Transcript United States Power Squadrons ® Advanced Piloting Course Chapter 8 Effects of Winds & Currents.

United States Power Squadrons
®
Advanced Piloting Course
Chapter 8
Effects of Winds & Currents
Currents
 Boat operates on a platform - the Water
• Imagine the water as a table
• Move the table
 Everything moves including the boat
– Water is moving with respect to the ground below
– In addition to the boat moving across the water
• This is what current does
 Tidal currents form in coastal areas
• Where there are restrictions to tidal flow
• Flow in (flood) and out (ebb)
 Currents have direction
• Set (direction)
• Drift (speed)
Slide 2
Current Effects on Boat
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Slide 3
Winds
 Winds do not effect water platform
 Other than to create waves
• Effects only those objects that have ‘windage’
 Forces boats to move across water with wind
 Leeway
• Boat’s path is altered by wind (angle)
 Seaman’s Eye
• Look at boat’s wake
 Represents boat’s real motion through the water
 Compare with boat’s heading
– Difference is Leeway
Slide 4
Wind Effects on Boat
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Slide 5
Navigating with Current or Wind
 Need to adjust for
• Wind or Current:
 set and drift
 Leeway
(vector from DR to Fix)
(angle between course steered and COG)
• Chapter 9
Slide 6
Tide & Currents on Digital Charts
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Slide 7
Currents shown on Paper Chart
Current Vectors Printed
on some charts
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Slide 8
Tides on Digital Charts
bar and number
show height of
tide in real time
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chart shows
Height of tide
before and after
the current time
Slide 9
Currents on Digital Charts
Arrow shows
direction & speed
of current
in real time
Chart shows
direction & speed
of current before and
After the current time
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Slide 10
Current Characteristics
 Flood & Ebb not symmetrical
•
•
•
•
Different maximum speeds
Uneven Shape
Caused by uneven shape of bodies of water
Most uneven in narrow channels
Slide 11
Predicting Currents - Rule of Twelfths
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Slide 12
On-Line Current Tables
Slide 13
Estimating Current
 Use variation of “rule of twelfths”
• Called “50-90” rule
• ¼ of cycle at a time



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Slack to max flood
Max flood to slack
Slack to max ebb
Max ebb to slack
• Determine time interval
 Divide by 3
 Determine maximum
 Use percentages
– Slack to Max:
– Max to Slack:
0, 50%, 90%, 100%
100%, 90%, 50%, 0
Slide 14
Data Needed
 Current tables
• Determine times
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Slack before flood
Max Flood
Slack before ebb
Max Ebb
• Selected Bracket ¼ cycle
 Determine time interval
– Slack to max, or
– Max to slack
• Divide time interval by 3
Slide 15
Slide 16
Reference Station
Date
New Moon
Time of Slack
Speed at Max (E or F)
Time of Max
Slide 17
Subordinate Stations
 Time differences
• From reference station
 Speed differences
• Ratio of speed at reference station
 Local information
• Direction
 Flood and Ebb
Slide 18
Subordinate Stations
Slide 19
Subordinate Stations
Reference Station
Station No.
