Vertical clearance ??
Steve
I can't seem to find this information in any of my reference books.
What I find in Chapman's, in a section on chart "Plane of Reference", states
"Different planes are on different charts of various boating areas. For
charts along the Atlantic coast the National Ocean Survey uses mean low
water as the datum for soundings. On the Pacific coast it is the mean lower
low water that is used for the reference plane....."
No mention of vertical clearance.
Here in the Pacific NW where tide difference are signicant the chart datum
is not metioned in my Maptech Reg. 15 portfolio (or I just can't find it).
Example:
Today the vertical clearance under two different bridges was just too close
to take a chance on and it was a +8 ft tide.
I ended up playing it safe and had the draw span opened on the Hood Canal
floating bridge and went out of my way to avoid going through the the Port
Townsend Canal with an overhead bridge span.
In each case the "stated" Vertical Clearance was enough for my 54 ft 7 inch
requirement, but I had no idea what tidal state their datum was based on.
(Hood Canal Bridge, east span was 55ft vert. clearance while P.T. Canal
bridge was 58ft.)
Sure hope the 10-20 min traffic delay for some 200 cars and trucks wasn't
due to my cautions and lack of knowledge on this matter..
Please enlighten me.
Steve
s/v Good Intentions
Steve
in the Maptech portfolio, I see that vertical clearance is from MHW (mean
high water), as stated in the title block of each NOAA chart.
It seems Maptech 'washes out' the title block as they copy and crop the
charts for their book pages.
But, that still leaves me with the question: How do you calculate or adjust
the stated Vertical Clearance when all you have is a chart and a tide
table.
In my example; The charts show a vertical clearance of Hood Canal Bridge,
east span, as 55 ft and the tide table shows a height of 8 ft at the time I
want to pass under it. My mast height is 54' 7", including the VHF
antenna...
Do I need the NOAA tide tables or can this be calculated from the usual,
locally reproduced, convience tables??
I realize, after the fact, that there would have been sufficient clearance
yesterday, but there may well come a day when I arrive at the bridge at
something slightly higher than MHW.
Steve
s/v Good Intentions
Dave Skolnick
> Were is vertical clearance measured from (what datum)?
Should be Mean High Water.
>
> I can't seem to find this information in any of my reference books.
Could have sworn it was in Chapman's, but my copy is on my bedside table
and my wife is still asleep.
Chart 1 (http://www.carolinaglobalmaps.com/products/nautical/chart1.htm)
says: the shoreline is "usually the mean high water line" (p 4) and
heights of landmarks and structures are "referred to the shoreline plane
of reference" (p8).
regards,
dave
M/V Auspicious
"a sailor fallen from grace"
Dave Skolnick
> But, that still leaves me with the question: How do you calculate or adjust
> the stated Vertical Clearance when all you have is a chart and a tide
> table.
>
> In my example; The charts show a vertical clearance of Hood Canal Bridge,
> east span, as 55 ft and the tide table shows a height of 8 ft at the time I
> want to pass under it. My mast height is 54' 7", including the VHF
> antenna...
You have to know MHW. If it isn't on your tide table, I'd ask the bridge
tender. If he doesn't know, I'd ask him to open up.
Jeff Morris
http://pollux.nss.nima.mil/NAV_PUBS/APN/Chapt-09.pdf
Yes, you do have to be careful. Astronomical predictions often show tides a foot or two
high than MHW in some locations. Also, a storm or even a persistent wind can do the same.
And both can occur at the same time.
And, although most bridges are actually a few inches higher than listed, some are actually
lower, or have gear hanging down.
"Steve" <est...@hctc.com> wrote in message news:vj4dvg9...@corp.supernews.com...
Ron Thornton
Does this bridge not have a water height scale? I thought all bridges
on navigable water had them.
Ron
Jim Woodward
significant exception is small scale charts where the datum may be
Mean Sea Level. Such charts rarely show bridge clearances, so the
difference isn't an issue.
The exact datum varies from country to country. On US domestic charts,
it's always Mean High Water (the average of all high tides). On
foreign charts, including US charts made from foreign data, it's
usually a higher datum, Mean Higher High Water (the average of the
higher of the two high tides each day), Mean High Water Springs (the
average of all spring tides), Highest Astronomical Tide (the highest
the water ever gets without help from a storm) or maybe something
else.
May I suggest that you buy a copy of Bowditch (The American Practical
Navigator -- NIMA Pub 9) which will answer this question and every
other question you ever had about navigation? Bowditch just had its
200th anniversary of being continuously in print, through about 75
editions. It's the standard source of basic information on all kinds
of navigation. There's no need to buy it new -- any edition from the
last forty years will do fine. I might add, that as a serious
navigator, I own two copies of Bowditch (1962 and 1977) and don't own
Chapman.
