[Ozee Adventure]

Tides; Tidal Heights; How much water under the boat; Tidal curves at standard ports

Repeat...[Same deal... I'm not asking anyone to do my homework for me - I'll be searching for a paragraph & picture for each thing I have to learn. I'll leave it here for others who are learning too.
If you have anything to add - thanks in advance.
OA]
Eeek :( I have to go take a deep breath & think about how to pull this one off!

[mad]

This should get you started.



In most places there is a delay between the phases of the Moon and the effect on the tide. Springs and neaps in the North Sea, for example, are two days behind the new/full Moon and first/third quarter. This is called the age of the tide.[13]

The exact time and height of the tide at a particular coastal point is also greatly influenced by the local bathymetry. There are some extreme cases: the Bay of Fundy, on the east coast of Canada, features the largest well-documented tidal ranges in the world, 16 metres (53 ft), because of the shape of the bay.[14] Southampton in the United Kingdom has a double high tide caused by the interaction between the different tidal harmonics within the region. This is contrary to the popular belief that the flow of water around the Isle of Wight creates two high waters. The Isle of Wight is important, however, as it is responsible for the 'Young Flood Stand', which describes the pause of the incoming tide about three hours after low water. Ungava Bay in Northern Quebec, north eastern Canada, is believed by some experts to have higher tidal ranges than the Bay of Fundy (about 17 metres or 56 ft)[citation needed], but it is free of pack ice for only about four months every year, whereas the Bay of Fundy rarely freezes.

Because the oscillation modes of the Mediterranean Sea and the Baltic Sea do not coincide with any significant astronomical forcing period the largest tides are close to their narrow connections with the Atlantic Ocean. Extremely small tides also occur for the same reason in the Gulf of Mexico and Sea of Japan. On the southern coast of Australia, because the coast is extremely straight (partly due to the tiny quantities of runoff flowing from rivers), tidal ranges are equally small.

[Ozee Adventure]

mad, on Feb 26 2008, 09:13 PM, said:

...bathymetry...

Jesus wept – hope I don’t have to spell that in an exam… I need a few more breaths & I’ll take the paper bag! :lol:

[Simon T]

Ozee Adventure, on Feb 26 2008, 11:24 AM, said:

Jesus wept – hope I don’t have to spell that in an exam… I need a few more breaths & I’ll take the paper bag! :lol:

I think if you put down Bathymetry in a Day Skipper exam, they'll kick you out for being an RYA ringer.

[johnnysaint]

Ozee Adventure, on Feb 26 2008, 11:24 AM, said:

Jesus wept – hope I don’t have to spell that in an exam… I need a few more breaths & I’ll take the paper bag! :lol:

You won't have to go into all the previous posters detail. All you need to know is "the rule of 12ths" . How to read tide tables, so you can work out how much water you will have at any given place, so you don't "hit the bricks" or go aground on your own anchor.
Are you in Airlie or Hammo? You are in QLD I've worked out.

[Ozee Adventure]

What are the different types of Tides
When the sun and moon are aligned, there are exceptionally strong gravitational forces, causing very high and very low tides which are called spring tides, though they have nothing to do with the season.
When the sun and moon are not aligned, the gravitational forces cancel each other out, and the tides are not as dramatically high and low. These are called neap tides.

Spring Tides
When the moon is full or new, the gravitational pull of the moon and sun are combined. At these times, the high tides are very high and the low tides are very low. This is known as a spring high tide. Spring tides are especially strong tides (they do not have anything to do with the season Spring). They occur when the Earth, the Sun, and the Moon are in a line. The gravitational forces of the Moon and the Sun both contribute to the tides.
Spring tides occur during the full moon and the new moon.

