Cruisers Forum
 


Join CruisersForum Today

Reply
 
Thread Tools Rate Thread Display Modes
Old 23-06-2015, 13:34   #31
Senior Cruiser
 
hpeer's Avatar

Cruisers Forum Supporter

Join Date: Dec 2008
Location: Philadelphia
Boat: Murray 33-Chouette & Pape Steelmaid-44-Safara-both steel cutters
Posts: 3,896
Re: Mast Height, Bridge Clearance, Tides.

Quote:
Originally Posted by Tingum View Post
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.
__________________

__________________
hpeer is offline   Reply With Quote
Old 23-06-2015, 13:45   #32
Registered User

Join Date: Oct 2014
Boat: Shopping
Posts: 426
Re: Mast Height, Bridge Clearance, Tides.

Quote:
Originally Posted by category4jay View Post
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%.
__________________

__________________
Cottontop is offline   Reply With Quote
Old 23-06-2015, 15:13   #33
cruiser

Join Date: Jan 2012
Location: Ocean Springs, MS
Posts: 279
Re: Mast Height, Bridge Clearance, Tides.

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.
__________________
jaybird1111 is offline   Reply With Quote
Old 23-06-2015, 15:31   #34
Senior Cruiser
 
Cheechako's Avatar

Cruisers Forum Supporter

Join Date: Aug 2006
Location: Skagit City, WA
Posts: 19,334
Re: Mast Height, Bridge Clearance, Tides.

Quote:
Originally Posted by hpeer View Post
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!
__________________
"I spent most of my money on Booze, Broads and Boats. The rest I wasted" - Elmore Leonard











Cheechako is offline   Reply With Quote
Old 23-06-2015, 18:48   #35
Registered User

Join Date: Dec 2010
Location: W Carib
Boat: Wildcat 35, Hobie 33
Posts: 7,908
Re: Mast Height, Bridge Clearance, Tides.

Quote:
Originally Posted by category4jay View Post
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.
__________________
belizesailor is offline   Reply With Quote
Old 23-06-2015, 19:24   #36
Senior Cruiser
 
senormechanico's Avatar

Cruisers Forum Supporter

Join Date: Aug 2003
Boat: Dragonfly 1000 trimaran
Posts: 5,821
Re: Mast Height, Bridge Clearance, Tides.

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!
__________________
Memento,homo, quia pulvis es, et in pulverem reverteris.
senormechanico is online now   Reply With Quote
Old 23-06-2015, 19:37   #37
Registered User
 
markpierce's Avatar

Join Date: Aug 2010
Location: Central California
Boat: M/V Carquinez Coot
Posts: 3,413
Re: Mast Height, Bridge Clearance, Tides.

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


__________________

__________________
Kar-KEEN-ez Koot
markpierce is online now   Reply With Quote
Reply

Tags
mast

Thread Tools
Display Modes Rate This Thread
Rate This Thread:

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Bridge Clearance on the Intracoastal Waterway 2divers Navigation 33 30-12-2015 18:49
bridge clearance j8502 Seamanship & Boat Handling 11 07-12-2008 22:27
Tombigbee - Mobile Rivers Bridge Clearance Jerry Grass Sailor Logs & Cruising Plans 4 31-08-2008 21:38
Leopard 45, Bridge Deck Clearance isi Multihull Sailboats 4 04-02-2007 22:16
What is bridge clearance ona cat? vilanomark Multihull Sailboats 33 16-03-2006 03:40



Copyright 2002- Social Knowledge, LLC All Rights Reserved.

All times are GMT -7. The time now is 21:53.


Google+
Powered by vBulletin® Version 3.8.8 Beta 1
Copyright ©2000 - 2017, vBulletin Solutions, Inc.
Social Knowledge Networks
Powered by vBulletin® Version 3.8.8 Beta 1
Copyright ©2000 - 2017, vBulletin Solutions, Inc.

ShowCase vBulletin Plugins by Drive Thru Online, Inc.