Answering the question in your number 3, on US domestic charts* there
is a box labeled "Tidal Information", which shows tide heights at
several important points one the chart. On a random East Coast chart,
it shows MHW, Mean Tide Level, MLW, and Extreme Low Water for each
point, which allows you to figure bridge clearance =
stated clearance above MHW
plus difference between MHW and MLW
less present height of tide referenced to MLW
I put an asterisk on my generalization in the first sentence --
although we own about 600 charts (did a circumnav a while ago), we
don't have any for the US West Coast, so it's just an informed guess
that on West Coast charts the box shows both the depth datum and the
height datum.
Remember, too, that this kind of calculation has a lot of room for
error, particularly with local wind conditions, which can change the
water height by several feet, and with local error -- clearance
numbers aren't always right.
If I were going through a bridge for the first time and was within
three feet of the calculated clearance, and didn't have good local
knowledge available, I'd absolutely send someone up the mast to watch.
This assumes conditions under which you have complete control of the
boat, preferably with a small current against you, as the worst
possible outcome would be to be forced under the bridge and lose the
stick with a person at the top.
An alternative might be to put someone ashore and have him or her
watch from the bridge.
Jim Woodward
www.mvfintry.com
otnmbrd wrote in message ink.net...
First off, I must confess, that out on the West Coast, I've just used
the existing tide tables and bridge clearance numbers and compared the
two to find my clearance and MLW/MLLW be damned, since most of the
bridges that I've passed under, had enough clearance, that it normally
wasn't a concern.
However, a couple things on Jack's post:
1. MHW is used to discuss bridge clearances in the US, unless otherwise
noted (we noted some differences).
2. In dealing with charted depths and tide tables, MLW is the datum for
the East (and I believe Gulf - correction any one?) Coast (with
exceptions), while MLLW is the datum for the West Coast.
3. This causes the problem (and it may or may not be). If your tide
tables are based on MLLW, how do you apply those readings to MLW/MHW
(used for bridge clearance) to get the closest possible reading?
(realizing that all of these readings are subject to error due to many
factors of weather, etc.. )
otn
Jack Dale wrote:
On Fri, 08 Aug 2003 18:07:42 GMT, otnmbrd
wrote:
Chuck Bollinger wrote:
But something bothers me about Mean Tidal Level being half way between
MHW and MLW, especially where there are two diurnal highs and lows.
Can't put my finger on it, but that seems like one of those shortcuts
that can introduce errors. Kind of like those situations where computing
from the results of a computation introduces error. Another thing to
research.
This is part of my problem with this. If we need to find the height of
MHW and our tide datum is based on MLLW, I'm not sure how we can
directly convert with any certainty from the info given.
Also:
Diurnal - Single high and low
Semi Diurnal - two high and low
Mixed - Variations/inequalities in highs and lows .... what we have on
the West Coast, with variations in local
This is one area I've always been weak on, so BG hopefully this old
dog can learn some new tricks.
There is no need to convert anything. They are different
measurements.
(?)
On US charts use MWH to deal with clearances and heights. The
clearance is normally the minimum clearance available under a bridge,
overhead lines, etc.. Use your tide tables to determine if you have
additional clearance.
Understood, however, see above
Mean High Water (MHW): A tidal datum. The average of all the high
water heights observed over the National Tidal Datum Epoch. For
stations with shorter series, simultaneous observational comparisons
are made with a control tide station in order to derive the equivalent
datum of the National Tidal Datum Epoch.
(http://co-ops.nos.noaa.gov/mapfinder/mhw.html)
Again, understood
Use MLLW to deal with depths on US charts. MLLW will normally be the
shallowest that the water will be. Use your tide tables to determine
how much water you have under you on that day at that time. Also
this information will let know how much additional depth you have over
underwater rocks that are a danger to navigation, how much water is
over rocks awash and whether or not drying rocks are visible.
Disagree with using MLLW for all US Charts as the datum, the rest
understood.
Mean Lower Low Water (MLLW): A tidal datum. The average of the lower
low water height of each tidal day observed over the National Tidal
Datum Epoch. For stations with shorter series, simultaneous
observational comparisons are made with a control tide station in
order to derive the equivalent datum of the National Tidal Datum
Epoch. (http://co-ops.nos.noaa.gov/mapfinder/mllw.html)
Understood
Always read the title block to establish datum for clearances and
depths, and ensure that you use the appropriate tide tables. Canadian
datum is based on Lowest Normal Tide, clearances are based on Higher
High Water, Large Tides. For US charts use US tide tables, use
Canadian tide tables for Canadian charts.
BTW - the space between MWH and WLLW on US charts is the green stuff
(foreshore).
A couple of years I attempted to create an online lesson for reading
tide tables
(http://www.acs.ucalgary.ca/~jodale/e...t/content.html).
Jack
__________________________________________________
Jack Dale
Swiftsure Sailing Academy
Director/ISPA and CYA Instructor
http://www.swiftsuresailing.com
Phone: 1 (800) 470-SAIL (toll free)
__________________________________________________