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Vertical clearance ??
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) __________________________________________________ |
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