Inlet Currents

Cruisers along the Intracoastal Waterway (ICW) are made keenly aware of one thing -- namely tidal currents. The currents can be very strong and very significant for your progress. A sailboat like Tarwathie cruises at about 5 knots under power. With a 2 knot tidal current, our net speed may be 3 knots with current against us, and 7 knots if it is with us. That's more than a 2:1 difference!

Another feature of the ICW is that it is like a narrow river, separated from the ocean by barrier islands. Every once in a while, there is a gap between the barrier islands forming an ocean inlet. Boats may or may not be able to navigate the inlet, but the tidal waters surge in an out at these points. Those surges are what causes the tidal currents in the ICW.

The two pictures below, illustrate a passage between two neighboring inlets. During flood tide, water rushes in and during ebb tide it flows out. In the picture, you can also see boats, being depicted as traveling fast or slow. You get the picture.


Now, consider the following two assertions. First, when a boat passes an inlet, the relative current flips. If you had current with you before, it is now against you. Second, although the average current speed is zero, you spend much more time with current against you than with you. Call that the pessimist's rule.

What's with this pessimist's rule? Isn't that urban legend? No, it's real. I'll explain. Let us consider a simplified tidal profile. The current runs at a constant speed of 5 knots in one direction for 6 hours, then it instantly flips to 5 knots the other way for 6 hours. Further suppose that your boat speed is 5 knots. Then, with current against you, your net speed is zero. You don't move at all, and you might as well anchor for those 6 hours. Then, when the current flips, you are whisked to your destination (or to the next inlet where current flips again) at the breakneck speed of 10 knots.

Take a numeric example. Suppose inlets are 1 mile apart. Just after you pass the first inlet, the current turns against you at 5 knots. You sit there making zero progress for 6 hours. Finally, the current flips again and you reach the second inlet in another 6 minutes. As you pass the second inlet, the current flows against you once again for another 5 hours and 54 minutes. No wonder boaters are pessimists.

There is an important exception to the pessimist's rule. Suppose you pass the inlet at the right during flood tide. You'll have current with you. Then suppose that your timing is such that halfway between the inlets, the tide switches to ebb. You will have had current with you the whole time. It is like pumping your legs at the right speed on a swing set.

I recall once when we hit it just right. We left the Saint Johns River near Jacksonville Florida, and entered the ICW. We had a very fast trip at 7 knots all the way to Saint Augustine, 36 nautical miles away. We made it in about 5.5 hours. With no tide at all, it would have taken 7 hours. If our timing had been such to suffer the full pessimist's rule, the same passage would have taken 9.5 hours.

Is there anything a boater can do to improve his net speed without increasing fuel consumption? Yes, he can ease the throttle when the current is with him and use more throttle when current is against him. However, the net gain is small and most of us don't do that; we travel with the throttle set at cruise speed 100% of the time.

What can you do to minimize fuel consumption? That's easy. Anchor when the current is against you and drift with no power when it is with you. Your average speed is one half the current speed and your fuel consumption is zero. With a little bit of wind in your sails, you can achieve a much higher average speed.

Of course, the more power you have, the less you are hindered by those pesky currents. It is a problem mostly for slow, underpowered, sail boats where current speed is a significant fraction of cruising speed.