Neap Tides
During the moon's quarter phases the sun and moon work at right angles, causing the bulges to cancel each other. The result is a smaller difference between high and low tides and is known as a neap tide. Neap tides are especially weak tides. They occur when the gravitational forces of the Moon and the Sun are perpendicular to one another (with respect to the Earth). Neap tides occur during quarter moons

 tidechart2.jpg (15.91K)
Number of downloads: 1  tidechart3.jpg (14.48K)
Number of downloads: 1

[johnnysaint]

Ozee Adventure, on Feb 27 2008, 07:17 AM, said:

What are the different types of Tides
When the sun and moon are aligned, there are exceptionally strong gravitational forces, causing very high and very low tides which are called spring tides, though they have nothing to do with the season.
When the sun and moon are not aligned, the gravitational forces cancel each other out, and the tides are not as dramatically high and low. These are called neap tides.

Spring Tides
When the moon is full or new, the gravitational pull of the moon and sun are combined. At these times, the high tides are very high and the low tides are very low. This is known as a spring high tide. Spring tides are especially strong tides (they do not have anything to do with the season Spring). They occur when the Earth, the Sun, and the Moon are in a line. The gravitational forces of the Moon and the Sun both contribute to the tides.
Spring tides occur during the full moon and the new moon.

Neap Tides
During the moon's quarter phases the sun and moon work at right angles, causing the bulges to cancel each other. The result is a smaller difference between high and low tides and is known as a neap tide. Neap tides are especially weak tides. They occur when the gravitational forces of the Moon and the Sun are perpendicular to one another (with respect to the Earth). Neap tides occur during quarter moons

tidechart2.jpg tidechart3.jpg

Here's a good question for you OA.

Why, when there is no moon (when the moon and sun are both almost in line on the same side of the earth - i.e. a spring tide) is there a corresponding spring high tide on the opposite side of the earth when there is nothing there to pull the water?

I have been trying for years to find a satisfactory answer. I have a theory but it's probably way out.

[Ozee Adventure]

johnnysaint, on Feb 27 2008, 05:34 PM, said:

Here's a good question for you OA.

Why, when there is no moon (when the moon and sun are both almost in line on the same side of the earth - i.e. a spring tide) is there a corresponding spring high tide on the opposite side of the earth when there is nothing there to pull the water?

I have been trying for years to find a satisfactory answer. I have a theory but it's probably way out.

Does your theory look anything like this?

The second tidal bulge
The cause of the second bulge on the opposite side of the Earth to the Moon, is generally not well understood. The second bulge is due to the lack of gravitational attraction by the Moon on the waters on the far side of the Earth. The solid Earth is closer to the Moon than the water on the far side of the Earth and is thus attracted more strongly towards the Moon than the water on the far side of the Earth. This difference in attraction between the solid Earth and the water shows up as the second bulge as the water gets left behind!

Resource: TEACHERS' ONLINE PRIMARY SCIENCE SITE
© State of Victoria (Department of Education, Employment and Training)

[johnnysaint]

Ozee Adventure, on Feb 27 2008, 07:41 AM, said:

Does your theory look anything like this?

The second tidal bulge
The cause of the second bulge on the opposite side of the Earth to the Moon, is generally not well understood. The second bulge is due to the lack of gravitational attraction by the Moon on the waters on the far side of the Earth. The solid Earth is closer to the Moon than the water on the far side of the Earth and is thus attracted more strongly towards the Moon than the water on the far side of the Earth. This difference in attraction between the solid Earth and the water shows up as the second bulge as the water gets left behind!

Resource: TEACHERS' ONLINE PRIMARY SCIENCE SITE
© State of Victoria (Department of Education, Employment and Training)

I can't quite get my head around that. It doesn't quite gel, and as they said "is generally not well understood."

My theory (and it is only mine) is that while the moon rotates around the earth, the earth to a very small degree rotates around the moon. I.E. They rotate around each other as a unit with the centre of rotation of the unit being slightly off the earths centre. This (if it is how it works) would cause a centrifugal force, causing the water to bulge on the opposite side to the moon.

I obviously spend too much time sitting on my boat, gazing at the moon & stars, drinking rum and thinking.
A bloke shouldn't think. Its dangerous. Drinking rum is not.

