Tide Tables and Charts

[DrB]

Simple question maybe.

The NOAA CHarts report depths as Mean Low Low Water, which to me means a the average of depth of the water at a low low tide level.

When I read the tide chart, I get the time of the tide and the height, so say high tide is at 11 PM with a height of 8.63 ft.

Now here is the question, if the chart has a 6.0 ft MLLW reported reading, what is the actual depth if the tide reported at Low tide is -1.52 feet.

The reason for the question is that I am ready to pull my boat and they want me to leave it at the end of the dock, which they say has a depth of 6.5 ft at low tide. I don't know what that means. Is it a 6.5 ft MLLW value? If so and -1.52 feet, I can't leave it there due to my 6 ft draft. If the tide NOAA charts averages in the neg tide values, to get its MLLW, then I might be safe for a day or two, especially if the tides for the next couple of days are positive values of MLLW.

I never really think about this as I am on a mooring where there is planty of water around me even at low tide.

DrB.

[jackdale]

Charted depth +/- height of tide = depth of water
6.0 -1.52 = 4.48 feet

Jack

[Tempest]

The charts are listed as MLLW and your tide charts show you have a negative number for that day, you need to subtract from the 6.5 ft, if 6.5 is the MLLW number.

But, the marina owners should know what the depth at their docks are better than the charts. If they say they have 6.5 feet at low tide then you could be ok. I would double check with them, we had some very low tides at my marina last week and some boats were sitting on the bottom.

[Maxboatspeed]

Cool,
here it is
MLLW is the datum that the chart and the tide refer to. In other words, "0" on the tide table does mean that the depth should be what is printed on the chart. Depth=charted dept + tide level, 6.5+(-1.5)=5'. You draw 6'. Aground in a perfect world. Good logic.
Charts are not that accurate. Even large scale charts. If there is a 6.5(mllw) near that area, 6.5 may be marked there - even when it's 9' there.
If you really want to know about that spot: go there when there's water. Drop a leadline(measure the depth around your boat). Calculate what the depth will be as the tide changes.
OR tell the yard dude your concern. Read the guys response. It's likely he's a pro that knows. It's possible he's young and stupid.
Evaluating yard guys is part of it.
Max

[jackdale]

DrB

To be absolutely sure of the depth, use a lead line.

[PorFin]

Originally Posted by Tempest

But, the marina owners should know what the depth at their docks are better than the charts.

This is one case where the word "should" ought to be underlined and italicized, just to clearly differentiate it from "will."

I've had several cases where the marina staff were fairly clueless as to actual dockside depths. One time I was assigned a slip the supposedly had "at least 7 feet," but I a littel skeptical so I checked it before I moved from the fuel dock. Turned out that while it had 7' right at the fairway, it quickly shoaled to 4.5' about 1/3 of the way in.

Bottom line: If there's any doubt, check it with a handheld depthsounder, leadline or a long probe.

[sailingdog]

There are times where having a boat with a draft of 15" boards up is very useful.

[ottos]

Other considerations -

Is there ANY wave action at the dock? You could lose another 6" or more right there.

What about blow out tides? We get them around here, and easily lose another 1 - 1 1/2 feet.

And don't forget that "Mean Lower Low Water" is an average and not an absolute.

edit - just found a good illustration of that at NOAA

[Omatako]

I always thought that depths reflected on the charts were the lowest observed. In other words it doesn't get shallower than that.

But that doesn't work in marinas because the bottom of a marina generally experiences a lot of movement and silt ans such stuff and corners, nooks, crannies and even main canals can vary considerably.

I don't believe there is any chart that you can rely on for the depth at the slip in your marina. You have to check it out or, more sensibly go in at a higher stage of tide which will guarantee water under your boat.

[Minnewaska]

I could swear that I've seen charts that say it was the 1950s since they last took soundings. Don't have any handy.