Uphill and Downhill

Today we made a long 50 nm run from outside of Charleston to Beaufort SC. The weather was good, and winds were light, so creeks, rivers and sounds were calm, as you can see above. On the other hand, lunar tides were high with 9 ft tides, which we haven't seen since Maine, and the resulting tidal currents were a big factor in our travel.

In this beautiful marshland the ICW winds towards the ocean, then away from the ocean, and repeats. We started at the John's Island Oxbow (photo above) where we had arrived yesterday after a short 12 mile run from Charleston, at roughly high tide. Thus most of our travel today was on an ebbing tide, so when we were headed towards the ocean we had assisting current (downhill run) and when we veered away from the ocean we had opposing current (uphill run).

As we cruised I wondered whether the effect of currents averaged out and so didn't matter. Here's a thought experiment that might edify :

- Lets run Spray at a constant 1500 rpm, which yields a speed of 6 knots in still water. We run a 12 nm route. If the route is in still water it's a 2 hour trip.

- Now split the route in half and add some current. The first 6 nm of the route will have a 2 knot assisting current and the last 6 nm will have a 2 knot opposing current. Will the trip still take 2 hours? What say you?

We need to do the math on each half of the route. The downhill portion covers 6 nm at a rate of 8 knots. Start with Distance = Rate x Time and solve for Time = Distance/Rate = 6 nm/8 knots = 0.75 hours.

The uphill portion takes T = D/R = 6 nm/4 knots = 1.5 hours. Both portions together take 1.5 + 0.75 = 2.25 hours, or 1/4 hour LONGER than with zero currents.

That's why today's run seemed so frustrating. We saw 'speed over ground' range from 9 knots for some downhill runs, down to 4.5 knots for some uphill runs. The downhill runs are over quickly while some of the uphill runs are brutal.

[An interesting extreme case for this exercise uses current speed of 6 knots. Then the downhill run is done at 12 knots and takes a quick 1/2 hour, but for the uphill run Spray is actually stopped and takes infinite time to complete the route.]

Nevertheless, we did end up reaching lovely Beaufort SC at about 4 pm. We had time to explore the town (really nice waterfront park) but we'll spend another day here to see more of it. Stay tuned!

 

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