Right, but those are not entered on a chart are they?

Cheers MC

Donal wrote:

"Scott Vernon" wrote in message
...

Yes, those of us with a functioning brain knew that. I believe that
charted depths are given at MLW and it's up to the sailor to know the tide
table and apply it. Maybe that's why that site gave it that way?

Scotty


Around here, heights are given above chart datum, which is traditionally the
same as LAT. This can vary from one port to another. Today, at Low Water,
Portsmouth will have 2m (6'6") above CD.

http://www.portsmouth.co.uk/ViewArti...rticleID=55658
2


Also, we have MLWS, and MWLN but we don't have a plain MLW.


Regards


Donal
--

"The_navigator©" wrote in message
...
Right, but those are not entered on a chart are they?


MHWS & HLWS? No, they are not mentioned on the chart. I've just checked
one of my charts, and it explains that CD is "about" LAT - but, it doesn't
say where??? I wonder if that means that there are multiple CD's for a
chart that covers a 50 mile stretch of shoreline?

On the back of the chart, they have printed all sorts of useful information,
including definitions of MHWS, MHWN and MHW!!!

I don't remember learning about MHW, and I cannot think of any practical use
for the figure.

Perhaps Bob can explain what use one could make of the MHW figure??

Regards

Donal
--




Cheers MC

Donal wrote:

"Scott Vernon" wrote in message
...

Yes, those of us with a functioning brain knew that. I believe that
charted depths are given at MLW and it's up to the sailor to know the
tide
table and apply it. Maybe that's why that site gave it that way?

Scotty


Around here, heights are given above chart datum, which is traditionally
the
same as LAT. This can vary from one port to another. Today, at Low
Water,
Portsmouth will have 2m (6'6") above CD.


http://www.portsmouth.co.uk/ViewArti...rticleID=55658
2


Also, we have MLWS, and MWLN but we don't have a plain MLW.


Regards


Donal
--

MHW is very important on US charts. RB might hurt himself trying to figure out why -
should we give him a hint?

As to your other question, yes the Chart Datum for Soundings is not absolute, it is
relative to the local tide. Usually not a factor, but if you're looking at a chart of the
Cape Cod Canal you have rather different tide domains at either end - a 10 foot range on
the CC Bay side, about 3 feet in Buzzard's Bay - hence a rather severe current.

-jeff
"There is a tide in the affairs of men / Which, taken at the flood, leads on to
fortune." - William Shakespeare


"Donal" wrote in message
...

"The_navigator©" wrote in message
...
Right, but those are not entered on a chart are they?


MHWS & HLWS? No, they are not mentioned on the chart. I've just checked
one of my charts, and it explains that CD is "about" LAT - but, it doesn't
say where??? I wonder if that means that there are multiple CD's for a
chart that covers a 50 mile stretch of shoreline?

On the back of the chart, they have printed all sorts of useful information,
including definitions of MHWS, MHWN and MHW!!!

I don't remember learning about MHW, and I cannot think of any practical use
for the figure.

Perhaps Bob can explain what use one could make of the MHW figure??

Regards

Donal
--




Cheers MC

Donal wrote:

"Scott Vernon" wrote in message
...

Yes, those of us with a functioning brain knew that. I believe that
charted depths are given at MLW and it's up to the sailor to know the
tide
table and apply it. Maybe that's why that site gave it that way?

Scotty


Around here, heights are given above chart datum, which is traditionally
the
same as LAT. This can vary from one port to another. Today, at Low
Water,
Portsmouth will have 2m (6'6") above CD.


http://www.portsmouth.co.uk/ViewArti...rticleID=55658
2


Also, we have MLWS, and MWLN but we don't have a plain MLW.


Regards


Donal
--

Heights on land and clearances are above MWHS -right?

Cheers MC

We don't often use "spring tides" here in the States - people like RB would be saying they
only sail in Summer.

We just use MHW - that leads to excitement since the bridges will likely be lower than
their reported clearances once a day!

