Can you solve this tidal problem?

Posted by Director of Education on January 15, 2015 under About NauticEd, Bareboat Charter, Coastal Navigation, Crew, Skipper | Comments are off for this article

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Tidal Exercise

On Wednesday October 16th you are going to sail past this port in the morning. There is a shallow area you’d like to pass over. The tidal information you have obtained is as such.

low tide 1.6 m high tide 3.3 m

The chart says the depth of the water of the shallow area is 1 meter. You draw one and a half meters and you would like 1 meter below the keel for safety. Summer daylight savings is in effect.

The Tidal Curve for this port is as follows: (click image for downloadable PDF)

 

Tide Curve

Between what times in the morning can you safely pass over the area?

ANSWER:

If you’re just reading this for the first time – think about the question and try to answer it  before cheating and dropping straight to the answer.

>>>>>>>>>>

Sorry about this but it was sort of a trick question to get you thinking. Many who emailed us before the deadline of Jan 31st 2015 to win $10 credit towards a NauticEd class got it right – congrats.

Next – apologies for a little ambiguity – the problem did not list if the tide height took into account daylight savings in the tide height listings. Normally they don’t but sometimes they do so Kudos to those who accounted for/discussed  this in their answer. We developed the problem using tide heights of NOT using daylight savings – and thus 0131 is 0231, 0752 is 0852, 1427 is 1527 and 2039 is 2139. Actually it doesn’t matter in the answer really because we were only discussing the morning.

To solve: (don’t cheat if you have not answered yet)

First off you need the water to be 2.5 meters deep. 1.5 meters in draft and 1 meter for safety.

Next you need to realize that the reported tide height numbers in a tide table are always listed as the height above the MLLW (mean low low water) datum. So at 0231  the height of the low tide water was 1.6 m above the MLLW datum.

Next realize that chart depths are always listed as the depth of the water at the MLLW datum.

So tide heights and chart depth numbers use the same datum.

Note: USA uses MLLW while most other places use LAT (lowest Attainable Tide). Regardless both are using the same datum in this problem.

This means that at 0231 (low tide) the height of the water in the shallow area was 2.6 m (1 meter depth plus 1.6 meters tide). This is already deeper than the required depth of 2.5 m.

So since 0231 is low tide and the problem asked in the morning then for any  time midnight to 0231 the water height is higher than this (0231 is low tide).

Next take a look at the next low tide of 1527 (1427) which is 1 meter. This would not fit the 2.5 meter depth requirement as the depth of the water in the shallow area would be only 1m tide plus 1 meter depth = 2.0 meters. But again the question was “in the morning”. So the question is would there be a time prior to noon where the tide height is  lower than 1.5 m?

See below for the plot or the ebbing tide. The tide drops from 3.3 meters to 1.0 meters. And the curve to use is neaps since the difference between high and low is 2.3 meters (close to the mean range for neaps at 2.5m) (neaps are when the tide range is less due to the sun and moon not combining their effects – spring is when the sun and moon combine their effects to make the tides higher).

Draw a sloped line on the chart from 3.3 meters to 1 meter. Now drop a line down from 1.5 meters to the sloped line. Bring this intersection point across to the neaps tide curve for the descending tide. Add in the times starting with 0852 being the high tide and adding 1 hour per section. When you hit the neaps curve drop down to the time. The time shows 1301 (each tick is 10 minutes). Thus at 1301 the tide height will be 1.5 meters which is the threshold. Thus you’d have to be clear of the shallow area before 1301.

Since the question asked for the morning – the answer is anytime in the morning. If you made the assumption that the tide height time did include daylight savings then your answer would have been 1201 which still meets the question answer for anytime in the morning.

ANSWER – ANYTIME IN THE MORNING!

Tide Curve

Answer to the tide curve problem is anytime in the morning.

Understanding tides is essential.

Common mistakes made in the sent in answers to this question were:

  • Using the rule of twelves (no the tide curve was provided)
  • Not knowing how to use a tide curve
  • Assuming linear dropping of the tide
  • Not realizing that the same datum for tides and chart depth are the same.
  • Not reading the problem properly
  • And pure not understanding the concept of tides e.g. neaps, springs, sinusoidal type rise and fall

Here is a great Comment regarding extra practical thoughts around this problem given to us by Michael Sisley Instructor and free lance yacht skipper. Thanks Michael!

