will slam you into the abutments if you are trying to cut it close.
Strong currents will also create eddies and vortexes that will swirl just behind large fixed objects. These swirling forces can pull you in and throw you off course. Your keel is especially susceptible to these phenomenon and will pull you along helplessly.
Encountering other vessels is also tricky, as the typical rules about right of way are skewed to accommodate for the current. The vessel traveling downstream will have right of way over all vessels traveling upstream because the downstream vessel has less maneuverability. If you are approaching a bridge pass, the downstream vessel can not stop as the current will pull them through the pass, so the vessel traveling upstream must stop and wait; even if the downstream vessel is under power and the upstream vessel is under sail.
Currents are fun, but they can be a little stressful in tight quarters where the speed takes away your ability to stop the vessel with the normal controls, such as heaving to or putting the motor in neutral. The current will carry you along and push you where it wants, and currents in narrow areas run faster as the water has to rush through the narrow opening in a hurry. This can add to the stress of the situation if not managed well.
If you do not feel comfortable navigating with a strong current, you will be forced to wait for slack water to occur before transiting tight areas. While it might seem intimidating at first, an important thought to keep in mind is that anchors still work with currents. If you ever feel like a current is going to slam you into a bridge or wall, the anchor will hold the bottom and keep your yacht from traveling at the mercy of the current.