Tides
 

I know that the peak or trough of a tide is not linear with time but I can't
remeber if it is 1/6, 2/6, and 3/6 for each of the three segments or if it
si different. Can anyone help, please.

Fred Miller wrote:
I know that the peak or trough of a tide is not linear with time but I can't
remeber if it is 1/6, 2/6, and 3/6 for each of the three segments or if it
si different. Can anyone help, please.


Yes, you have it sort of right, but not quite. Its often called the
"Rule of Twelves." One twelfth of the height come in in the first
hour, two twelfths in the second, three in the third, three in the
forth, two in the fifth, one in the sixth. Thus the fastest change will
occur at mid tide.

Sometimes the Rule of Twelves is applied to currents, but that is often
not correct. When the current is caused by large bodies of water of
differing heights connected by a narrow channel, the current gets up to
speed rather quickly - sometimes reaching half speed in less than 30
minutes from slack tide.

Jeff Morris wrote:
Fred Miller wrote:
I know that the peak or trough of a tide is not linear with time
but I can't
remeber if it is 1/6, 2/6, and 3/6 for each of the three segments
or if it
si different. Can anyone help, please.


Yes, you have it sort of right, but not quite. Its often called the
"Rule of Twelves." One twelfth of the height come in in the first
hour, two twelfths in the second, three in the third, three in the
forth, two in the fifth, one in the sixth. Thus the fastest change
will
occur at mid tide.

Good info, thanks for posting.


Sometimes the Rule of Twelves is applied to currents, but that is
often
not correct. When the current is caused by large bodies of water of
differing heights connected by a narrow channel, the current gets up
to
speed rather quickly - sometimes reaching half speed in less than 30
minutes from slack tide.

So, and please excuse a really stupid question, but slack tide refers
to the hour or so on both sides of low tide when the tide rising or
falling most slowly?
I've always seen this term but never been precisely sure of the
definition.

richforman

Correct, but it applies to high tide as well. Slack tide normally
is a time when the current is slower as well so one can make better
progress and not fight against the current.

Doug
s/v Callista


wrote in message
oups.com...

Jeff Morris wrote:
Fred Miller wrote:
I know that the peak or trough of a tide is not linear with time
but I can't
remeber if it is 1/6, 2/6, and 3/6 for each of the three segments
or if it
si different. Can anyone help, please.


Yes, you have it sort of right, but not quite. Its often called the
"Rule of Twelves." One twelfth of the height come in in the first
hour, two twelfths in the second, three in the third, three in the
forth, two in the fifth, one in the sixth. Thus the fastest change
will
occur at mid tide.

Good info, thanks for posting.


Sometimes the Rule of Twelves is applied to currents, but that is
often
not correct. When the current is caused by large bodies of water of
differing heights connected by a narrow channel, the current gets up
to
speed rather quickly - sometimes reaching half speed in less than 30
minutes from slack tide.

So, and please excuse a really stupid question, but slack tide refers
to the hour or so on both sides of low tide when the tide rising or
falling most slowly?
I've always seen this term but never been precisely sure of the
definition.

richforman

Here's the deal on current timing compared to tide timing. Some people say
"slack tide" but they really mean "slack current". The current at any given
location depends on the relative height of the tide at a location on either
side of that location. In a large enough body of water the water in a big
circle (say a half mile) around a boat will be pretty much all at the same
height. All this water will be flowing in a big mass toward or away from
the closest pathway to a bigger body of water (such as an ocean). Slack
current will take place when the tide is either highest or lowest in the
general area.

However, in narrow passageways the whole idea changes. Suppose you are in a
canal that connects two larger bodies of tidal water. The slack will take
place when the tide level in those two bodies is the SAME, not necessarily
when either is high or low. The exact point in time when they will be the
same depends on the time delay as the water enters each body of water from
typically two different routes.

In many cases the time of equal tide height for each large body might be at
the moment when the tide is rising or falling fastest (midpoint between high
and low) and therefore the slack in the canal will be very short.

An example of this is Deception Pass in Washington State. This is a narrow
channel that is the only separation between the mainland and the very large
Whidbey Island. To the west is a very direct route to the Pacific Ocean,
through the Strait of Juan de Fuca. To the east is a small bay formed
between Whidbey and the mainland. It takes many hours for the rising tide
from the ocean to reach the inside area because it must flow all the way
around Whidbey to fill that bay up. Truly slack current in Deception pass
lasts for only a few minutes and safe passage can be made in small boats
only for a half-hour or so around slack. At maximum current there can be 7+
knots with whirlpools and overfalls that can get the better of 100 foot
Naval vessels.

