Your Chart is Probably Out of Date--Updated

PS: I just spent the weekend with club friends in a little cove off the Manatee River. There's a very high spot shown on both paper and electronic charts that's supposed to only be 3.3' deep at mean low tide.

I had difficulty getting the anchor up, and in the process, according to my chart plotter my boat drifted all over that spot, but the chart plotter, which I checked frequently because I knew about that supposed bump, showed I never had less than 9..4' water to work in. There's another example of how wrong charts can be. Someone had told me with great confidence that this "bump" wouldn't change because there was no current in there, but of course, since it's on the south side of the river, a lot of storms will stir that water up. In any event, the bump seems to be gone. This is actually the most dramatic case of change I've come across so far.


I've shown pictures of floating buoys in a major channel in the Tampa Bay area before:

 

This photo faces east. This is Channel Marker #10 in the Sunshine Skyway Channel. You can see exposed sand beyond. Channel Markers #8 and #12 are significantly farther east than this one is. The Channel is shoaling, or filling in with loose sand, at this spot. I took this picture some months ago. Since then I've seen a sailboat aground, in the channel, north of this spot. In addition, to the south, another piling has been taken down and another floating buoy moved significantly west. The eastern edge of this channel is starting to get a bit zig-zaggy!

I ran lightly aground between markers 2 and 4 a couple of weeks ago. We had a lower than average low tide, partly influenced by the 25 mph north wind that apparently blew a fair amount of Tampa Bay's water to Venezuela. I got off fairly easily; sand that has just moved hasn't had a chance to compact yet and gives way fairly easily. I've also "skipped bottom" at other spots in the channel.

I don't know when the powers that be will get around to redredging this channel, but probably it takes a while. Meanwhile, what do I have to go by? One thing is for sure: my chart still shows both channel markers in their old places. So does my chart plotter.

Meanwhile, we've had a run of unusually low tides here. The lowest point doesn't typically go below mean low tide (except for that one very windy day), but we're not having full high tides. Since the typical distance from low to high tide around here is only two feet, when the tide only rises one foot "high tide" isn't a lot of help in very shallow places.

But the problem with charts and chart plotters lagging behind real life is more complicated than just where the channel markers are. At the southern end of the Skyway Bridge Channel, it appears that there is deeper water -- 8' - 10' -- just to the west of Channel Marker 2. So it would *appear* that one could just bypass the southern entrance by going west of it, and then enter the channel past that shoaling spot.

However, if the channel is filling in to that degree, perhaps the area to the west of it is filling in as well.

When faced with such situations, local knowledge can be a great help. Local fishermen may know, or local dockmasters, charter captains, etc. You can get on the radio and see if you can raise someone. or put someone, preferably someone with experience in shallow water, on the bow. Proceed very slowly, so if you do run aground, your boat's speed doesn't make it worse. 

For now, I'm just avoiding that patch of water at low tide. I'm not going to go through there at night by myself unless I have a full-out high tide, and even then it wouldn't be my first choice, because being aground in a narrow channel at night is not exactly an ideal situation. 

So don't trust your chart plotter 100%. Don't completely trust the routes you've plotted on it unless it's all through known deep water. Keep your eyes open, and reduce your speed at night. Keep a log of such instances so you're not left scratching your head next time, thinking, "Now where was that high spot?" You don't want it under your keel!

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