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 Sport Phillip Marine has free Tide Charts of Port Phillip Bay available.

TIDAL FORCES     The moon and sun exerts a gravitational force on the earth - the effect of the moon being more significant. Since the water of the oceans is fluid it moves toward the moon. Furthermore the earth moon combination is rotating in space causing a net centrifugal force on the earth away from the moon and the oceans thus also move away from the moon. The net effect is to cause higher water (a high tide) on those parts of the earth both closest to the moon and on the opposite parts furtherest from the moon The high tide will be at a maximum directly underneath the moon and get gradually lower at points further away from directly underneath.
     As the earth also rotates relative to the moon once every twenty four hours fifty minutes we have a semi-diurnal tide. This means that every half day (12h 25 min) we have a complete cycle of high and low water. Added to this motion is the constantly changing latitude of the moon (called the declination). This causes a difference in tidal heights between consecutive high waters.
     There is also an effect by the sun which has only about half the influence of the moon. The sun sets up another cycle of 12 hours similar to that of the moon but with the sun's declination cycle taking a year to complete.
When the two patterns are superimposed there is a significant change in the resultant tide pattern. This is quite a simplistic view and within this there are many complex cycles. For example the moon's phases repeat every 19 years - the metonic cycle and within this there are a further 10 cycles and one even larger one taking 600 years

 


SPRING & NEAP TIDESSPRING TIDES

 

     When the effects of the moon & sun are combined they sometimes work together and at other times work in opposition. Twice in a lunar month the sun & moon are in line with each other producing higher and

 

lower tides. This occurs at the full moon and the new moon and the tides so produced are called spring tides and are not related to the seasons. When the moon is at A it is said to be in conjunction and at B in opposition. The tidal force is the same whether in conjunction or opposition.

 

NEAP TIDESWhen the sun and moon are working in opposition to each other the net effect makes the high water relatively lower and the low water relatively higher. These are called neap tides

 


METEOROLOGICAL EFFECTS      To calculate tides based just on the effects of the sun and moon with the attendant cycles is no mean feat. Superimposed on this is the effect of land masses disrupting the flow of the ocean - this is predictable and local knowledge allows accuracy to be preserved. However meteorological conditions are far less regular or predictable. 
     Tides are computed for normal seasonal barometric pressure. When the actual pressure is outside this there will be a commensurate change in sea level. A difference of 34 mb will change the predicted sea level by about 0.3 m. This effect is neither immediate or local. It would need a change in pressure over a long time and over a wide area for it to become noticeable - except in the case of a sudden and very intense low pressure system where the effect could be immediate. A low pressure will produce a high on a high tide and on its own the effect is not a problem. However coupled with storm surges and / or strong winds the effect can be important.
    The effect of wind blowing onshore will cause an increase in the heights of tides and an offshore wind will produce the opposite effect. Winds blowing along a coastline can set up long waves travelling along the coast which can have quite disastrous results when coupled with other effects.
     A combination of low pressure and strong wind blowing along the coastline can set up a storm surge. These effects can be seen in large bights where due to shallow water effect the tidal wave is slowed down to the same speed as that of the storm. The tidal wave is then amplified and can give heights in excess of 3 metres on top of the predicted high water.
    Seismic waves are usually caused by under-sea earthquakes and have no connection with tidal forces. Known asTsunamis these waves can have speeds up to 500 Knots and can be very devastating when they reach shallow water where they can build up to over 15 m in height.

 


 


TIDE HEIGHT & THE RULE OF TWELFTHS     There is a simple method of calculating the height of the tide by first knowing the height and times of the successive high and low tides or vice versa. Assuming a time interval of 6 hours between the top and bottom tide and knowing the time of the last high or low tide by applying the rule of twelfths in the table the approximate water height can be found for any time.

 

HOUR or SEGMENT No

 

1

 

2

 

3

 

4

 

5

 

6

 

TWELFTHS / HR
(Rise or Fall)

 

1

 

2

 

3

 

3

 

2

 

1

 

TOTAL CHANGE

 

1/12

 

3/12

 

6/12

 

9/12

 

11/12

 

12/12

 

     For example if the tide range is to be 1.2 M and the water height is required for 4 hours after the last high tide the level will be Nine-Twelfths of 1.2 M below the high   ie 0.9 M below the last high water.
     Where the difference between high and low tides is significantly different from 6 hours divide the time difference into six equal segments and then apply the rule to each segment.

Sport Phillip Marine services the boating, yachting & fishing needs of the Mornington Peninsula from Frankston, Daveys Bay, Mt. Eliza, Mornington, Mt. Martha, Safety Beach, Martha Cove, Dromana, Rosebud, Rye, Blairgowrie, Sorrento and Portsea on Port Phillip Bay to Yaringa, Warneet, Somers, Hastings, and Flinders on Westernport Bay.

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