NASA would have the answer somewhere but I don't feel inclined to spend hours trying to find out.

Anyone else got a theory? or maybe an answer?

[BalticBandit]

Bear with me, this might explain things

There is no such thing as "centrifugal force". There _IS_ [a href="http://en.wikipedia.org/wiki/Centripetal_force"]Centripetal Acceleration[/a] but that's actually backwards from what most people think.

So whats the difference? What people CALL "centrifugal force" is the "pull to the outside" of a spinning object. But if there is such a pull, then it would pull you straight out from the center of the circle. Yet anyone who has whirled a ball on a string knows that when you let go of the string, the ball does not fly straight outwards, but instead goes off at a 90deg angle to the radius. If there were Centrifugal force, it could not instantly stop the moment you let go of the string so something else MUST be going on.

That something else is Centripetal Acceleration. Newtons First Law of Motion says that unless an object is acted on by an unbalanced force, it will either stay at rest or in the motion it currently has. A better way to say this is that the object will follow whatever relative velocity VECTOR the object has, be it zero or non-zero.

So in the case of the ball on the string, when you let go, you cease imparting a force on the ball, so it keeps its existing velocity vector, which is a straight line TANGENT to the circle you were spinning it in. What keeps the ball moving in a circle is that at every instant you are exerting a force 90deg to the existing velocity vector of the ball TOWARDS THE CENTER of the circle (that's the diff, the actual FORCE is TOWARDS THE CENTER, not from the center outwards).

OK so how does this apply to the second bulge.

So imagine a fairly heavy ball. you spin it so that on one side of you it goes a bit faster and the string straightens out, but on the other side you pull a little less hard and spin a little slower and it droops. Less CENTRIPETAL Accel - ie less force TOWARDS THE MIDDLE.


Now instead of a ball you have water, and instead of string it is gravity. Without the sun and the moon, there would be no tides. So first lets use only the moon.

If you combine the mass of the moon plus the earth, the Center of Gravity would end up not in the center of the earth but off to the side the moon is on. Clearly water will be pulled to this side of the earth.

But what about the water on the completely opposite side of the earth? Gravity pulling water towards the earth is clearly less since the force of gravity varies as the inverse CUBE of the distance from the object. So if there is less gravity pulling the water, that is the same as not pulling as hard on the string with the ball. IE there is less Centripetal Force or Acceleration towards the center of the earth.

Less pull means that it piles up higher and bulges. And hence the high tide opposite the moon's position.


Now in a spring tide, the sun and moon are aligned so the combined center of gravity is even FURTHER to one side of the earth, and hence the pull on the sun side is even stronger (higher tides) and the gravity is proportionally even weaker on the far side.

In a normal tide, the moon is off to one side so there is some change in the center of combined gravity but not nearly as much.

And on a neap tide the moon is in opposition to the sun and the center of combined gravity is closest to the physical center of the earth and hence all the bulges are the most evened out.


Did I just make this clear as mud?

Key thing to think about THERE IS NO SUCH THING AS CENTRIFUGAL FORCE, there is only Centripetal Acceleration TOWARDS THE CENTER of the circle or the earth. Everything else follows from that.

[sockeye]

If i weigh x at low tide( a spring tide OK) do I weigh x+.00000000000x at high tide?

It is somewhat imponderable to attribute the action of the tides to the motion of the moon and sun in the sky, but the observable relation is undeniable. the problem I have with the and the gravity is proportionally even weaker on the far side. part of your statement is that it is not proportionlly much further away. remember that a spaceship orbiting the earth every 90 minutes takes about a week to get to the moon when you let go of the string. I think that there is a more einsteinian explanation. Some of that gravitaional effect is masked or diminished by the earth itself. the bending of light and gravity is as well demonstrated as the tides.

[johnnysaint]

HUH ??

