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Calculating Tidal Streams Part 1: The 50-90 Rule & Rule of Thirds!
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When calculating the effect of tidal streams, sea kayakers need to use the most detailed/accurate
sources of information in their paddling area. These sources may include tidal stream atlases, tidal
diamonds, pilots, paddling guides and charts. Sometimes the information is detailed enough to
allow precise calculations of tidal speed and direction for specific times of the day; more often,
however, we have only the direction and maximum rate of the tide, combined with slack water
times that mark the time of day when the tide turns in a particular location.!
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For example, we may be planning a sea kayak journey around South Stack, a headland on the
west coast of Anglesey. Study of the available guides and pilots reveals the following information: !
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The N-going (flood) stream begins 4hrs 50mins after HW Liverpool!
The S-going (ebb) stream begins 1hr 10mins before HW Liverpool!
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The maximum tidal stream (Mean Springs) is 6kts
The tidal stream atlas for the area contains hourly information for the area, but too far offshore to
be directly related to the speed of the tidal stream immediately off the headland. !
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The nearest Tidal Diamond, located 0.5 NM north west of North Stack, is too distant to be directly
relevant to South Stack. We therefore need another method of calculating the speed of the tidal
stream at South Stack at various times of the day.!
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One method, the 50-90 Rule, is a useful way of figuring out what is happening at a specific
location, based on the slack water times and maximum tidal rate. As with all ‘rules of thumb’, it
needs to be treated with a little caution, but is a very useful practical tool when other information is
lacking. !
The essential elements of the rule are presented in the following table:
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The key to this rule is that it provides a reasonably accurate indication of the speed of the tidal
stream at specific moments in the tidal phase. Using a simple model with a 6-hour period between
LW and HW, and a maximum rate of 4 kts, the 50-90 Rule tells us:!
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Time of Day
Time of Tide
% of Max Rate
Speed at this time
1200
LW Slack
0%
0 kts
1300
1hr after LW Slack
50%
2 kts
1400
2hrs after LW Slack
90%
3.6 kts
1500
3hrs after LW Slack!
3hrs before HW Slack
100%
4 kts
1600
2hrs before HW Slack
90%
3.6 kts
1700
1hr before HW Slack
50%
2 kts
1800
HW Slack
0%
0 kts
Remember that the 50-90 Rule tells us what is happening at the end of each hour of the tidal
phase, not what is happening during the periods between these times. !
It is possible to use the rule to figure out the speed of tidal movements at other times, for example:!
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30mins after LW Slack = !
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1hr 30 mins after LW Slack = !
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25% of max. rate (4kts) = !
70% of max. rate (4kts) = !
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1kt!
2.8kts!
The following diagram presents the same information in a more visual way.!
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It’s unusual for consecutive LW and HW times to be exactly 6 hours apart, and specific
locations around the coastline often have longer periods of flood or ebb. Using the 50-90
Rule is therefore a little more involved in practice - but it’s not complicated, as the following
example shows:!
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Returning to the example of South Stack on the west coast of Anglesey, let’s use the 50-90 Rule to
calculate the speed of the tidal stream at different times of a specific day.!
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Step 1:!
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The Standard Port for the Tidal Constants at South Stack is Liverpool:!
Monday 19 March 2012 - Liverpool Alfred Dock!
All times GMT!
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HW 0900!
LW 1600!
HW 2135!
8.5m!
1.7m!
8.5m!
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Step 2:!
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The tidal differences for South Stack are:!
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N-going stream begins 4hrs 50mins after HW
Liverpool!
The tidal range for each tidal phase is 6.8m
- halfway between Springs and Neaps
Tidal streams are described in terms of
their direction: !
a NW stream is going in a NW direction
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S-going stream begins 1hr 10mins before HW
Liverpool!
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Max rate (Springs) for NW-going stream = 6kts!
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Max rate on 12 March 2012 = 4.5kts!
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The max rate (Neaps) is typically 50% of
the Spring rate, in this case 3kts. !
Halfway between Spring and Neaps, the
maximum rate for South Stack on 19 March
2012 is therefore 4.5kts.
