We made fifty miles today after uncoiling our way out of Baby Bear Cove over a shallow bar on a falling tide. Then we powered our way through a current that was less contrary that the tide tables had us believe.
We have a meter that shows the boat's speed through the water as measured with a small paddle wheel under the hull. Since the water itself is moving, the actual Speed over Ground (SOG) is measured with GPS. The difference tells the the speed of the current.
Even with the tide table predictions of current, it is usually difficult to predict what the current, or the SOG, will be. This is especially true when the flow in the channel is turbulent. Channels with smooth contour lines are said to be laminar (not turbulent). The thing that makes some places so treacherous is not just the velocity of the flow, but the topography of the bottom. A lot of topological irregularity creates turbulence.
It was interesting to watch our boat knots and SOG as we headed up Peril Straight this morning. Because the center of the channel generally has a higher rate of flow, we favored a line closer to the shore. The shoreline itself was fairly irregular and the topography of the bottom was also uneven (as all channels are to some degree). Shortly after exiting our cove and heading up into the current, we found ourselves flying along at 7.5 kts SOG with 6.0 kts speed through the water. That suggests that the current was going with us at 1.5 kts. A minute later our SOG was 5.0 kts. The speed through the water remained fairly constant, but the SOG drifted around considerably, suggesting rather turbulent conditions. All in all, we didn't see the 1.5-2.0 kts against us that was predicted by the tide tables, probably because we weren't in the center of the channel. As the current slackened over time and the channel widened, we did see our SOG slowly come up to parity with the speed through the water around the time we expected to see slack water.
This is all part of the learning most of us hope to achieve: by paying close attention the particulars of the channel we're in: width, depth, bottom and shoreline contours, we can take note of the resulting turbulence in terms of its effect on our boat speed. And we can learn from it. Whether fishing around Johnstone Straight, or river rafting the Skykomish, there are people who have a lot of experience at this kind of thing and they're really good at it. They're better than I'll ever be, but it's fun to give it the attention it deserves. And being more current aware makes for safer passages.
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