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.