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.

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.

That's not a backpedal. Tidal current is discrete from hydraulic current
and the two sum as I said.

Cheers

Jeff Morris wrote:

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.

You mean a line parted and it wasn't even chafing on the boat or dock??? Good
Grief!!!

This is the most pathetic confession since jaxie admitted he didn't know how to
use a compass!



"Nav" wrote in message
...
That's not a backpedal. Tidal current is discrete from hydraulic current
and the two sum as I said.

Cheers

Jeff Morris wrote:

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.

Sorry Nav, that was obviously meant for Booby.

Your backpedal was pretty pathetic, also. You most pathetic since claiming to
be a charter member of the "Save the Constitution" committee!





"Jeff Morris" wrote in message
...
You mean a line parted and it wasn't even chafing on the boat or dock???
Good
Grief!!!

This is the most pathetic confession since jaxie admitted he didn't know how
to
use a compass!



"Nav" wrote in message
...
That's not a backpedal. Tidal current is discrete from hydraulic current
and the two sum as I said.

Cheers

Jeff Morris wrote:

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.

Sure. There was I thinking you would discuss a scientific subject
intelligently. Silly me.

Cheers


Jeff Morris wrote:
Sorry Nav, that was obviously meant for Booby.

Your backpedal was pretty pathetic, also. You most pathetic since claiming to
be a charter member of the "Save the Constitution" committee!





"Jeff Morris" wrote in message
...

You mean a line parted and it wasn't even chafing on the boat or dock???

Good

Grief!!!

This is the most pathetic confession since jaxie admitted he didn't know how

to

use a compass!



"Nav" wrote in message
...

That's not a backpedal. Tidal current is discrete from hydraulic current
and the two sum as I said.

Cheers

Jeff Morris wrote:


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.

Sorry Nav, I missed that. Perhaps you should have a special annotation for when
you're trying to be intelligent.



"Nav" wrote in message
...
Sure. There was I thinking you would discuss a scientific subject
intelligently. Silly me.

Cheers


Jeff Morris wrote:
Sorry Nav, that was obviously meant for Booby.

Your backpedal was pretty pathetic, also. You most pathetic since claiming
to
be a charter member of the "Save the Constitution" committee!





"Jeff Morris" wrote in message
...

You mean a line parted and it wasn't even chafing on the boat or dock???

Good

Grief!!!

This is the most pathetic confession since jaxie admitted he didn't know how

to

use a compass!



"Nav" wrote in message
...

That's not a backpedal. Tidal current is discrete from hydraulic current
and the two sum as I said.

Cheers

Jeff Morris wrote:


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.

If you look at a curve of the "tidal streams" of a given location, or
at the ATSA, (Admirallity Tidal Stream Atlas), it incorporates the
effect of landmasses and sea-bottom topography on the water-flow.
Doesn't it ?
Otherwise it wouldn't be very useful !

Peter S/Y Anicula

"Nav" skrev i en meddelelse
...
That's not a backpedal. Tidal current is discrete from hydraulic
current
and the two sum as I said.

Cheers

Jeff Morris wrote:

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.

Of course. While topography can have big effects in estuaries (except at
their end and mouth) the sea bottom has almost no effect on the time of
peak tide current along the coast. That is determined by the addition of
tide and ocean currents. Did you wonder why charts show tide and ocean
currents with separate symbols and often only a flood arrow? Could these
pieces of information be used by a navigator to estimate current at
other times than high tide?

Cheers

Peter S/Y Anicula wrote:

If you look at a curve of the "tidal streams" of a given location, or
at the ATSA, (Admirallity Tidal Stream Atlas), it incorporates the
effect of landmasses and sea-bottom topography on the water-flow.
Doesn't it ?
Otherwise it wouldn't be very useful !

Peter S/Y Anicula

"Nav" skrev i en meddelelse
...

That's not a backpedal. Tidal current is discrete from hydraulic

current

and the two sum as I said.

Cheers

Jeff Morris wrote:


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,

Why wouldn't you use a RMS relationship if it close to a sine wave?

Ole Thom