[Ozee Adventure]

More... (I don't claim to have a position yet!)...
The Earth experiences two high tides per day because of the difference in the Moon's gravitational field at the Earth's surface and at its center. You could say that there is a high tide on the side nearest the Moon because the Moon pulls the water away from the Earth, and a high tide on the opposite side because the Moon pulls the Earth away from the water on the far side. The tidal effects are greatly exaggerated in the sketches.

 mtid.gif (11.68K)
Number of downloads: 5

[Ozee Adventure]

Folklore often alludes to the "pull of the Moon" -- probably because coastal civilizations have always noticed that the ocean's rhythmic rise and fall mostly follows the Moon's, rather than the Sun's, motion and position. Although the Sun's gravitational influence has an immense effect on us (we orbit the Sun and not the Moon), tides move to the tune of the Moon. But why does the Sun have the greater gravitational pull and the Moon the greater tidal influence?

Tides are caused by the difference between the Moon's gravitational pull on one side of Earth compared to its pull on the other because of its proximity to Earth. The Sun is so distant -- 400 times farther away from us than the Moon -- that there isn't much difference in its gravitational pull at different points on Earth. In other words, the Sun's gravitational influence is almost the same in Russia as it is in Australia as it is in Canada, and so on. The Moon, however, is so close to us that there's a big difference between the gravitational pull on the side of Earth nearest the Moon and on the side of Earth farthest away. This difference in the Moon's gravitational effect is the tidal effect.

Since oceans are fluid and uncontained, they flow toward the side of Earth nearest the Moon. This creates a high-water bulge. A second high-water bulge occurs simultaneously on the side of Earth opposite the Moon. The second bulge exists because the Moon doesn't orbit the center of Earth. Rather, Earth and the Moon swing as a unit around the center of their combined mass. The midpoint of their weight is located much closer to Earth, which is 81 times heavier than the Moon. This center of mass, or barycenter, :o sits about a thousand miles beneath Earth's surface at whatever point is facing the Moon at any particular time.

As both Earth and the Moon complete their orbit around the barycenter every 27.32166 days, the side of Earth farthest from this point experiences the fastest motion. Whirled around as if on a carnival ride, the oceans there are whipped centrifugally and rise upward as if being partially hurled away, creating the second high-water bulge.

Because Earth rotates under these high-water bulges, high tides move around the world in a daily cycle: About every 12-1/2 hours, there's a new one, which gives every beach on Earth roughly 2 high tides a day. Since the Moon is also moving around Earth, the high tides arrive almost an hour later each day throughout the lunar cycle.

Thus we have two high-water bulges, and two high tides -- one on the side of Earth facing the Moon and the other on the opposite side. Neither is caused by the Moon actually pulling on water.
http://www.almanac.com/tides/index.php

[Ozee Adventure]

If gravity is always pulling towards the moon, what causes the bulge on the opposite side of the earth?
Most people think the moon rotates round the earth. In reality, the earth and the moon rotate about a common centre just inside the earth's surface (indicated by the light blue dot on the diagram). At the centre of the earth the two forces acting: gravity towards the moon and a rotational force away from the moon are perfectly in balance. They have to be otherwise the earth and moon would not stay in this orbit.

The 'tide-generating' force is the difference between these two forces. On the surface of the earth nearest the moon, gravity is greater than the rotational force, and so there is a net force towards the moon causing a bulge towards the moon. On the opposite side of the earth, gravity is less as it is further from the moon, so the rotational force is dominant. Hence there is a net force away from the moon. It is this that creates the second bulge away from the moon. On the surface of the earth, the horizontal tide generating forces are more important than the vertical forces in generating the tidal bulges.
Web site of the Proudman Oceanographic Laboratory (POL)
 earthmoon1.gif (9.13K)
Number of downloads: 1  bulge.gif (5.63K)
Number of downloads: 1
Just realized its Wednesday & I'm posting pics titled "bulge" - sick, sick chick :lol:

[mad]

Good to see this is keeping you busy OZ, looks like your going pass this thing with no problems

[Ozee Adventure]

mad, on Feb 27 2008, 08:56 PM, said:

Good to see this is keeping you busy OZ, looks like your going pass this thing with no problems

Hope its not multiple choice... by the end of this I'll have attitude & opinions!

