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

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Sometimes…

•

You need to travel across shallow areas

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You must do so when there is sufficient water

•

You will need to anticipate depth

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At some time other than MLLW Slide 2

What causes Tides

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Gravitational pull

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Moon and sun

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Sun’s influence only 46% that of moon

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Moon

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24 hr 50 min (apparent orbit of Earth)



Tides delayed ~ 50 min every day

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Centrifugal force

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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

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Most of Earth

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Two bulges per day = two tides per day Called “semi-diurnal” tides (diurnal means daily)

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Bulges not equal

• •

Due to geometry Means one daily tide greater than other

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Some cases

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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

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Vertical datum

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Reference level on chart

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Typically MLLW for depths MHW for vertical clearance

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Soundings

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Title block – indicates references “Soundings in feet” from MLLW (typical - coastal charts)

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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

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Internet

•

NOS (NOAA)

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Software

• • •

WXTide (freeware) Tidetool (Palm) Tideware

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GPS

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Built-in tide data Chartplotters

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Tables

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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

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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

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Rule of Twelfths

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Need to know time of high and low Need to know tidal range

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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

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Look-up Specified Date (reference station)

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Bracket desired time

• •

High or Low Tide just before

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Time Height Low or High Tide just after

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Time Height Slide 28

Tide Height Calculations

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Slide 29

Determine Range for Location

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Look-up – Subordinate Station

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Note Time Differences

• •

Adjust time of high Adjust time of low (ref H + or - = local high) (ref L + or - = local low)

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Note Height Differences

• •

Adjust height of high Adjust height of low (ref ht x ratio * (ref ht x ratio * = local high) = local low)

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Compute Actual Range

•

Tidal Range = (Local High – Local Low)

* Note: some subordinate stations provide differences (+) or (-) – add or subract rather than multiply

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Determine the Depth

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Apply Rule of Twelfths

•

Hourly increments

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0, 1hr, 2hrs, 3hrs, 4hrs, 5hrs, 6hrs

•

Compute Change in Tide Height

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High to Low: Ht of High Tide 0, 5, 25, 50, 75, 95, 100% of Range

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Low to High: Ht of Low Tide + 0, 5, 25, 50, 75, 95, 100% of Range

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Determine Depth

Depth Clearance = Sounding + Height Tide - Draft (from chart) (Rule 12ths) (boat specs)

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Alternate Method

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If predicted heights for day not available

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Use “Mean Range”

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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

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Exercise 7-1 - Tide Calculation

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Location: Chappaquoit Point West Falmouth Harbor, MA

  

Date: Sept 1, 2004 Desired Time: 0500 EST Draft: 2.5 feet



Determine Depth below the Keel

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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

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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)

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Determine if you can clear the bridge

•

Vertical clearance of bridge: 27 ft

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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

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Exercise 7-3 - Tide Calculation

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Location: Date: Monument Beach Aug 22, 2004 Desired Time: 1930 EST

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Determine height of tide

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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

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Exercise 7-3

Solution to Exercise 7-3

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Exercise 7-4 - Tide Calculation



Location: Budd Inlet Olympia Shoal, WA

 

Date: Aug 22, 2004 Desired Time: 1930 PST



Determine Height of Tide

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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

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Exercise 7-5 – Bridge Clearance

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Location: Sneeoosh Point

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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

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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?

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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