Today there was a slack in Deception Pass at 7:51 AM. At that time the tide
at the western side of the pass was very near its high. However, the
eastern side tide height was 1.5 feet below and over two hours below the
high tide level. Waiting for the high tide at Cornet Bay on the eastern
side would have you going through the pass at 10:04 AM at which time the
current was ebbing at 6.64 knots!

Many boaters have gotten in trouble by assuming that tide tables are all
they need to deal with currents. There are a lot of places like this around
this region, and a good set of current tables is a very important tool. If
you use a Palm OS PDA there is a very nice little freeware program called
TideTool that provides both tide and current data for a huge number of
locations in the US and Canada (both coasts) and shows the data in both
table and graph formats. www.toolworks.com

Don

"Jeff Morris" wrote in message
...
wrote:


So, and please excuse a really stupid question, but slack tide refers
to the hour or so on both sides of low tide when the tide rising or
falling most slowly?
I've always seen this term but never been precisely sure of the
definition.


As mentioned, "slack tide" (or "slack water"} occurs at both high and low
tide. In most locations slack occurs within 5 or 10 minutes of high or
low tide.

However, the times do not always coincide. In fact, there are sometimes
major differences. Tonight at the RR Bridge in the Cape Cod Canal slack
water will be at 7:46 pm, but low water is over an hour later, at 8:51. At
the time of low, the current will already be running at 3 knots, so it
could be a serious problem if one presumed that low and slack were the
same.

A good description. I'm so used to thinking of "slack" as slack
current, that I forgot that "slack tide" refers precisely to high of
low, not "slack current."
Don wrote:
Here's the deal on current timing compared to tide timing. Some people say
"slack tide" but they really mean "slack current". The current at any given
location depends on the relative height of the tide at a location on either
side of that location. In a large enough body of water the water in a big
circle (say a half mile) around a boat will be pretty much all at the same
height. All this water will be flowing in a big mass toward or away from
the closest pathway to a bigger body of water (such as an ocean). Slack
current will take place when the tide is either highest or lowest in the
general area.

However, in narrow passageways the whole idea changes. Suppose you are in a
canal that connects two larger bodies of tidal water. The slack will take
place when the tide level in those two bodies is the SAME, not necessarily
when either is high or low. The exact point in time when they will be the
same depends on the time delay as the water enters each body of water from
typically two different routes.

In many cases the time of equal tide height for each large body might be at
the moment when the tide is rising or falling fastest (midpoint between high
and low) and therefore the slack in the canal will be very short.

An example of this is Deception Pass in Washington State. This is a narrow
channel that is the only separation between the mainland and the very large
Whidbey Island. To the west is a very direct route to the Pacific Ocean,
through the Strait of Juan de Fuca. To the east is a small bay formed
between Whidbey and the mainland. It takes many hours for the rising tide
from the ocean to reach the inside area because it must flow all the way
around Whidbey to fill that bay up. Truly slack current in Deception pass
lasts for only a few minutes and safe passage can be made in small boats
only for a half-hour or so around slack. At maximum current there can be 7+
knots with whirlpools and overfalls that can get the better of 100 foot
Naval vessels.

Today there was a slack in Deception Pass at 7:51 AM. At that time the tide
at the western side of the pass was very near its high. However, the
eastern side tide height was 1.5 feet below and over two hours below the
high tide level. Waiting for the high tide at Cornet Bay on the eastern
side would have you going through the pass at 10:04 AM at which time the
current was ebbing at 6.64 knots!

Many boaters have gotten in trouble by assuming that tide tables are all
they need to deal with currents. There are a lot of places like this around
this region, and a good set of current tables is a very important tool. If
you use a Palm OS PDA there is a very nice little freeware program called
TideTool that provides both tide and current data for a huge number of
locations in the US and Canada (both coasts) and shows the data in both
table and graph formats. www.toolworks.com

Don

"Jeff Morris" wrote in message
...

wrote:


So, and please excuse a really stupid question, but slack tide refers
to the hour or so on both sides of low tide when the tide rising or
falling most slowly?
I've always seen this term but never been precisely sure of the
definition.


As mentioned, "slack tide" (or "slack water"} occurs at both high and low
tide. In most locations slack occurs within 5 or 10 minutes of high or
low tide.

However, the times do not always coincide. In fact, there are sometimes
major differences. Tonight at the RR Bridge in the Cape Cod Canal slack
water will be at 7:46 pm, but low water is over an hour later, at 8:51. At
the time of low, the current will already be running at 3 knots, so it
could be a serious problem if one presumed that low and slack were the
same.