Mast Height, Bridge Clearance, Tides.

[hpeer]

Originally Posted by Tingum

Bridges are also rated at low steel, meaning the lowest part you could manage to hit. The rise of the bridges arch adds some height to what is on the chart and measure board.

So "low steel" would be measured at the edge of the channel?

I'm probably going down the ditch in the spring. 63' high and 6' 6" low.

[Cottontop]

Originally Posted by category4jay

Can someone tell me why in the world I would ever use a high tide chart based on Mean High Water? Or conversely a depth chart showing depths calculated from Mean Low Water? I would think that charts should be based on the worst case scenario given the situation - both height to the bottom of a bridge and depth to the bottom of the sea/river? I assume that would be Admiralty charts which, I assume, reflect depths and heights using High Actual Tide and Low Actual Tide data. Am I out to lunch on this or what?

I live on an estuary emptying in to the Gulf of Mexico. Every once in a while (not even once a year), a fierce north wind will blow the water out of the estuary. What was 4 or 5 feet becomes just about zero. Surely you wouldn't expect the charts to show the estuary as dry land (there are thousands of boats here).

No, the charts have to use something normal, what you can count on absent unusual conditions. Some authorities may decide to use what is present, I don't know, say, 95% of the time, others, 97%.

[jaybird1111]

From
Tidal Datums - NOAA Tides & Currents

Tidal Datums
In general, a datum is a base elevation used as a reference from which to reckon heights or depths. A tidal datum is a standard elevation defined by a certain phase of the tide. Tidal datums are used as references to measure local water levels and should not be extended into areas having differing oceanographic characteristics without substantiating measurements. In order that they may be recovered when needed, such datums are referenced to fixed points known as bench marks. Tidal datums are also the basis for establishing privately owned land, state owned land, territorial sea, exclusive economic zone, and high seas boundaries. Below are definitions of tidal datums maintained by the Center for Operational Oceanographic Products and Services.

HAT
Highest Astronomical Tide
The elevation of the highest predicted astronomical tide expected to occur at a specific tide station over the National Tidal Datum Epoch.

MHHW*
Mean Higher High Water
The average of the higher high water height of each tidal day observed over the National Tidal Datum Epoch. For stations with shorter series, comparison of simultaneous observations with a control tide station is made in order to derive the equivalent datum of the National Tidal Datum Epoch.

MHW
Mean High Water
The average of all the high water heights observed over the National Tidal Datum Epoch. For stations with shorter series, comparison of simultaneous observations with a control tide station is made in order to derive the equivalent datum of the National Tidal Datum Epoch.

DTL
Diurnal Tide Level
The arithmetic mean of mean higher high water and mean lower low water.

MTL
Mean Tide Level
The arithmetic mean of mean high water and mean low water.

MSL
Mean Sea Level
The arithmetic mean of hourly heights observed over the National Tidal Datum Epoch. Shorter series are specified in the name; e.g. monthly mean sea level and yearly mean sea level.

MLW
Mean Low Water
The average of all the low water heights observed over the National Tidal Datum Epoch. For stations with shorter series, comparison of simultaneous observations with a control tide station is made in order to derive the equivalent datum of the National Tidal Datum Epoch.

MLLW*
Mean Lower Low Water
The average of the lower low water height of each tidal day observed over the National Tidal Datum Epoch. For stations with shorter series, comparison of simultaneous observations with a control tide station is made in order to derive the equivalent datum of the National Tidal Datum Epoch.

LAT
Lowest Astronomical Tide
The elevation of the lowest astronomical predicted tide expected to occur at a specific tide station over the National Tidal Datum Epoch.

GT
Great Diurnal Range
The difference in height between mean higher high water and mean lower low water.

MN
Mean Range of Tide
The difference in height between mean high water and mean low water.

DHQ
Mean Diurnal High Water Inequality
The difference in height of the two high waters of each tidal day for a mixed or semidiurnal tide.

DLQ
Mean Diurnal Low Water Inequality
The difference in height of the two low waters of each tidal day for a mixed or semidiurnal tide.

HWI
Greenwich High Water Interval
The average interval (in hours) between the moon's transit over the Greenwich meridian and the following high water at a location.

LWI
Greenwich Low Water Interval
The average interval (in hours) between the moon's transit over the Greenwich meridian and the following low water at a location.