Jim Woodward
www.mvfintry.com
"Steve" <est...@hctc.com> wrote in message news:<vj3o7js...@corp.supernews.com>...
Jeff Morris
are online at:
http://pollux.nss.nima.mil/pubs/pubs_j_apn_sections.html?rid=187
There are two versions of the latest Bowditch available in hard copy - the one from the
government includes a CD which has a wrapper program to do some of the navigation for you.
It also has a calculator and a few other features. I don't know if the "commercial"
version - roughly the same price, also has that.
Old editions of Bowditch are fun - I have several, including a first and one from 1870 - I
like to see what the practices and terminology was back then.
"Jim Woodward" <jameslw...@attbi.com> wrote in message
news:b3a8520c.03080...@posting.google.com...
Steve
(seat of the pants) since he is about a mile from the fixed span I was
interested in passing under.
There is a clearance scale on the fixed span but according to my cruising
guide, it has some error (on the conservative side), several feet less than
the vert. clearance on the charts.
Even if I could trust these scale markers, I wouldn't hate to get in close
enough to read them only to find there wasn't enough clearance, with several
knot of current running.. Bad situation for a sail boat, trying to come
about with a deep draft.
Many of these scales that I have seen in New England, were badly faded or
covered with marine growth/bird droppings, etc.
I'm going to search out a Gov. tide table and see what the charted datum is
for both of these bridges.
Thanks again for the good response on this question. A good exercise.
40+ years of boating but not to proud to ask the dumb questions.
Steve
s/v Good Intentions
otnmbrd
the clearance based on the stage of the tide at the time you will pass
under.
The tables or a program will give you the mean tide ranges (or in the
case of a tide program it will frequently tell you MHW) from which you
can calculate YOUR height above or below MHW for the particular time or
stage of the tide, in question.
If the bridge clearance is 55', and you find that MHW for a particular
bridge ( or nearest datum point)is 5' and you will be passing under the
bridge at a time when the tide is at +2', you (technically <EG>) should
have 58' of clearance ..... or if you are in some extreme tides and the
tide at the particular time will be +8', you will only have 52' of
clearance.
Always approach these numbers with caution and check any local "tide
gauge" on a bridge pier when available.
Rodney Myrvaagnes
>Were is vertical clearance measured from (what datum)?
>
>I can't seem to find this information in any of my reference books.
I am quite sure Chart 1 says bridge clearance is from mean high water.
Rodney Myrvaagnes NYC J36 Gjo/a
"Accordions don't play 'Lady of Spain.' People play 'Lady of Spain."
Paul L
Paul L
"Steve" <est...@hctc.com> wrote in message
news:vj4dvg9...@corp.supernews.com...
Paul L
Paul L
"Steve" <est...@hctc.com> wrote in message
news:vj4dvg9...@corp.supernews.com...
Paul L
Paul L
"Steve" <est...@hctc.com> wrote in message
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otnmbrd
You need to check your chart or Particular Coast Pilot, etc., if you
have any doubts ..... For instance, I believe the Mississippi river,
above a point will use "river stage".
If in doubt, <G> use the one that gives you the least clearance.
otn
Bill
Bridge heights are measured from Mean High Water - although I can't
remember where I saw the reference...
Bill
"Steve" <est...@hctc.com> wrote in message news:<vj3o7js...@corp.supernews.com>...
Chuck Bollinger
> Were is vertical clearance measured from (what datum)?
The vertical clearance is measured from 'Mean High Water' in places with a
single diurnal tide. In the Pacific Northwest, it is measured from 'Mean Higher
High Water' (MHHW).
The tide tables provide 'Mean Tide' for each subordinate station, and 'Mean
Range'. You can figure out what MHHW is for a place (within a foot or so) by
taking 'Mean Tide level' and adding one-half the Mean Tidal Range. When I do
that, I come within a foot of the MHHW given in my 'Tides and Currents Pro'
program, and the error results in a number less (more safe) than the listed one.
The vertical clearance of the east span of Hood Canal Bridge is 55 ft.
Yesterday the highest tide was 8.8 feet (at 1500). The Mean Tide for Port
Gamble is listed as: 6.10 ft. The Mean Range is listed as: 6.70. My
computation results in a figure of 9.45 feet for the MHHW.