Our sport is fun when we plan in order to make it safe! 
1)Read the question. – in a harbour right? Flat water. 1m clearance to allow for uncalculated variables such as atmospheric pressure, on shore wind, shifting sands – good seamanship built into the plan. Yes 2m waves on the sandbar leading to the harbour entrance, an ebbing tide and a strong on shore wind would lead to a very different contingency. 
It’s all a matter of preparation and planning. 
1) Before setting out, check the your depth echo sounder with a lead line. 
2) Harbours silt up and sand bars shift with the tide. So contact the harbour master and ask “Where is the shallow patch now?” “How deep?” 
3) The hydrographical service publishes valuable information – use it! You can then use your calculation to help decide when it is safe to go. – And enjoy!

If you learned something in this exercise, perhaps you should take our NauticEd Coastal Navigation Course or take the NauticEd RYA Day Skipper Course and earn your ICC.

Huge congrats to those who solved the problem using the tide curve plot.

If you liked this problem – make sure you friend us on facebook – we will announce there when we post a new one. 

Here is the next one

http://www.nauticed.org/sailing-blog/another-tide-problem-to-solve/

Those who solved the problem were:

Instructors

  • Martin Silk
  • Michael Sisley

Students

  • Gregg K
  • Ted D
  • Markus R
  • Alain C
  • Paul W
  • Mike R
  • John B
  • James S
  • Ed J
  • Greg F
  • Abdellatif E

Thanks everyone else for being brave and giving it a go. I’m assuming you learned something and glad we were able to contribute to your knowledge.

Read our blog on the Rule of Twelves for tides

Tides and Tidal Currents

Posted by Director of Education on February 16, 2012 under Bareboat Charter, Coastal Navigation, Crew, Skipper, weather | Comments are off for this article

Tides and tidal currents came to mind today as I strolled along the harbor’s edge watching the behaviour of some sailboats racing. Remembering a recent race regatta series I participated in in the Auckland New Zealand harbor last winter also brought up this topic of tides and tidal current. In one race in the series, we were racing back up the harbor while the tide was ebbing (going out). Consequently, the current was racing in the other direction. Our tactic was to stick to the sides of the harbor as close as possible where the current is the least. Unfortunately all the other boats knew to do this as well and this created a pretty big mess of all the fleet tacking on top of each other. “Starboard” was the call of the day as each boat established their stand-on position over the other. Every now and then one boat would break out and try to brave the current instead of the tacking mess only then to rejoin the fleet as they were dragged backwards. It was pretty exciting actually, although our skipper was stressing a little.

There was pretty much nothing we could do except tack tack tack and keep a very diligent watch for traffic ducking and tacking to give way when required. The skippers were trading expletives with each other across the water more in this race than I’d seen in any other. LOL

How Tidal Flow Works in a Harbor

In a channel, current will run strongest in the deepest parts typically towards the center, unless there is a bend in the channel then the current will run strongest on the outsides. Just think about the last time you watch water flowing in a river to visualize. So your best bet when trying the go against the tidal current is to hug close to the sides and on the inside turn if possible. At an extreme case I had a friend in Sydney harbor who won a race by waiting out the worst part of the tidal current by throwing down the anchor. Not sure if that’s against any official race rules but it’s pretty funny.

Also take note that current flows “relative” to the tide period but slack water does not necessarily match high and low tide times especially in harbors. Tidal current is determined by the local effects of the upstream harbour shape and weather, not just the sinusoidal tidal period. That comes as a big revelation to some. In fact, I physically had to show my skipper prior to the start of a race one day last winter.

Real Example of Tides in a Harbor

Observe the following which is Auckland New Zealand harbour, one of the more heavily raced harbors in the world.

Chart of  in Auckland Harbour

Chart of in Auckland Harbour

 

Now look at today’s tidal period;

Sinusoidal Tide in Auckland Harbour

Sinusoidal Tide in Auckland Harbour

 

  • High tide: 3:52 am
  • ½ tide at 6:55 am ebbing (going out)
  • Low tide: 9:57 am
  • ½ tide 1:07pm flooding (coming in)
  • High tide: 4:16 pm
  • ½ tide: 7:22 pm ebbing (going out)

You might assume that minimum current occurs at high and low tides ie 3:52 am and 9:57 am and 4:16 pm while the max current occurs at ½ tides at 6:55 am and 1.07 pm

But now look at today’s current predictions:

Tidal Current in Auckland Harbour

Tidal Current in Auckland Harbour

 

  • Min current flow was at 3:15am (45 minutes before high tide)
  • Max current flow ebbing was at 5:16 am (1 hour 39 minutes before ½ tide ebbing)
  • Min current flow was at 8:15 am (1 hour 45 minutes before low tide)
  • Max current flow flooding at 10:54 am (2 hours 13 minutes before 1/2 tide flooding)
  • Min Current flow at 3:51 pm (25 minutes before high tide)
  • Max current flow ebbing at 5:57 pm (1 hour 25 minutes before ½ tide ebbing)

A quick analysis of this shows that the current matches in time the flooding tide more than the ebbing tide. This empirically supportis the statement above about how the upstream shape determines the current flow out.