Station Datum
A fixed base elevation at a tide station to which all water level measurements are referred. The datum is unique to each station and is established at a lower elevation than the water is ever expected to reach. It is referenced to the primary bench mark at the station and is held constant regardless of changes to the water level gauge or tide staff. The datum of tabulation is most often at the zero of the first tide staff installed.

National Tidal Datum Epoch
The specific 19-year period adopted by the National Ocean Service as the official time segment over which tide observations are taken and reduced to obtain mean values (e.g., mean lower low water, etc.) for tidal datums. It is necessary for standardization because of periodic and apparent secular trends in sea level. The present NTDE is 1983 through 2001 and is actively considered for revision every 20-25 years. Tidal datums in certain regions with anomolous sea level changes (Alaska, Gulf of Mexico) are calculated on a Modified 5-Year Epoch.

[Cheechako]

Originally Posted by hpeer

So "low steel" would be measured at the edge of the channel?

I'm probably going down the ditch in the spring. 63' high and 6' 6" low.

You should be good at 63 except for anamolies. I was 6-6 draft also. I did some "plowing" at times in the ditch!

[belizesailor]

Originally Posted by category4jay

Can someone tell me why in the world I would ever use a high tide chart based on Mean High Water? Or conversely a depth chart showing depths calculated from Mean Low Water? I would think that charts should be based on the worst case scenario given the situation - both height to the bottom of a bridge and depth to the bottom of the sea/river? I assume that would be Admiralty charts which, I assume, reflect depths and heights using High Actual Tide and Low Actual Tide data. Am I out to lunch on this or what?

Vertical clearances of bridges are normally charted at MHW to give conservative clearance. Water depths on charts are normally charted at MLW, or even MLLW, for the inverse reason...to give conservative depths. So, no, you would not use or even be likely to find a chart with water depths projected at MHW.

[senormechanico]

Here in the PNW, most charts list vertical clearance from MMHW.
Yesterday we sailed on a reach with the current under the East end of the Hood Canal Bridge which lists clearance at 50 feet.
As the tide was at only + 1.5 I thought "No problem" even though it looked awfully close.
Our mast height with 2 antennas (VHF and 12db wifi) is 53 ft.
Even though we went under the shoreside end do the span which I estimated about 6 feet higher than the end with the sign, as we went under, I became convinced we would hit something for sure.
Thankfully we missed, but the math didn't add up.
Today I called the bridge number and was told the measurement is from ZERO TIDE!

Doing the math, 50+6-1.5=54.5 feet.
There was a lot of wave action as well.
Next time, I'll call for an opening!

[markpierce]

Istanbul skippers have boat/bridge clearance down to a science.

[Paul Eisenberg]

Originally Posted by senormechanico

Here in the PNW, most charts list vertical clearance from MMHW.
Yesterday we sailed on a reach with the current under the East end of the Hood Canal Bridge which lists clearance at 50 feet.
As the tide was at only + 1.5 I thought "No problem" even though it looked awfully close.
Our mast height with 2 antennas (VHF and 12db wifi) is 53 ft.
Even though we went under the shoreside end do the span which I estimated about 6 feet higher than the end with the sign, as we went under, I became convinced we would hit something for sure.
Thankfully we missed, but the math didn't add up.
Today I called the bridge number and was told the measurement is from ZERO TIDE!

Doing the math, 50+6-1.5=54.5 feet.
There was a lot of wave action as well.
Next time, I'll call for an opening!

This is an old thread however this information may help. We live right by the Hood Canal Bridge and keep our boat at Port Ludlow. The charted data is frustrating so I called the WSDOT engineer and also the NOAA guy who does the charting. The NOAA guy told me that the chart shows 50' at any tide because the western end of the east span floats with the tide, and the landward eastern end of the eastern span is fixed. The engineer had looked up the bridge specs and gave me dimensions. I drew the bridge with a CAD program and did a spreadsheet. That information and my calcs suggest that the clearance at the floating end is always 50.7', but at mid span it varies from 50.7' at a +12.00' tide to about 57.2' at a -1.00 tide. It gets very shallow towards the landward end so it is risky to pass too close to shore. The above boat at 53' tall AWL, at a +1.5' tide would have cleared at mid span by about 2.7'. The math is so complicated on this that NOAA decided to just state the clearance as 50' at all tide levels and mid span. We are 49'-6" so we are good to go unless the tide goes above 12.0 when the mid span clearance starts to drop below the 50.7' at the floating end, because the landward end of the span starts to get lower than the floating end. To be safe, if you are more than 50' AWL, or the tide is higher than +12.0 have them open the bridge.