Inlet Currents on Steriods

Vero Beach
No LL

The analytical engineer in me couldn't let that problem with the inlet currents in the previous blog alone. I know that the simple example in that previous post with 5 knots current and 5 knots speed and 1 mile between inlets was unrealistic. So what would realistic numbers really look like. Therefore I wrote a little simulator program to calculate the numbers for me. The results shown below are surprisingly complex. No wonder I couldn't do the math in my head.

Here is the basic problem. Suppose that inlets are 30 miles apart, and that the strongest tidal current is 2.5 knots. Those are realistic numbers for the ICW. Then suppose that a boat leaves at a certain time and with a calm water cruising speed of 3, 5, or 7 knots, and that the trip continues for 24 hours. How will the speed vary through the day?

The results are shown below for two times; departure against peak current and departure with peak current.




















Why are the curves so wiggly and wild? We see the interaction of three things. The tide varies with time with a period of 11.5 hours. The boat moves from inlet to inlet interacting first with the inlet behind and then to the inlet ahead. Finally, as the boat passes an inlet, the relative current direction abruptly flips.

I think it is astounding that such complex behavior can arise from such a simple problem. No wonder I couldn't do the math in my head.

What about the pessimism factor I mentioned in the previous post? We know that at constant engine RPM, the boat spends more time moving against the current than with it, but by how much? See the table below. 50% means equal times with current and against it. I guess that the pessimism factor is not as large as I imagined.