[johnnysaint]

"A second high-water bulge occurs simultaneously on the side of Earth opposite the Moon. The second bulge exists because the Moon doesn't orbit the center of Earth. Rather, Earth and the Moon swing as a unit around the center of their combined mass. The midpoint of their weight is located much closer to Earth, which is 81 times heavier than the Moon. This center of mass, or barycenter, sits about a thousand miles beneath Earth's surface at whatever point is facing the Moon at any particular time."


I can't believe it. I was actually on the right track.

Just goes to show that wisdom can be gained from a bottle of rum

[steveromagnino]

Ozee Adventure, on Feb 27 2008, 05:37 PM, said:

More... (I don't claim to have a position yet!)...
The Earth experiences two high tides per day because of the difference in the Moon's gravitational field at the Earth's surface and at its center. You could say that there is a high tide on the side nearest the Moon because the Moon pulls the water away from the Earth, and a high tide on the opposite side because the Moon pulls the Earth away from the water on the far side. The tidal effects are greatly exaggerated in the sketches.

sorry, this will screw u up... not all of the world experiences two tides per day.

It might be two opposing forces cancelling things out to get just one tide per day but for sure the nett result is...one tide per day in some parts of the world like here in Thailand.

diurnal tides I think they are called.

It is bizarre as in NZ the tide kind of flows in and out fairly constantly with an ebb I think it is called in the middle.

Here it can be almost all the way in, then suddenly over a few hours go out...and then stay like that for ages..then rush back in. It is totally weird.

Kind of like Matthew McCounaughy's hair.

[Ozee Adventure]

steveromagnino, on Feb 27 2008, 09:13 PM, said:

sorry, this will screw u up... not all of the world experiences two tides per day.
It might be two opposing forces cancelling things out to get just one tide per day but for sure the nett result is...one tide per day in some parts of the world like here in Thailand.
diurnal tides I think they are called.
It is bizarre as in NZ the tide kind of flows in and out fairly constantly with an ebb I think it is called in the middle.
Here it can be almost all the way in, then suddenly over a few hours go out...and then stay like that for ages..then rush back in. It is totally weird.
Kind of like Matthew McCounaughy's hair.

Oh great - I bet there's more "B" words too!
I'm going to bed & I'm taking Matthew with me...

[Ozee Adventure]

To interpolate between high and low water heights we use the Rule of Twelve.
We assume the tidal curve to be a perfect sinusoid with a period of 12 hours.
The height changes over the full range in the six hours between HW and LW.

• During first hour after HW the water drops 1/12th of the full range.
  
• During the second hour an additional 2/12th.
  
• During the third hour an additional 3/12th.
  
• During the fourth hour an additional 3/12th.
  
• During the fifth hour an additional 2/12th.
  
• During the sixth hour an additional 1/12th.

Hence, two hours after the HW the water has fallen 3/12 of the full range.

The Rules of Twelfths
1 Hours of Tide Cycles 1 2 3 4 5 6
Hourly Rise 1/12 2/12 3/12 3/12 2/12 1/12
Cumulative Rise 1/12 3/12 6/12 9/12 11/12 12/12
Percent of Rise 8% 16% 25% 25% 16% 8%

To interpolate between spring and neap tides we use the Rule of Seven.
Since the change from spring range to neap range can be assumed linear (instead of sinusoid), each day the range changes with 1/7th of difference between the spring and neap ranges.
Hence, the daily change in range is (spring range - neap range)/7.

[Ozee Adventure]

Before you decide where to anchor there are a few considerations to take in to account.