Location
Time Differences
Slacks, Flood & Ebb
Speed Ratios
Flood & Ebb
Ave. Speeds
& Directions
Slide 20
Determine Specific Current Speed
 Look-up Specific Date
(reference station)
 Bracket desired time
• Slack (or Max) just before
 Time
 (Max Speed)
• Max (or Slack) just after
 Time
 Max Speed
Slide 21
Calculating Current
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Slide 22
Determine Range for Location
 Look-up – Subordinate Station
 Note Time Differences
• Adjust time of Slack
• Adjust time of Max
(ref S + or - = local slack)
(ref M + or - = local max)
 Note Speed Ratio
• Adjust speed of Max
(ref max x ratio = local max)
 Note Time Interval
• Slack to Max (or Max to Slack)
• Divide interval into thirds
Slide 23
Compute Speed
 Apply 50-90 rule
• Determine Time Interval
 Interval:
– time from Slack to Max, or Max to Slack
• Divide interval into 3 equal time increments
• Compute Speed at each time increment
 Slack to Max:
 Max to Slack:
Max Speed x
0, 50, 90, 100%
Max Speed x 100, 90, 50,
0%
Slide 24
Alternate Method
 If data is limited
• Need times of slack
• Need max flood or ebb current
 Use 50-90 rule
• Determine interval: slack to slack
• Divide interval by 6
• Apply: 0, 50, 90, 100, 90, 50, 0%
 Caution
• Does not handle asymmetry
• Currents can be in error – 50% points - most
Slide 25
Current
Form
Slide 26
Sample Current Calculation
 Location:
 Date:
 Time:
Marblehead Channel
May 18, 2004
1400 EST
 Determine current speed and direction
 Need
• Prediction of Current and Direction form
• Current Table
• Subordinate Station (Table 2) (#996)
Slide 27
1st Part
bracketing 1300
Slide 28
2nd Part
4 x 60 + 3 =
+ 81
Slide 29
final part
Slide 30
Solution for Example
Slide 31
Unpredictable Currents
 Rain Runoff
• Causes changes to speed of current in rivers and
streams
 Wind Driven
• Particularly on large lakes
 Seiches
• Can result from changes in barometric pressure
across large lakes
 Similar to sloshing water in a bowl
Slide 32
Seiche on Lake Erie
+ 6 ft
5 hrs
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Slide 33
Questions ? … Comments
Slide 34
Homework
Chapter 8 - Solutions
Exercise 8-1 - Current Calculation
 Location:
West Chop
0.2 nm West
 Date:
 Time:
Sept 1, 2004
1000 EST
 Determine current speed and direction
 Need
• Prediction of Current and Direction form
• Current Table – Pollack Rip
• Subordinate Station (Table 2) (#1941)
Slide 36
Slide 37
Slide 38
Solution to Exercise 8-1
Exercise
8-1
Slide 39
Exercise 8-2 - Current Calculation
 Location:
 Date:
 Time:
Waquoit Bay Entrance
Aug 26, 2004
1100 EST
 Determine current speed and direction
 Need
• Prediction of Current and Direction form
• Current Table – Pollack Rip
• Subordinate Station (Table 2) (#1926)
Slide 40
Slide 41
Slide 42
Solution to Exercise 8-3
Exercise
8-2
Slide 43
Exercise 8-3 - Current Calculation
 Location:
 Date:
 Time:
Nobska Point
Sept 24, 2004
0100 EST
 Determine current speed and direction
 Need
• Prediction of Current and Direction form
• Current Table – Pollack Rip
• Subordinate Station (Table 2) (#1946)
Slide 44
Slide 45
Slide 46
Solution to Exercise 8-3
Exercise
8-3
Slide 47
Exercise 8-4 - Current Calculation
 Location:
.8 mi E of Yellow Bluff
Golden Gate Bridge, CA
 Date:
 Time:
Sept 1, 2004
1000 PST
 Determine current speed and direction
 Need
• Prediction of Current and Direction form
• Current Table – Golden Gate Bridge
• Subordinate Station (Table 2) (#551)
Slide 48
Golden Gate Bridge
Slide 49
Table 2
Slide 50
Solution to Exercise 8-4
Exercise
8-4
Slide 51
Exercise 8-5 - Current Calculation
 Location:
Richardson Bay Entrance
Golden Gate Bridge, CA
 Date:
 Time:
Aug 26, 2004
1100 PST
 Determine current speed and direction
 Need
• Prediction of Current and Direction form
• Current Table – Golden Gate Bridge
• Subordinate Station (Table 2) (#601)
Slide 52
Golden Gate Bridge
Slide 53
Table 2
Slide 54
Solution to Exercise 8-5
Exercise
8-5
Slide 55
Exercise 8-6 - Current Calculation
 Location:
.75 mi E of Angel Island
Golden Gate Bridge, CA
 Date:
 Time:
Sept 24, 2004
0100 PST
 Determine current speed and direction
 Need
• Prediction of Current and Direction form
• Current Table – Golden Gate Bridge
• Subordinate Station (Table 2) (#586)
Slide 56
Golden Gate Bridge
Slide 57
Table 2
Slide 58
Solution to Exercise 8-6
Exercise
8-6
Slide 59