I just looked up Bowditch because I was surprised at the generality you
implied. Bowditch is clearly talking about hydraulic current which
_adds_ to the tidal current. The overall effect is as I described in my
other response.


Cheers



Jeff Morris wrote:

Then you're disagreeing with Bowditch:

"A slight departure from the sine form is exhibited by
the reversing current in a strait, such as East River, New
York, that connects two tidal basins. The tides at the two
ends of a strait are seldom in phase or equal in range, and
the current, called hydraulic current, is generated largely
by the continuously changing difference in height of water
at the two ends. The speed of a hydraulic current varies
nearly as the square root of the difference in height. The
speed reaches a maximum more quickly and remains at
strength for a longer period than shown in Figure 914b, and
the period of weak current near the time of slack is considerably
shortened."


"Nav" wrote in message
...

That sounds like an estimate based on an energy argument. However, I
don't think that case could be considered to be a closed system so such
energy balance need not apply.

Cheers

Jeff Morris wrote:


"Thom Stewart" wrote in message
...


Scott,

You've seem to have forgotten "The Gulf Stream Master" Jax. I'm sure he
can help you ride the eddies:^) Maybe even find you a stray current
heading south (g)


Jax also insisted that the time of high water and slack current must be the
same, to be otherwise would violate the laws of physics (on his planet).






Now, for what's it worth. The Tide Flow is a sine wave. Us common folk
understand that slack water is at the top and bottom of the wave. So
slack is, roughly about a hour and it is roughly in the upper 30%
counting the end of the rising tide and the beginning of the Ebb. 15% to
the HW and 15% roughly to the Max Flow. That means about two hours to
max flow, 8 hours of max flow and then another couple of hour of
diminishing flow to LW.


While the current flow in "simple harbors" may follow a sine curve, in cuts
between two bodies of water the flow is different. The curve is more
"squarish," that is, the flow ramps up quicker and stays high longer. The
duration of slack is reduced. The best example is Hell Gate in New York.

IIRC, the flow rate is proportional to the square root of the different of
height of the two bodies.

"Nav" wrote in message
...
Bowditch? How about Google?

Did you find anything?



This is basic seamanship on tides. The most likely time of max
stream is
when tide height changes most rapidly.


Well, duh! but when is the fastest heighth change?

Scooter


Beers

Scott Vernon wrote:
"Nav" wrote in message
...

Bowditch? How about Google?


Did you find anything?



This is basic seamanship on tides. The most likely time of max

stream is

when tide height changes most rapidly.



Well, duh! but when is the fastest heighth change?


It depends on the type of tide. Many tides do not have simple sine wave
forms. Dougs 7/12ths was his typical BS. But one can make the generality
that in the open sea and at the end of estuaries it is most likely to be
at half tide -OK?

Cheers

Nav wrote:
Tide tables are hardly rocket science!



Well actually they are. Mankind does not yet posses the full knowledge
to produce exact tide predictions.

Have a look at:

Tidal Science - 1996, proceedings of a symposium held in London, 21-22
October 1996. Progress in Oceanography, Elsevier Press. 'Special
issue', vol. 40, Issue 1-4, 1998

This should give an overview of the outlook for tidal science just
before the start of the new millennium.

Peter S/Y Anicula



"Nav" skrev i en meddelelse
...
You should complain to the hydrographic offic. Tide tables are
hardly
rocket science!

Cheers

DSK wrote:


One of the most frustrating things around here is that the tide
tables
are usually in error by at least half an hour. The tidal current
in
Snow's Cut (for example) is very strong, and so far I have yet to
see it
run on schedule. It's annoying after planning a departure at an
inconvenient time so as to arrive at slack, only to find the
current
running like a champ with no sign of slacking.

Fresh Breezes- Doug King

Nav wrote:
Tide tables are hardly rocket science!



Well actually they are. Mankind does not yet posses the full knowledge
to produce exact tide predictions.

Have a look at:

Tidal Science - 1996, proceedings of a symposium held in London, 21-22
October 1996. Progress in Oceanography, Elsevier Press. 'Special
issue', vol. 40, Issue 1-4, 1998

This should give an overview of the outlook for tidal science just
before the start of the new millennium.

Peter S/Y Anicula



"Nav" skrev i en meddelelse
...
You should complain to the hydrographic offic. Tide tables are
hardly
rocket science!

Cheers

DSK wrote:


One of the most frustrating things around here is that the tide
tables
are usually in error by at least half an hour. The tidal current
in
Snow's Cut (for example) is very strong, and so far I have yet to
see it
run on schedule. It's annoying after planning a departure at an
inconvenient time so as to arrive at slack, only to find the
current
running like a champ with no sign of slacking.

Fresh Breezes- Doug King

Nav wrote:
What a sad little man you are.

???


So bound up in jealousy.

???

What am I supposed to be jealous of? Your superior knowledge about the
USS Constitution?

DSK

Thom Stewart wrote:
Doug,

Pardon me if I sound argumentative. I'm not trying to be.

What are you doing, *not* arguing? Wanna get kicked out of the club??