Therefore, at the high tide (daylight hours) there was 55.6 feet under the east
span high rise. Of course, prudence leaves 2' of wiggle room to make up for
unplanned things like wind effect and such, or some dot.com yahoo with more boat
than brains zipping through at 55. But if your boat height over water is 53' or
less, well, just don't go on rec.auto.vacationing and mention it. ;=)
This information comes from Chapman, but also from U.S. Power Squadron courses
in Piloting and Advanced Piloting. Not a bad investment in time and a few bucks.
otnmbrd
Chuck Bollinger wrote:
> Steve wrote:
>
>> Were is vertical clearance measured from (what datum)?
>
>
> The vertical clearance is measured from 'Mean High Water' in places with a
> single diurnal tide. In the Pacific Northwest, it is measured from
> 'Mean Higher
> High Water' (MHHW).
Interesting ..... Can you show me where this information comes from? In
looking at CP 7, it list all heights as above MHW, unless otherwise
stated, and the only major change to this I can find is for the Columbia
River, which uses MLLW below Harrington Point, and "Columbia River
Datum", between there and Bonneville Dam.
>
> The tide tables provide 'Mean Tide' for each subordinate station, and 'Mean
> Range'. You can figure out what MHHW is for a place (within a foot or
> so) by
> taking 'Mean Tide level' and adding one-half the Mean Tidal Range. When
> I do that, I come within a foot of the MHHW given in my 'Tides and
> Currents Pro' program, and the error results in a number less (more
> safe) than the listed one.
On the right track, but be careful that the meaning of "mean Tide" and
"mean range", given in the program and tables you are using, mean <G>
what you want. Many define "Mean tide" as "the level half way between
mean high water and mean low water" and "mean range" as "the difference
in height between MHW and MLW" .... see the problem?
My particular tide program, gives me MHHW and "Mean Tide". In this case,
I would take the "mean Tide" X 2 and apply it to Zero tide (MLLW) and
use this as MHW .... It should, normally, give a built in safety factor.
At any rate, as I said before, be careful. There are many variables
which can come into play, and you should NEVER push the envelope too
closely.
BTW, I think the program you are using gives MHHW
otn
C. A. La Varre
> Were is vertical clearance measured from (what datum)?
In the European canal system it is measured from PHEN, which is a French
acronym for "the highest navigable waters"(les plus hautes eaux
navigables). When the water gets higher than that they cancel all
navigation.
So a posted vertical height is always the worst case.
Cheers, Andy
Paul L
say MHW. It seems like MHHW would make more sense out here.
(sorry for the multiple posts earlier - Outrage acting up).
Paul
"otnmbrd" <otn...@earthlink.net> wrote in message
news:hVwYa.573$Nf3...@newsread4.news.pas.earthlink.net...
Chuck Bollinger
>
>
> Chuck Bollinger wrote:
>
>> Steve wrote:
>>
>>> Were is vertical clearance measured from (what datum)?
>>
>>
>>
>> The vertical clearance is measured from 'Mean High Water' in places
>> with a
>> single diurnal tide. In the Pacific Northwest, it is measured from
>> 'Mean Higher
>> High Water' (MHHW).
>
>
> Interesting ..... Can you show me where this information comes from? In
> looking at CP 7, it list all heights as above MHW, unless otherwise
> stated, and the only major change to this I can find is for the Columbia
> River, which uses MLLW below Harrington Point, and "Columbia River
> Datum", between there and Bonneville Dam.
>
We're just going out and tomorrow going to Port Ludlow. It will be Monday
evening before I'll be on the internet again. Working on it.
>>
>> The tide tables provide 'Mean Tide' for each subordinate station, and
>> 'Mean
>> Range'. You can figure out what MHHW is for a place (within a foot or
>> so) by
>> taking 'Mean Tide level' and adding one-half the Mean Tidal Range.
>> When I do that, I come within a foot of the MHHW given in my 'Tides
>> and Currents Pro' program, and the error results in a number less
>> (more safe) than the listed one.
>
>
> On the right track, but be careful that the meaning of "mean Tide" and
> "mean range", given in the program and tables you are using, mean <G>
> what you want. Many define "Mean tide" as "the level half way between
> mean high water and mean low water" and "mean range" as "the difference
> in height between MHW and MLW" .... see the problem?
Frankly, no. One is a tide level and the other is a range.
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.
> My particular tide program, gives me MHHW and "Mean Tide". In this case,
> I would take the "mean Tide" X 2 and apply it to Zero tide (MLLW) and
> use this as MHW .... It should, normally, give a built in safety factor.
> At any rate, as I said before, be careful. There are many variables
> which can come into play, and you should NEVER push the envelope too
> closely.
Hmm. More later on that. Literally have to go.
>
> BTW, I think the program you are using gives MHHW
Yes.
>
>
> otn
>
otnmbrd
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.
otn
Jack Dale
wrote:
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.
