Why Mean Low Tide?

[valhalla360]

It's a starting point for estimating the depth. As long as your chart and tide table use the same datum, it could use the height of Everest as the datum.

Using a 20yr event (particularly in areas with big tides) will close off a lot of viable channels and anchorages because you will look at the chart and if you don't pull out your tide table, you will assume, it's too shallow.

If you are operating somewhere with big tides, it's poor seamanship not to be keeping track of the expected tides and accounting for them.

[carstenb]

Originally Posted by valhalla360

It's a starting point for estimating the depth. As long as your chart and tide table use the same datum, it could use the height of Everest as the datum.

Using a 20yr event (particularly in areas with big tides) will close off a lot of viable channels and anchorages because you will look at the chart and if you don't pull out your tide table, you will assume, it's too shallow.

If you are operating somewhere with big tides, it's poor seamanship not to be keeping track of the expected tides and accounting for them.

well, not sure I agree with you. using MLT means I can be lured into a channel where there isn't enough water.

LAT tide says what is the worst you can expect to see.

[a64pilot]

Originally Posted by carstenb

LAT tide says what is the worst you can expect to see.

Not really, there are for example weather events that can greatly influence tidal swings, even wind can make more difference than the difference between these tidal datums.

Knowing the average tidal swing and where you are reference to spring tides makes more sense, for instance if I know there is a tidal swing of one meter and I have two under my keel, I don’t worry about it.
But if the tidal swing is three, then I do.

Extreme example, the tides on the Great Lakes are apparently 2 inches, but winds have caused as much as a 22 foot difference in water level.
https://www.canr.msu.edu/news/great_...des_kinnunen15

[jackdale]

Originally Posted by Benz

Which makes it even less useful as a datum. What's the lowest yearly called? There's too many acronyms to really keep it straight.

LAT should be standard for chart datum, all tide eights in tables would then be positive. Meteorological factors such as wind and BP cannot be predicted in advance, but can be factored in my weather offices when necessary.

There are too many national standards for chart datum.

[boatman61]

Coz its real Mean when it catches you out..

[sailorboy1]

Originally Posted by boatman61

Coz its real Mean when it catches you out..

and LOW

For me I am fine with MLLW use. If it is close to my draft I only go in on a raising high tide and it has to be high enough to believe the tide guess number for me to be ok with it.

[TomThumb]

As many have noted, chart datum is a baseline and could be anything.

Anyone not checking a chart for what that datum is, just like the GPS datum or currency of the chart is asking for trouble. Similarly anyone passing under a bridge not checking what datum is used for clearance heights, usually MHHW.

But in terms of which tidal stats are used, various agencies and countries have chosen one for a given logic that suits them and unfortunately it isn't consistent. The Baltic having no tides just uses Mean Sea Level.

Lowest Astronomical Tide (LAT) is defined as the lowest tide level which can be predicted to occur under average meteorological conditions and under any combination of astronomical conditions. Many national charting agencies, including the United Kingdom Hydrographic Office and the Australian Hydrographic Service, use the LAT to define chart datums. Note this is 'average' and 'predicted'.

One advantage of using LAT for chart datums is that all predicted tidal heights must then be positive (or zero) avoiding possible ambiguity and the need to explicitly state sign. Calculation of the LAT only allows for gravitational effects so lower tides may occur in practice due to other factors (e.g. meteorological effects such as high pressure systems).

Mean Lower Low Water
The United States' National Oceanic and Atmospheric Administration uses Mean Lower Low Water (MLLW), which is the average height of the lowest tide recorded at a tide station each day during a 19-year recording period, known as the National Tidal Datum Epoch. MLLW is only a mean, so some tidal levels may be negative relative to MLLW. The 19-year recording period is the nearest full year count to the 18.6-year cycle of the lunar node regression, which has an effect on tides.

Mean Higher High Water
Similarly, the Mean Higher High Water (MHHW), is the average height of the highest tide recorded at a tide station each day during the recording period. It is used, among other things as a datum from which to measure the navigational clearance, or air draft, under bridges. Using MHHW is a sort of worst case for minimum clearance which makes sense.

The Mean Low Tide (or Mean Low Water) is the average height of all low tides in a given place, deriving from a long series of observations (NTDE) of all levels of low tide in that spot. Branching off this concept, are MLWN (Mean Low Water Neaps) and the MLWS (Mean Low Water Springs), which are the average of all low neap tides and all low spring tides, respectively.

Clear as mud.

[valhalla360]

Originally Posted by carstenb

well, not sure I agree with you. using MLT means I can be lured into a channel where there isn't enough water.

LAT tide says what is the worst you can expect to see.

If you get "lured" in, it means you aren't paying attention.

Will you use the same excuse when your mast hits a bridge because LAT suggested you had sufficient air clearance but you didn't bother to check the tide first and ignored the boards on the bridge piers?

If it were half as big an issue as you suggest, they already would have changed.

[valhalla360]

Originally Posted by a64pilot

Extreme example, the tides on the Great Lakes are apparently 2 inches, but winds have caused as much as a 22 foot difference in water level.

Yep, went to bed one night in a marina on the west end of Lake Eire.

By the time a long sleepless night was over (adjusting the lines every hour or two), in the morning I had to reach over my head while standing on the cabin top to tie off a rope ladder to the dock, so we could get off the boat.

If you had tried to go into that marina, you would have run aground about a mile off the charted shoreline if you stuck to the channel.

[requiem]

Originally Posted by TomThumb

Mean Higher High Water
Similarly, the Mean Higher High Water (MHHW), is the average height of the highest tide recorded at a tide station each day during the recording period. It is used, among other things as a datum from which to measure the navigational clearance, or air draft, under bridges. Using MHHW is a sort of worst case for minimum clearance which makes sense.

Sounds to me that this is no different from the LAT / MLLW debate, since HAT could be argued as the "better worst case".

I do wish that charts would more clearly relate MSL / MHHW / HAT to the sounding datum.

[StuM]

Originally Posted by TomThumb

Branching off this concept, are MLWN (Mean Low Water Neaps) and the MLWS (Mean Low Water Springs), which are the average of all low neap tides and all low spring tides, respectively.

What no Mean Lower Low Water Springs?

How can I live without that?

[StuM]

Originally Posted by TomThumb

Mean Higher High Water
Similarly, the Mean Higher High Water (MHHW), is the average height of the highest tide recorded at a tide station each day during the recording period. It is used, among other things as a datum from which to measure the navigational clearance, or air draft, under bridges. Using MHHW is a sort of worst case for minimum clearance which makes sense.

I wouldn't call it "worst case". It's a mean (average) of the highest tide on each day.


Which means that on roughly 50% of the days, it was higher than that at some point during the day.


HAT would be much more sensible since then you only ever have to consider weather effects for minimum clearance.

[boatman61]

Wot.!!! no App to resolve basic maths problems.. the worlds gone to hell in a basket.

[requiem]

Originally Posted by boatman61

Wot.!!! no App to resolve basic maths problems.. the worlds gone to hell in a basket.

Quick, the depth on the chart is 10 feet, the bridge clearance on the chart is 40 feet, and the current height of tide is 5 feet: what's the tallest vessel that will fit?

[boatman61]

Originally Posted by requiem

Quick, the depth on the chart is 10 feet, the bridge clearance on the chart is 40 feet, and the current height of tide is 5 feet: what's the tallest vessel that will fit?

So.. MLW is 10ft, you have 5ft of tide.. thats kinda like silly unless one also knows the tide range.. it could be anything from 5ft to 30ft.
If its 5ft you have 40ft minus your mast top gadgets.. if its 30ft you have 65ft minus your gadgets.