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To give the student an understanding of:
` The main causes of Tides;
` The creation of Spring and Neap Tides;
` Causes of differences between PredictedHeights of Tides and Actual Heights of Tides;
` Calculations involving Underwater and
Overhead Obstructions, Shoreside Structures,
Leadlines & Echo Sounders.
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The Student will be able to:
` Explain how tides are created;
` Explain how Spring and Neap Tides are created;
` Explain the causes of the variation betweenPredicted and Actual Heights of Tides
` Perform Calculations involving Underwater and
Overhead Obstructions, Shoreside Structures,
Leadlines and Echo Sounders
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` The main factors causing tides is the combined effect of the
gravitational forces exerted on the earth by the moon and also by the
sun
` The approximate ratio of these forces can be represented as 7 : 3respectively
` Tide is also caused by the centrifugal forces produced by the earth
and the moon
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HW
LW
Point A
Under ideal conditions the earth is considered to have a uniform covering ofwater with no landmass
The difference in the gravitational and centrifugal forces exerted on the earths
surface by the moon causes the water to pile up towards the moon and also inthe hemisphere opposite to the moon
The earth is effectively rotating in an ellipsoid of water and point A will becarried round as the earth rotates
As the earth rotates once every 24 hours we would expect point A to see twoHWs and two LWs. This also applies to every part of the earth
LW
HW
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However, as the moon is in orbit around the earth, each daythe moon moves further along in its orbit and the earth
takes an extra 50 minutes to be back in conjunction withthe moon
Therefore a Lunar day is 24hours 50mins
HW
LW
P
LW
HW
P
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The time between two HWs is approximately 12hrs 25mins
The time between successive HW and LW is approximately
6hrs 12mins
This means that on some days it is possible to experience
only one HW or LW
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Spring Tides The tides with maximum range are known as Spring Tides
Spring tides occur at fortnightly intervals
They occur under the following circumstances:-
Opposition Conjunction
P SUN
New Moon
+7
+3
Full Moon
+7
The above figure represents the relative positions of the Sun and Moon at Spring tides
This is when the tidal generating forces of the Sun & Moon are acting together toproduce the highest High tides and the lowest Low tides
The Moon is said to be either in conjunction or opposition
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Neap Tides The tides with minimum range are known as Neap Tides
They also occur at fortnightly intervals
They occur under the following circumstances:-
Quadrature
Quadrature
PSUN
+3
1st. Quarter
+7
+7
LW
HW
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Neap Tides
1st. Quarter
P
SUN
+7 +3
+7
The figure represents the relative positions of the Sun and Moon at Neap tides
This is when the tidal generating forces of the Sun & Moon are acting at right angles toeach other so that effectively a lower High water and a higher Low water is produced
The Moon is said to be at Quadrature
The Moon exerts agreater influencein the ratio 7 : 3
The effect of theSun is to reducethe resultantheight of tide
HW
LW
Last Quarter
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` The predicted heights and times are shown in the Admiralty Tide Tables(ATT)
` The predictions are based on observations over at least one year and
are calculated for average meteorological conditions
` Other factors which affect heights of tide but which cannot be takeninto consideration in the tables are barometric pressure and wind
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Accuracy of Predictions - Survey
Predictions are usually based on a
survey period of at least 1 year,
preferably 3 years can be as short as3 weeks.
How recent the survey is.
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` Sailing Directions give the average barometric pressure for various
areas and if the actual pressure is less than this the sea level will tend
to be higher
` When the pressure is greater than the average the sea level will tend
to be lower
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` An onshore wind will pile up the water against the coast
`
An offshore wind will push the water away from the coast andtherefore tend to lower the coastal water level
` Winds blowing along the coast will set up long waves (storm surges)
which will raise or lower the level depending upon the position of the
crest or trough
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Exercises involving UnderwaterExercises involving Underwater
RestrictionsRestrictionsWaterWater InformationInformation Ship InformationShip Information
Shoal
WL
Chart Datum
Charted
Depth
draught
UKC
HoT
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Exercises involving UnderwaterExercises involving Underwater
RestrictionsRestrictions Example 1Example 1
oal
5.0m
4.0m
U
2.0m
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Exercises involving UnderwaterExercises involving Underwater
RestrictionsRestrictions Example 1Example 1
oal
5.0m
4.0m
U
2.0m
arted ept 5.0 m
Ht of Tide +2.0 mTotal ept 7.0 m
raug t -4.0 m
learance 3.0 m
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Exercises involving UnderwaterExercises involving Underwater
RestrictionsRestrictions Example 1Example 1
Charted Depth . m
Ht of Tide + . mTotal Depth 7. m
Draught - . m
Clearance 3. m
Shoal
WL
CD
. m
. m
UKC
. m
The Underkeel clearance is 3.0m
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Exercises involving UnderwaterExercises involving Underwater
RestrictionsRestrictions Exercise 1Exercise 1
oal
6.2
.6
U
.9
arted ept 6.2
Ht of Tide + .9
Total ept 1 .1raug t - .6
learance 2.