That reminds me - Happy Spring!


"The_navigator©" wrote in message
...
Heights on land and clearances are above MWHS -right?

Cheers MC

Hmmm. That's a worry. Might mean you'll need tide tables to decide if
you can pass an overhead...

Cheers MC

Jeff Morris wrote:

We don't often use "spring tides" here in the States - people like RB would be saying they
only sail in Summer.

We just use MHW - that leads to excitement since the bridges will likely be lower than
their reported clearances once a day!

That reminds me - Happy Spring!


"The_navigator©" wrote in message
...

Heights on land and clearances are above MWHS -right?

Cheers MC

Yes - In case anyone is following this, let me explain:

Here in the States we use Mean High Water (MHW) as the datum for bridge clearances. This
means that, on average, half the tides will be higher than MHW, so the bridge will be
lower than expected. To make matters worse, with the common semi-diurnal tide, one tide
(the "higher high") will often be significantly higher than the other, so the discrepancy
can be large.

From chart 13270 of Boston harbor there is a table:

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

Thus, the average "higher high" is 0.4 feet above the datum used for bridge clearance.
However, today the Higher High is at 10.22 feet. almost a foot higher than the datum.
Anyone thinking they have a foot of clearance could be in for a nasty surprise!

I should add that a strong onshore breeze can often add an extra foot to the predicted
tides.

-jeff



"The_navigator©" wrote in message
...
Hmmm. That's a worry. Might mean you'll need tide tables to decide if
you can pass an overhead...

Cheers MC

Jeff Morris wrote:

We don't often use "spring tides" here in the States - people like RB would be saying
they
only sail in Summer.

We just use MHW - that leads to excitement since the bridges will likely be lower than
their reported clearances once a day!

That reminds me - Happy Spring!


"The_navigator©" wrote in message
...

Heights on land and clearances are above MWHS -right?

Cheers MC

Yup, here on the Bay, a strong South wind combined with high tide can make
for higher than MHHW.

SV


"Jeff Morris" jeffmo@NoSpam-sv-lokiDOTcom wrote in message
...
Yes - In case anyone is following this, let me explain:

Here in the States we use Mean High Water (MHW) as the datum for bridge
clearances. This
means that, on average, half the tides will be higher than MHW, so the
bridge will be
lower than expected. To make matters worse, with the common semi-diurnal
tide, one tide
(the "higher high") will often be significantly higher than the other, so
the discrepancy
can be large.

From chart 13270 of Boston harbor there is a table:

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

Thus, the average "higher high" is 0.4 feet above the datum used for
bridge clearance.
However, today the Higher High is at 10.22 feet. almost a foot higher than
the datum.
Anyone thinking they have a foot of clearance could be in for a nasty
surprise!

I should add that a strong onshore breeze can often add an extra foot to
the predicted
tides.

-jeff



"The_navigator©" wrote in message
...
Hmmm. That's a worry. Might mean you'll need tide tables to decide if
you can pass an overhead...

Cheers MC

Jeff Morris wrote:

We don't often use "spring tides" here in the States - people like RB
would be saying
they
only sail in Summer.

We just use MHW - that leads to excitement since the bridges will
likely be lower than
their reported clearances once a day!

That reminds me - Happy Spring!


"The_navigator©" wrote in message
...

Heights on land and clearances are above MWHS -right?

Cheers MC

Well, DUH!

"The_navigator©" wrote ...
Hmmm. That's a worry. Might mean you'll need tide tables to decide if
you can pass an overhead...

Well put, Scotty.

But MC does have a point - under the US system you have to check to see if the current
tide is much higher than MHW; in other countries where the datum for bridge clearance is
the Spring Tide level, or the Large Tides, its a bit easier.

However the best thing to do is to read the gauge board on the bridge and ask the
tender.Can


"Scott Vernon" wrote in message
...
Well, DUH!

"The_navigator©" wrote ...
Hmmm. That's a worry. Might mean you'll need tide tables to decide if
you can pass an overhead...