Changes in depth: When you anchor in an area with a rise and fall of tide, it is essential that you know that there will be sufficient depth for the vessel at low tide.
This is ensured by calculating the height of tide when you arrive, subtracting the height of low water to find the fall of the tide, then adding this to the draught of the boat and the required clearance below the keel at low water. See the example below:


Height of tide on arrival = 5.0m

- Height of low tide = 1.5m

Fall of the tide = 3.5m

+ Draught of vessel = 1.4m

+ Clearance required below the keel at low water = 1.0m

Minimum depth of water in which to anchor = 5.9m

This has been called the FUD rule. This stands for FALL+UNDER+DRAUGHT.

The yacht then motors around the area, surveying the depths with the echo-sounder and the skipper selects a spot where the depth of water is 5.9m or more. Obviously, the chart can be consulted first to find an area where the depths are likely to be suitable for anchoring, but the final spot is chosen by the above method (when checking the depth, remember to allow for the swing of the boat).

 scope.gif (4.59K)
Number of downloads: 1

[BalticBandit]

This assumes that your Depth Sounder is calibrated to the waterline of the vessel. In vessels with variable draught, your F+U+D, D needs to be normalized to the calibrated waterline of the vessel. Thus if your vessel normally draws 20', and the sounder is calibrated for thism but the vessel is extra heavily loaded and now draws 24', your normal F+U+D calculation will be off by 4' because the sounder will be measuring from 4' deeper.

My dad had a case where a slightly heavier load, plus extra "squat" caused by a higher than normal speed, caused a ship to bottom out on a shoal. The ship's owner claimed the CG had mismeasured/mismarked the chart. It turned out to be Skipper error.

[BalticBandit]

steveromagnino, on Feb 27 2008, 11:13 AM, said:

sorry, this will screw u up... not all of the world experiences two tides per day.

It might be two opposing forces cancelling things out to get just one tide per day but for sure the nett result is...one tide per day in some parts of the world like here in Thailand.

diurnal tides I think they are called.

It is bizarre as in NZ the tide kind of flows in and out fairly constantly with an ebb I think it is called in the middle.

Here it can be almost all the way in, then suddenly over a few hours go out...and then stay like that for ages..then rush back in. It is totally weird.

Kind of like Matthew McCounaughy's hair.

We see this some of the time up here in Puget Sound as well. Its a combination of the Bathyogrpahy (deep and long sound with lots of rivers and "benches" near the entrance that reduce flow). Essentially its the inertial mass of the water that will cause this. When there is a big ebb or flood, the momentum of the water flowing out of the sound 60miles to the south will take time to turn around, so the intervening tide will be somewhat overwhelmed by that flow

[Ozee Adventure]

TIDAL TERMS:

Mean Sea Level (MSL)
The average level of the sea surface over a long period or the average level which would exist in the absence of tides.

Mean High Water Springs (MHWS) and Mean Low Water Springs (MLWS)
The average of the levels of each pair of successive high waters, and of each pair of successive low waters, during that period of about 24 hours in each semi-lunation (approximately every 14 days), when the range of the tide is greatest (Spring Range).

Mean High Water Neaps (MHWN) and Mean Low Water Neaps (MLWN)
The average of the levels of each pair of successive high waters, and of each pair of successive low waters, during that period of about 24 hours in each semi-lunation (approximately every 14 days), when the range of the tide is least (Neap Range).

Chart Datum (CD)
A water level so low that the tide will but seldom fall below it. When meteorological conditions are such that sea level is lowered, the tide will fall below the predicted low water heights, and at a place where Chart Datum is at a comparatively high level, the actual depths at or near low water may be considerably less than charted.

Highest and Lowest Astronomical Tide (HAT AND LAT)
The highest and lowest tidal levels which can be predicted to occur under average meteorological conditions over 18 years. Modern chart datums are set at the approximate level of Lowest Astronomical Tide (LAT) and Tide Tables list the predicted height of tide above Chart Datum. It should be noted that water level may fall below the level of LAT if abnormal meteorological conditions are experienced.
 hield.jpg (31.14K)
Number of downloads: 3