7/12 isn't the way I figure the Tides. I use the RMS and the 12 hour
cycle. I round the 707 to 70%. This leaves 30% for the curve at the top
and bottom and since we are only thinking of a 1/2 sine wave 15% on the
top and 15% on the bottom. So, .15x12=1.8 hours from the middle of
Slack Tide, 8.4 hours max flow, 1.8 to Slack again (1.8+8.4+1.8=12 hour
Tide Run)

That's a bit more complicated but still simple enough to do in your
head. A good method.


That leaves only (g) the problem of Mid Slack.This I think is where we
start to lay the blame on the Tide Tables. There can be HW by sight for
at least an hour. I really don't know how to find this mid point and an
hour difference at a inlet can be the difference of passage or no.

Up there, heck yeah. I was very intimidated by tides, currents, and
rocks, the times we chartered in the PacNW. Fortunately it's a lot
easier around here. You might find the southeast coast boring.


Allow time and be early. I let my boat "Lady Bug" until the right time.

I like GF's explanation, but I still don't know what that means.

Fresh Breezes- Doug King

Holy Backpedal, Nav!

Of course the total current is the sum of a variety of effects. However, these
are certain places where the hydraulic current dominates the flow. Snow's Cut,
I suspect, is one. There are others, and they tend to be very significant for
sailors.

Most simple tidal currents around here max at under 2 knots, while the
hydraulics are frequently over 4, sometimes 6 or more. For simple currents, the
"50-90" rule, which approximates a sine curve, can be used. This says that the
current reaches half strength in the first third between slack and max, and 90%
in the second third.

However, when a hydraulic current dominates, the current changes very quickly,
often with virtually no slack. Cape Cod Canal this morning went from a .9 knot
ebb to a 1 knot flood in 10 minutes. It went from 50% ebb to 50% flood strength
in less than an hour, or twice as fast as a sine wave would predict. Hell Gate
changes even faster than that.

This is a very significant effect for the sailor, who often wishes to transit
such areas at slack. Perhaps they are not common where you are, Nav, but I'm
rather surprised you heard of it.

jeff





"Nav" wrote in message
...
I just looked up Bowditch because I was surprised at the generality you
implied. Bowditch is clearly talking about hydraulic current which
_adds_ to the tidal current. The overall effect is as I described in my
other response.


Cheers



Jeff Morris wrote:

Then you're disagreeing with Bowditch:

"A slight departure from the sine form is exhibited by
the reversing current in a strait, such as East River, New
York, that connects two tidal basins. The tides at the two
ends of a strait are seldom in phase or equal in range, and
the current, called hydraulic current, is generated largely
by the continuously changing difference in height of water
at the two ends. The speed of a hydraulic current varies
nearly as the square root of the difference in height. The
speed reaches a maximum more quickly and remains at
strength for a longer period than shown in Figure 914b, and
the period of weak current near the time of slack is considerably
shortened."


"Nav" wrote in message
...

That sounds like an estimate based on an energy argument. However, I
don't think that case could be considered to be a closed system so such
energy balance need not apply.

Cheers

Jeff Morris wrote:


"Thom Stewart" wrote in message
...


Scott,

You've seem to have forgotten "The Gulf Stream Master" Jax. I'm sure he
can help you ride the eddies:^) Maybe even find you a stray current
heading south (g)


Jax also insisted that the time of high water and slack current must be the
same, to be otherwise would violate the laws of physics (on his planet).






Now, for what's it worth. The Tide Flow is a sine wave. Us common folk
understand that slack water is at the top and bottom of the wave. So
slack is, roughly about a hour and it is roughly in the upper 30%
counting the end of the rising tide and the beginning of the Ebb. 15% to
the HW and 15% roughly to the Max Flow. That means about two hours to
max flow, 8 hours of max flow and then another couple of hour of
diminishing flow to LW.


While the current flow in "simple harbors" may follow a sine curve, in cuts
between two bodies of water the flow is different. The curve is more
"squarish," that is, the flow ramps up quicker and stays high longer. The
duration of slack is reduced. The best example is Hell Gate in New York.

IIRC, the flow rate is proportional to the square root of the different of
height of the two bodies.

Nav?

If the max flow is when the height distance is the greatest, wouldn't
the tide change wave be like a pyramid instead of a sine wave?

Guys, I think more thought needs to be given to what cause Tides (Moon)
and rotation of the earth. It isn't just a simple hydraulic solution.

Now a question; 1 point (to be awarded by Donal): If the Earth makes
one rotation in 24 hours and the Moon rotates around the Earth making
one revolution, why does high tide time vary daily?

Ole Thom

Thom Stewart wrote:

Nav?

If the max flow is when the height distance is the greatest, wouldn't
the tide change wave be like a pyramid instead of a sine wave?

Guys, I think more thought needs to be given to what cause Tides (Moon)
and rotation of the earth. It isn't just a simple hydraulic solution.

Now a question; 1 point (to be awarded by Donal): If the Earth makes
one rotation in 24 hours and the Moon rotates around the Earth making
one revolution, why does high tide time vary daily?

Ole Thom


Well Thom, the moon makes one revolution around the earth in 28 days, always keeping the same side facing the earth, thus the time of the overhead
position of the moon moves by about about 1/2 hour per day.

Cheers
Marty