The under-keel clearance is 2.5m
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Exercises involving UnderwaterExercises involving Underwater
RestrictionsRestrictions ExampleExample
Shoal
WL
CD
3. m
8. m
1. m
HoT
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Exercises involving UnderwaterExercises involving Underwater
RestrictionsRestrictions ExampleExample
oal
.0m
.0m
.5m
oTraug t .0 m
learance + .5 m
Total ept .5 m
arted ept - .0 m
t of Tide .5m
The Height ofTide is 6.5m above Chart Datum
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Exercises involving UnderwaterExercises involving Underwater
RestrictionsRestrictions ExerciseExercise
oal
6.
.
.5
oT
raug t .
learance + .5
Total ept .7arted ept -6.
t of Tide .
The Height ofTide is 3.4m above Chart Datum
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Exercises involving UnderwaterExercises involving Underwater
RestrictionsRestrictions Example 3Example 3
oal
4.0m
.5m
.5m
eig t of Tide
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Exercises involving UnderwaterExercises involving Underwater
RestrictionsRestrictions Example 3Example 3
Draught . mClearance 1. m
Total Depth . m
Drying Height (- -) +1. m
Ht of Tide 7. m.
Shoal
WL
CD
. m
1. m
1. m
Height of Tide
The Height ofTide is 7.0m above Chart Datum
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Exercises involving UnderwaterExercises involving Underwater
RestrictionsRestrictions Example 3Example 3
1. Charted depth of shoal 3. m Height of tide 3. m Draught . m. Find the clearance over the shoal.Charted Depth 3. m
Ht of Tide +3. mTotal Depth . m
Draught - . mClearance 1. m
The under-keel clearance is 1.0m
Shoal
WL
CD
3. m
1. m
.7m
Height of Tide
Draught 3. m
Clearance +1. m
Total Depth . m
Drying Height (- -) + .7 m
Ht of Tide 7.3 m.
The Height ofTide is 7.3m above Chart Datum
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Bill of Portland Lighthouse - Chart
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Bill of Portland Lighthouse - ALOL
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Springs and NeapsSprings and Neaps
NOAA National Ocean Service Education:NOAA National Ocean Service Education:Tides and Water LevelsTides and Water Levels
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MeasurementsMeasurements of Lighthousesof Lighthouses --
ALOLALOL
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Problems involving ShoresideProblems involving Shoreside
StructuresStructures
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Exercises involving ShoresideExercises involving Shoreside
StructuresStructures ExampleExample
CD
MHWS
8. m
3 . m
. m
SL
Heightof Light
Charted Height of light 3 . m. Height of MHWS 8. m. Height of tide . m.
Find the height of the light above the water.
Charted Ht of Lt 3 . m
MHWS + 8. m
Total Ht of Light .7 m
Height of Tide - . m
Ht of Lt above W/L 3 . m
The Height of the Light is 34.2m above the Sea Level
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Exercises involving ShoresideExercises involving Shoreside
StructuresStructures ExerciseExercise
CD
MHWS
.1m
. m
3. m
SL
Heightof Light
Charted Ht of Lt . m
MHWS + .1 m
Total Ht of Light .3 m
Height of Tide - 3. m
Ht of Lt above W/L .7 m
The Height of the Light is 45.7m above the Sea Level
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Exercises involving ShoresideExercises involving Shoreside
StructuresStructures ExampleExampleA Light is 1. m above the water. The Elevation of the light is charted as 37. m.The height of MHWS is 8.7m. Find the height of tide.
CD
MHWS
8.7m
37. m
HoT
WL
1. m
Elevation of Lt 37. m
MHWS + 8.7 m
Total Ht of Light . m
Ht of Lt above W/L 1. m
Height of Tide .3 m
The Height ofTide is 4.3m above Chart Datum
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Exercises involving ShoresideExercises involving Shoreside
StructuresStructures Exercise 5Exercise 5
CD
MHWS
1 . m
3 . m
. m
WL
Height
of Light
Height of light 3 . m. Height of MHWS 1 . m. Height of tide . m.
Find the height of the light above the water.
