United States Power Squadrons ® Advanced Piloting Course Chapter 7 Working with Tides Tides alter available depth When in doubt… • If charted depths are.
Download ReportTranscript United States Power Squadrons ® Advanced Piloting Course Chapter 7 Working with Tides Tides alter available depth When in doubt… • If charted depths are.
United States Power Squadrons ®
Advanced Piloting Course
Chapter 7 Working with Tides
Tides alter available depth
When in doubt…
• •
If charted depths are too low Don’t go that way
Sometimes…
•
You need to travel across shallow areas
You must do so when there is sufficient water
•
You will need to anticipate depth
At some time other than MLLW Slide 2
What causes Tides
Gravitational pull
•
Moon and sun
Sun’s influence only 46% that of moon
Moon
•
24 hr 50 min (apparent orbit of Earth)
Tides delayed ~ 50 min every day
Centrifugal force
•
Causes tides on side away from moon Slide 3
What Causes Tides?
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Slide 4
Spring & Neap Tides
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Slide 6
Local Tides
Most of Earth
• •
Two bulges per day = two tides per day Called “semi-diurnal” tides (diurnal means daily)
Bulges not equal
• •
Due to geometry Means one daily tide greater than other
Some cases
• •
Tides cancel each other = 1/day (Gulf of Mexico) Geometry = only one bulge at that latitude Slide 7
semi-diurnal
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Typical Tide Patterns mixed-tides diurnal Slide 8
Tidal Datum
Vertical datum
•
Reference level on chart
Typically MLLW for depths MHW for vertical clearance
Soundings
• •
Title block – indicates references “Soundings in feet” from MLLW (typical - coastal charts)
Different levels… Slide 9
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Various Tidal Levels
Charted Height Datum ≈ ½ Mean Range ≈ ½ Mean Range Charted Depth Datum
Slide 10
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Depth under the Boat
Different Tide Ranges Falmouth Cape Cod 1.5 ft tides
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Bar Harbor Maine 12 ft tides Slide 12
Sources of Tide Data
Internet
•
NOS (NOAA)
Software
• • •
WXTide (freeware) Tidetool (Palm) Tideware
GPS
• •
Built-in tide data Chartplotters
Tables
• •
NOS Local Slide 13
http://tidesonline.nos.noaa.gov/monitor.html
Slide 14
Actual vs. Prediction Slide 15
http://www.co-ops.nos.noaa.gov/tide_pred.html
Slide 16
Tide Predictions Slide 17
NOS - Predictions Slide 18
GPS Tide Charts Slide 19
Tide Tables
NOS
•
Official predictions
Reference/Primary Stations
– Times of High and Low for each day
Subordinate Stations
– Time and height differences – from Reference Station
Local Tables
•
Eldridge’s
•
Reed’s Slide 20
Slide 21
Reference Station
Date New Moon Time of Tide Height of Tide
Slide 22
Slide 23
Subordinate Stations
Reference Station Station No.
Location Time Differences High & Low Height Ratios High & Low Ranges Mean
Slide 24
Simple Prediction Techniques
Rule of Twelfths
• •
Need to know time of high and low Need to know tidal range
Quick calculation
• •
Divide time into hours Divide range into twelfths (or use %) Slide 25
Rule of Twelfths
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Slide 26
Tide Levels Vary over Time
15 days in Boston
1 st Quarter Neap Tides 8 feet Full Moon Spring Tides 10 feet Slide 27
Determine Specific Tide Levels
Look-up Specified Date (reference station)
Bracket desired time
• •
High or Low Tide just before
Time Height Low or High Tide just after
Time Height Slide 28
Tide Height Calculations
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Slide 29
Determine Range for Location
Look-up – Subordinate Station
Note Time Differences
• •
Adjust time of high Adjust time of low (ref H + or - = local high) (ref L + or - = local low)
Note Height Differences
• •
Adjust height of high Adjust height of low (ref ht x ratio * (ref ht x ratio * = local high) = local low)