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)
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.
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)
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/eder673/first/content.html).
Jack
__________________________________________________
Jack Dale
Swiftsure Sailing Academy
Director/ISPA and CYA Instructor
http://www.swiftsuresailing.com
Phone: 1 (800) 470-SAIL (toll free)
__________________________________________________
Jeff Morris
> Use MLLW to deal with depths on US charts. MLLW will normally be the
> shallowest that the water will be.
Since this is the average of the lowest tide for each day, roughly half the days will have
a lower tide. In some locations this might not be significant, but in Boston, for
instance, there are several days every month that are more than a foot below MLLW. There
are several days a year that are two feet or more below MLLW.
"Jack Dale" <jack...@nospam.telus.net> wrote in message
news:fc08jv0v8rr0u209j...@4ax.com...
Jack Dale
<jeffmo@NoSpam-sv-lokiDOTcom> wrote:
>Most of this is correct. However, I would take issue with the statement
>
>> Use MLLW to deal with depths on US charts. MLLW will normally be the
>> shallowest that the water will be.
>
>Since this is the average of the lowest tide for each day, roughly half the days will have
>a lower tide. In some locations this might not be significant, but in Boston, for
>instance, there are several days every month that are more than a foot below MLLW. There
>are several days a year that are two feet or more below MLLW.
>
Agreed - I was rather loose with the term "normally".
I am more used to Canadian tide tables and charts which use a
different chart datum. There can be a 1.5 meter difference. In
Volume 5 of Canadian Tide and Current Tables, Fulford Harbour in the
Gulf Islands of British Columbia shows only 7 days in 2003 with
negative tides ( 6 of 0.3 feet, 1 of 0.7 feet). Seattle and Port
Townsend, both in Washington, show significantly more because of a
different chart datum.
otnmbrd
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 <otn...@earthlink.net>
> 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
otnmbrd
Low water datum"
Also, just because you are using a particular datum which states that
your high might be, say 6.0', there is nothing that says you will not
get higher highs and lower lows, than this, at varying times.
Jeff Morris
says that the "US Datum" for tides is MLLW, and then all of its US tables list "US Datum."
It goes on to say the Canadian tides and charts use Lowest Normal Tides (LNT), which is
significantly different from the US, and is usually synonymous with Lowest Low Water,
Large Tides (LLWLT) - the average of the lowest water from each of the 19 years of
reference. Got that? There will be a quiz on Monday.
And yes, everyone should be aware that local weather conditions can add or subtract
several feet to the height of the tide.
"otnmbrd" <otn...@earthlink.net> wrote in message
news:5XWYa.1273$7z1...@newsread3.news.pas.earthlink.net...
otnmbrd
relates it as MLW (with an exception for Boston) (also the reason I
asked for confirmation).
My particular tide program does not have the East Coast, but again some
older stuff I was looking at did, and it used MLW for tide datum.
At any rate, you need to be sure which datum (as I think I've been
stressing, or at least should have) you are using, but it still doesn't
clear up the situation of how you change MLLW to MLW or MHW, from the
data given in the particular tide tables, which I think is the jist of
the discussion, when looking at bridge clearance, and I think Chuck was
having the same problem (although he came up with a workable solution),
that I do (although I have basically said "the hell with it" use what
you've got and be sure you have a built in safety factor).
otn
otnmbrd
Boston Harbor.
What is the "datum" being used for bridge clearance? MHW? MHHW?
I'm trying to get us all on the same reference plain.
otn
Jim Woodward
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 <otn...@earthlink.net> wrote in message news:<RRWYa.1271$7z1...@newsread3.news.pas.earthlink.net>...
Jeff Morris
chart 13270 of Boston harbor there is a table that lists various heights for "Boston
Light":
Height referred to datum of soundings (MLLW)
Mean Higher High Water 9.7 feet
Mean High Water 9.3 feet
Mean Low Water 0.3 feet
Extreme Low Water -3.0 feet
elsewhere it says:
HEIGHT
Heights in feet above Mean High Water
The tables that convert between the various heights is on a number of charts in my BBS
ChartKit, but the comment on bridge heights I couldn't find without going to an actual
chart.
I also have the same chart from 1867. It lists the minimum and maximum observed tides
from the "reference plane," plus the mean spring and neap low tides from the reference
plane, plus the mean range of the spring and neap tides. It doesn't list what the
"reference plane" is, nor does it have any bridge heights. It does have the "Corrected
Establishment" for determining the state of the tide relative to the full moon, and lists
longitude relative to the State House on Beacon Hill.