As a specific example, lets say it is 8:30am on the day shown. From a tidal analysis you would think that the tide is ebbing and so an early morning race out of the harbour you’d probably stick to the centre of the channel. However the prudent sailor doing a current flow analysis would see that the current has already turned to flood and would stick to the sides of the channel. All things else being equal, prudence would win.

In General:

  • Don’t assume that the current is slack at high and low tides
  • Stick to the edges of the harbor when going against the flow

Navionics Electronic Chart

In this article I used the Navionics iPhone app. I pressed and held my finger over the diamond shaped T to get the tidal info and the diamond shaped C to get the current info. When you have such an electronic chart, look for these diamond T’s and C’s scattered through out. On iPhone and iPad simultaneously push the home button and the power button to get a screen shot.

Tide and Current icons on an electronic navigation chart

Tide and Current icons on an electronic navigation chart

Rule of Twelve

While we’re on the tidal topic I might  as well discuss the rule of twelve regarding a sinusoid. It’s a good general piece of knowledge to know since tidal heights generally follow a sinusoid shape (except in weird tidal places in the world like the Solent in England where two high tides occur about 1 hour apart).

In the first 1/6th of the time between high and low tides, the height changes by only 1/12th of the full amount

In the next 1/6th the height changes by an additional 2/12 (=1/6)

In the 3rd 1/6th ie half tide the height changes by an additional 3/12 (1/4)

Adding 1/12 + 2/12 +3/12 = 1/2. So at half tide, the height has changed to ½. That makes sense but looking back and assuming a diurnal tide (6 hours between high and low), in the first hour the height has only changed by 1/12th. That’s insignificant. At the end of the 2nd hour the height has changed by a total of 3/12ths = ¼. That’s still pretty insignificant.

What this means is that if you’re relying upon the tide to increase the depth in a shallow area, then even with a 10 ft (3m) tide, 2 hours after low tide, it has only come up 2.5 ft (0.75m). Best you wait until half tide at least when the ½ of the height change has occurred (5 ft (1.5m) in this example).

NauticEd Coastal Navigation Sailing Course

For a full discussion on tides, tide table, how tides work and why there are two tides in one day when the earth and moon only rotate relatively about each other once per day, take the NauticEd coastal navigation sailing course. You’ll also be able to brush up on your navigational skills which isn’t at all a bad thing.

Coastal Navigation Sailing Course

Coastal Navigation Sailing Course

This article was written by Grant Headifen, Educational Director for NauticEd Online Sailing School

How the Tides Work

Posted by Director of Education on July 3, 2010 under Bareboat Charter, Coastal Navigation, Crew, Skipper, weather | Be the First to Comment

As a new sailor, just learning to sail, the thought of figuring out tides can be quite intimidating. We know that even some seasoned sailors tend to just wing it – a dangerous concern. This learn to sail series article will dispelled any concerns about understanding, reading and predicting tides.

Gravitational Pull
Tides took a lot of figuring out by some very smart people over the ages including Galileo.

But now that the principle causes of tides are understood and quite well mathematically modeled, they are pretty easy to understand by the average Joe when explained properly.

Tides exist solely because of the celestial bodies the moon and the sun. The moon is relatively close but comparatively small.  The sun is far away but a giant when it comes to mass. Gravitational pull increases linearly with mass but decreases inversely with the square of the distance. So when you balance these out and do the calculations, the sun’s effect is 46% that of the moon.

So the moon has the dominant and most noticeable effect on the earth’s tides. However the Sun still has a significant effect.

There are a couple of tricks to understanding the tides that took the physicists quite a while to figure out in the old days. The moon circles the earth about every 28 days. During this time the earth completes a rotation every 24 hours. So in effect  from a relative point of view the moon appears to rotate around us every 24 hours and 50.4 minutes. Given that you’d think that there would be one high tide every 12 and a bit hours. But for most of us it seems that the tide takes about 6  and a bit hours to go out and 6 and a bit hours to come back in. That’s a frequency of 12 and a half hours not 24:50 hours. What’s up with that?

Tide is actually a gravitational concept rather than specifically related to the movement of our oceans. All celestial objects are susceptible to tidal effects from other “relatively close” bodies. Consequently a planet with in gravitational range of another body will experience pull leading to stresses acting to deform the shape of the planet.