NW stream begins 0900 (HW L’pool) plus 4hrs 50mins (tidal constant South Stack): !
1350 GMT!
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NW stream ends 2135 (HW L’pool) minus 1hr 10mins (tidal constant South Stack):!
2025 GMT!
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Duration of NW-going (flood) stream: 6hrs 35mins!
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Step 3:!
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Applying the 50-90 Rule:!
Armed with slack water times and a maximum tidal stream, it’s a simple process now to calculate
the speed of the tidal stream at different times of the tidal phase. With a duration of 6hrs 35mins
between slack water times, the Slack-50%-90%-Max Rate intervals will now be 1hr 6mins apart, as
the table below shows.!
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When completing the table, it’s often a good idea to first write in the slack water (0%) times for the
tidal phase, followed by the max rate (100%) time, which is exactly halfway between slack water
times. You can then fill in the 50% and 90% times, checking your accuracy against the other
values.!
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!
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Time of Day
Time of Tide
% of Max Rate
Speed at this time
1350
LW Slack
0%
0 kts
1456
1hr after LW Slack
50%
2.25 kts
1602
2hrs after LW Slack
90%
4 kts
1708
3hrs after LW Slack!
3hrs before HW Slack
100%
4.5 kts
1812
2hrs before HW Slack
90%
4 kts
1918
1hr before HW Slack
50%
2.25 kts
2025
HW Slack
0%
0 kts
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Applications:!
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The table above looks quite detailed, with precise times and exact tidal speeds for each ‘hour’ of
the flooding tide. It’s important to remember, however, that the 50-90 Rule is a rule of thumb, and
needs to be used cautiously. It’s likely that times and speeds will vary slightly from those predicted
in the table. Nevertheless, it gives us a useful starting point for planning purposes.!
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It’s also useful for planning timings when leading groups in areas of stronger tides. With slack
water times and max rate rates for a zone of faster water, we can predict the likely speed of tide
30mins or 1 hour before and after slack water. We can also work out the time at which the fastest
tidal stream will occur, all of which will influence our trip planning on the day.!
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The Rule of Thirds:!
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This rule is also frequently adopted as a means of calculating the speed of tidal streams during a
part of the day. While the Rule of Thirds is useful for specific tasks, it fulfils a different role to the
50-90 Rule. !
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The key difference is that the Rule of Thirds deals with 1-hour blocks of information - in other
words, it tells us the average speed of a tidal stream over a 1-hour period. The table below
explains its function. !
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For example, in the first hour of the tide after Slack Water, the average speed of the tide will be one
third of the maximum rate. This information can be useful when travelling through a stretch of
coastline where the maximum rate is known but specific tidal information is lacking. !
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If such a coastline experiences a maximum rate of 4kts along its length, a 1-hour trip, with the tide,
during the first hour of the tidal phase, will experience one third of the maximum rate during that
hour:!
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Tidal stream during first hour = 1/3 of max rate (4kts) = 1.33kts!
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If paddling in the same direction as this tidal stream for this period of time, at a water speed of
3kts, the total distance travelled (according to the Rule of Thirds) will be 4.33 nautical miles.!
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Exercise:!
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Use the 50-90 Rule to calculate the speed of the tidal stream at different hours of the day, using
the following information:!
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Point Lynas, NE Anglesey!
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The SE-going (flood) stream begins 5hrs 30mins after HW Liverpool!
The NW-going (ebb) stream begins 0hr 30mins before HW Liverpool!
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The maximum tidal stream (Mean Springs) is 5kts
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The Standard Port for the Tidal Constants at Point Lynas is Liverpool:!
Wednesday 21 March 2012 - Liverpool Alfred Dock!
All times GMT!
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HW 1032!
LW 1723!
HW 2255!
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9.3m!
1.0m!
9.1m!
• Calculate the times at which the NW-going and SE-going streams begin and end!
• Calculate the time of maximum rate on the ebb and flood tides!
• Calculate the times of day when the tidal stream reaches 50% and 90% of its maximum rate!
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