Compute Actual Range
•
Tidal Range = (Local High – Local Low)
* Note: some subordinate stations provide differences (+) or (-) – add or subract rather than multiply
Slide 30
Determine the Depth
Apply Rule of Twelfths
•
Hourly increments
0, 1hr, 2hrs, 3hrs, 4hrs, 5hrs, 6hrs
•
Compute Change in Tide Height
High to Low: Ht of High Tide 0, 5, 25, 50, 75, 95, 100% of Range
Low to High: Ht of Low Tide + 0, 5, 25, 50, 75, 95, 100% of Range
Determine Depth
Depth Clearance = Sounding + Height Tide - Draft (from chart) (Rule 12ths) (boat specs)
Slide 31
Alternate Method
If predicted heights for day not available
•
Use “Mean Range”
Caution
• •
During Neap Tides
Using Mean Range may overstate depth at high tides
– Can be 25% of range
However, low tide depths may be greater than MLLW During Spring Tides
Depths at low tide may be less (minus tides)
– Can be 15% of range
However, high tide depths will be greater than predicted Slide 32
Vertical Charted Clearance
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Slide 33
Computing Clearance
Chart Datum
•
Generally MHW for clearances
Compute Clearance
• • •
Use Mean Range Add Mean Tide Subtract Height of Tide above MLLW Slide 34
Tide Form
Slide 35
Sample Tide Calculation Example 1
Location: Date: Rainsford Island – Nantasket Apr 18, 2004 Desired Time: 1400
Need:
• • •
Prediction of High and Low Tides (form) Tide Table (Newport, Rhode Island) April 2004 Subordinate Station (Table 2) (Station # 831)
demo
Slide 36
Sample Tide Calculation
Slide 37
Exercise 7-1 - Tide Calculation
Location: Chappaquoit Point West Falmouth Harbor, MA
Date: Sept 1, 2004 Desired Time: 0500 EST Draft: 2.5 feet
Determine Depth below the Keel
Need:
•
Prediction of High and Low Tides (form)
•
Tide Table (Newport, Rhode Island) Sep 2004
•
Subordinate Station (Table 2) (Station # 923) Slide 38
Newport, RI
Slide 39
Table 2
Slide 40
Exercise 7-1
Solution to Exercise 7-1
Slide 41
Questions ? … Comments
Slide 42
Homework
Chapter 7 - Solutions
Exercise 7-2 - Tide Calculation
Location: Pt. Judith Harbor of Refuge Date: Sep 1, 2004 Desired Time: 1200 EST
Height of Mast: 28 ft (vertical clearance)
Determine if you can clear the bridge
•
Vertical clearance of bridge: 27 ft
Need:
• • •
Prediction of High and Low Tides (form) Tide Table (Newport, Rhode Island) Sept 2004 Subordinate Station (Table 2) (Station # 975) Slide 44
Slide 45
Exercise 7-2
Solution to Exercise 7-2
Slide 46
Exercise 7-3 - Tide Calculation
Location: Date: Monument Beach Aug 22, 2004 Desired Time: 1930 EST
Determine height of tide
Need:
• • •
Prediction of High and Low Tides (form) Tide Table (Newport, Rhode Island) Aug 2004 Subordinate Station (Table 2) (Station # 925) Slide 47
Newport, RI
Slide 48
Exercise 7-3
Solution to Exercise 7-3
Slide 49
Exercise 7-4 - Tide Calculation
Location: Budd Inlet Olympia Shoal, WA
Date: Aug 22, 2004 Desired Time: 1930 PST
Determine Height of Tide
Need:
•
Prediction of High and Low Tides (form)
•
Tide Table (Seattle, WA Aug 2004
•
Subordinate Station (Table 2) (Station # 1135) Slide 50
Seattle, WA
Slide 51
Table 2
Slide 52
Exercise 7-4
Solution to Exercise 7-4
Slide 53
Exercise 7-5 – Bridge Clearance
Location: Sneeoosh Point
Date: Jul 4, 2004 Desired Time: 1500 PST
Determine Bridge Clearance
•
Charted Height of Bridge = 24 ft
•
Height of Mast on Boat = 25 ft
Need:
•
Prediction of High and Low Tides (form)
•
Tide Table (Seattle, WA Aug 2004
•
Subordinate Station (Table 2) (Station # 1161) Slide 54
Seattle, WA
Slide 55
Table 2
Slide 56
Exercise 7-5
Solution to Exercise 7-5
Slide 57
Exercise 7-6 - Tide Calculation Extra Exercise
Location: Sakonnet River Date: Jul 4, 2004 Desired Time: Mast Clearance: 1500 EST 25 ft Bridge Clearance: 24 ft
Can you clear the bridge?
Need:
• • •
Prediction of High and Low Tides (form) Tide Table (Newport, Rhode Island) Jul 2004 Subordinate Station (Table 2) (Station # 943) Slide 58
Newport, RI
Slide 59
Exercise 7-6
Solution to Exercise 7-4
Slide 60