"otnmbrd" <otn...@earthlink.net> wrote in message
news:zJZYa.1161$Nf3...@newsread4.news.pas.earthlink.net...
otnmbrd
with older publications and not having any charts here, which gave me
that info..... LOL shoulda known better.
At any rate, now we have a way to get MHW and can go back to an earlier
post for determining clearance, based on using the tide tables.
I haven't got one here .... anyone check a "Light List" for the info
Jeff has given? (I'm asking, to see if it's there, also).
otn
otnmbrd
bad info, let me try an example to hopefully clear up the mess.
Your tide datum is MLLW (also called Zero tide)
On your chart you have a box showing tidal data.
In this box, MHW is listed as +6.0'; MHHW is listed at +7.0'.
You are on a sailboat approaching a bridge. The height at your present
draft to the tip of an antenna at the masthead, is 50'. The clearance of
the bridge shown on the chart, is 50' at MHW.
AT Zero tide (MLLW), you will have 6' clearance between your antenna and
the bridge. At +6.0' tide (MHW), you will hear a slight "Ping" as the
tip of your antenna touches the bridge. At any stage of the tide which
your tables show to be greater than this (+7.0,8.0,9.0, etc.) your next
stop will be the nearest boatyard for repairs.
At any stage of the tide where the numbers show a minus reading (below
MLLW) you will gain additional clearance.
All warnings about accuracy of tables and predictions based on possible
weather, etc. conditions, still apply.
<BG>
otn
PS got my hands on a light list .....no tidal info in there for
particular lights, that I could see.
Jeff Morris
several other points that are not lights and I just used Boston Light as a well known
reference point in Boston. I could have picked the Chelsea Bridge, but few people would
have recognized it. Many of the charts in my ChartKit have similar tables, listing 3 or
4 reference points.
"otnmbrd" <otn...@earthlink.net> wrote in message
news:tqaZa.2011$Nf3...@newsread4.news.pas.earthlink.net...
Jack Dale
wrote:
>
>PS got my hands on a light list .....no tidal info in there for
>particular lights, that I could see.
The height of the light is from MHW on US charts. That is about all
the tidal information in that publication.
If you use the "dipping the horizon" technique for establishing a
circle of position, the height of the tide at the time of the
observation must be taken into consideration.
Jack
otnmbrd
multiple listings, do they relate to a tidal reference station, in some way?
otn
otnmbrd
navigation.
otn
Steve
I think the most important thing I have learned from this exercise is I
should dig out the actual NOAA charts to read all of this information..
No excuse, but this was a singlehanded, day trip (most all of my boating is
singlehanded). I was using a new Maptech Chartkit for convenience in the
cockpit and The Capt'n Voyager with Maptech BSB digital charts below on the
chart table. I can watch the chart and progress on the laptop from the hatch
but can't get away from the helm long enough to research this information on
the BSB charts.. It turns out that the answer to my question was on the BSB
charts and a paper chart that i had in my chart.drawer. However the Chartkit
didn't have it, or at least I could never find it.
In retrospect, in view of with all of the info here, I feel I was correct in
going through the draw span of the Hood Canal bridge, Too close to chance
on the East fixed span. However, with an additional 3 ft clearance on the
Port Townsend Canal span, I could have comfortablely cleard it and save
about 4 hours going around the Indian and Marrostone Islands.
Thanks again for all the good information.
Steve
s/v Good Intentions
Chuck Bollinger
otnmbrd wrote:
>
> Chuck Bollinger wrote:
>
>> The vertical clearance is measured from 'Mean High Water' in places
>> with a
>> single diurnal tide. In the Pacific Northwest, it is measured from
>> 'Mean Higher
>> High Water' (MHHW).
>
>
> Interesting ..... Can you show me where this information comes from? In
> looking at CP 7, it list all heights as above MHW, unless otherwise
> stated, and the only major change to this I can find is for the Columbia
> River, which uses MLLW below Harrington Point, and "Columbia River
> Datum", between there and Bonneville Dam.
>
And you are absolutely right. I cannot find anywhere that says that MHHW is
used in areas of semidiurnal tide. And yet, 4 out of 5 quite well educated
mariners swear up and down that they were told, or read, that information, but,
like me, cannot actually come up with the info.
That's great, and I appreciate your head check. Bullshit needs to be stomped,
no matter how well-intentioned.
I can't see why my tide program give me MHHW which isn't really very useful. To
use it risks being off by about a foot or so, in the wrong direction. But Mean
Tide and 1/2 the Mean Range isn't rocket science, so I'll continue in that
direction.
I see the conversation is raging away. Some pretty sharp people involved. I'll
take my discredited ass and go sit on the sidelines. {:-D