Charted Ht of Lt 3 . m
MHWS +1 . m
Total Ht of Light . m
Height of Tide - . m
Ht of Lt above W/L 3 . m
The Height of the Light is 34.5m above the Sea Level
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` Charts published prior to 2005 measurement ofBridges and Electrical Wires from MHWS
` Charts published after2005 measurement ofBridges and Electrical Wires from HAT
` SQA measurement of Bridges and ElectricalWires from MHWS
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Exercises involving ShoresideExercises involving Shoreside
StructuresStructures Exercise5
Exercise5
CD
MHWS
5. m
2. m
HoT
WL
.7m
Elevation of Lt 2. m
MHWS + 5. m
Total Ht of Light 8.2 m
Ht of Lt above W/L .7 m
Height of Tide 1.5 m
The Height ofTide is 1.5m above Chart Datum
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Problems involving OverheadProblems involving Overhead
ObstructionsObstructions
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Problems involving OverheadProblems involving Overhead
ObstructionsObstructions Example 6Example 6
Charted Ht of Bridge 17.0 m
MHWS + 7. m
Total Ht of Bridge24
. mClearance - 2.0 m
Air Draught - 22.0 m
Height of Tide 0. m
The Maximum Height of
Tide is 0.6m
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Problems involving OverheadProblems involving Overhead
ObstructionsObstructions Exercise 6Exercise 6
Charted Ht of Bridge 15.0 m
MHWS + 5.3 m
Total Ht of Bridge 20.3 m
Clearance - 1.0 mAir Draught - 18. 0 m
Height of Tide 1.3 m
The Maximum Height ofTide is 1.3m
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Problems involving OverheadProblems involving Overhead
ObstructionsObstructions Example 7Example 7
Charted Ht of Bridge 14. m
MHWS + 7.4 m
Total Ht of Bridge 22.0 m
Height of Tide - .3 m
Air Draught -1 2.5 m
Clearance 3.2 m
The Clearance between the Mast and the Bridge is 3.2m
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Problems involving OverheadProblems involving Overhead
ObstructionsObstructions Exercise 7Exercise 7
Charted Ht of Bridge 12. mMHWS + 5.3 m
Total Ht of Bridge 17. m
Height of Tide - 4.1 m
Air Draught - 8.3 m
Clearance 5.5 m
The Clearance between the Mast and the Bridge is 5.5m
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` Measures the Total Depth of Water
` If the HoT is subtracted, the Charted Depth is
found.
` HoT is therefore known as the Correction to theLeadline
` Useful for the creation of Charts
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Problems involving the LeadlineProblems involving the Leadline
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Problems involving the LeadlineProblems involving the Leadline
Example 8Example 8
Shoal
SL
Chart Datum
Charted
Depth
.7m
8.0mSounded Depth 8.0
mHt of Tide - .7 m
Charted Depth 1.3 m
The Charted Depth is 1.3m below Chart Datum
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Problems involving the LeadlineProblems involving the Leadline
Example 9Example 9
Shoal
SL
Chart at
Chart
th
5.
4.5
Soun th 4.5
Ht of Ti -5.
Chart th -1.2= rying H ight 1.2
The Drying Height is 1.2m above Chart Datum
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` Can be used to measure either:(a) Depth Under the Keel, or(b) Total Depth of Water.
` Care must be taken to ensure that theNavigator knows whether it is (a) or (b).
` If using Total Depth of Water, the correct up to
date draught must be entered into the EchoSounder
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Echo Sounder Measuring DepthEcho Sounder Measuring Depth
under the Keelunder the KeelWater Information Ship Information
Shoal
WL
Chart Datum
Charted
Depth
draught
Sounding
/UKC
HoT
Water Information Ship Information
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Echo Sounder Measuring TotalEcho Sounder Measuring Total
Depth of WaterDepth of Water
Shoal
L
Chart atum
Chart
th
Depthof
ater
oT
Water Information Ship Information
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` There may be several transponders, e.g. Fwd &Aft occasionally Port & Stbd.
` Care must be taken to know which Transponderis being used.
` If using the Fwd Transponder whilst the vesselhas a large stern trim the underkeel clearancecould be misleading!
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Problems involving theProblems involving the
EchoEcho--SounderSounder Example 10Example 10
Shoal
WL
Chart Datum
20m
5.2m
23.4m
HoT
Draught 5.2 mSounding +23.4 m
Total Depth 28. m
Charted Depth -20.0 m
Ht of Tide 8. m
The Height ofTide is 8.6m
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Complete Course Paper 1 starting in class,completing as homework.
Answers in Tides Course Papers Book at the frontof the Class
Answers also on Moodle
Good luck!