That means that as the moon circles the earth, the earth is deformed by the moon. Similarly, the moon also experiences deformation by the earth and thus is suffers a tide effect. The effect is called tidal force. Fortunately for life on earth, the deformation is small but it does lead to the rising water that we experience. Therefore, with out the moon, life on earth would be vastly different.

As non celestial body and gravitational experts we’ll probably struggle to understand the resultant effect because it happens in a way that we might not initially expect.

Double Bulge Phenomenom

It is suffice for this description of tides to state that the planet doesn’t just bulge towards the distant body but actually bulges on both sides in a line between the center of the two bodies because of the volumetric stress imparted upon the planet. Various arguments still exist on why, some are flat out wrong and some are right. As sailors we really don’t care too much other than knowing the resultant.

To gain a visual appreciation for this, although not in anyway accurate in science, take a calamari ring and hold in two points opposite each other. Now hold one side still and pull the other side. You’ll see the ring bulge on both sides in a line of the direction of pull.

That is a similar resultant action to what the earth is experiencing. The bulge occurs on both sides of the planet diametrically opposite each other and in the plane of  a line between the moon and the earth. So following this, we have a high tide on the same side of the planet as the moon and simultaneously on the opposite side. Thus leading to a theoretical tidal period twice for every rotation of the moon about the earth.

The double bulge concept has nothing to do with the spin and inertia of water on the earth, although you will find some arguments and textbooks reporting this as a contributing factor to the opposite bulge. Not so! In any case, again as sailors we don’t necessarily care too much. We just need to know and realize that the double bulge occurs to explain two tides in a day rather than matching the periodicity of one moon rotation per day.

The exact same concept is valid for the sun. A solar high tide exists on the same side of earth as the sun and simultaneously on the opposite side of the sun.

Theoretical Tide Heights

Now let’s look at a few theoretical mathematical numbers for comparison. If the earth’s oceans were uniform in depth and no landmasses existed and just considering the moon and if the moon was rotating around the earth every 24 hours, plus a few other ands and ifs, then the theoretical rise of the water at high tide would be 54 cm (21 inches). Now consider the suns effect in the same way. This would lead to a theoretical tide rise of 25 cm (9.8 inches). If the moon and the sun aligned or were directly opposite (from the bulge theory above) then we could add these heights together and get 79 cm (31 inches). If the moon and sun were acting against each other by being 90 degrees apart, then we would subtract the numbers and be left with a tidal height of 29 cm (11 inches).

The phenomenon then of a spring tide has thus been explained. Spring tide is when the sun and moon align or are diametrically opposite. The word spring does not come from a relationship to the season but from the concept of “force”. Meaning there is more force during this period of alignment, which occurs about every 14 days – full moon and new moon.

Consequently, a neap tide is the minimal tide effect from the action of the sun and moon opposing each other. When they are at 90 degrees from each other – not diametrically opposite each other due to the double bulge effect. Again, this occurs at a period of every 14 days.

In addition to the above, the sun and moon operate in elliptical orbits relative to the earth. You can then imagine as the sun and moon become at their closest points to the earth that a greater effect would take place. The effect is increased 18% for the moon and 5% for the sun. Since the earth rotates about the sun once per year, at a certain times of the year then you would expect all these phenomena to exist simultaneously and create a theoretical tidal height of 93 cm (37 inches).

You can also imagine a common high high tide condition existing when the moon is closest to the earth with the moon and sun aligned (or diametrically opposite). This is called a perigee spring tide and occurs every 7.5 moon cycles.

This can be further expanded to include the sun being at its closest point along with the moon and in a spring tide situation. This occurs every 18.6 years. For this reason tidal high high and low low measurements are recorded and reported over periods of 19 years by statistical governmental recording agencies.

There is one last effect to discuss and that is that the sun and moons orbits are not directly aligned with the pole of the earth. Both the sun and moon’s orbits are not in unison and change day by day. For this reason the height of each high tide will vary day by day.

And just in case you’re wondering, Venus provides the next greatest tidal effect on the earth. But it is less than 0.001% of the magnitude of the Sun’s effect.

We’ve added a rudimentary animation of the Moon’s and the Sun’s tidal forces on the Earth at http://www.youtube.com/watch?v=XZNnb9YP1xg. The animation shows the Moon’s larger tidal force in blue and the Sun’s smaller tidal force in yellow turning relative to the Earth with different frequencies. You’ll need to view it several times through to see that when the bodies are aligned either on the same side or diametrically opposite, that the tidal forces align creating a spring tide. You’ll see Neap tide occur when they are 90 degrees apart. Of course in reality, the earth rotates about the sun but for relativistic purposes of an observer standing on the Earth, this demonstrates the effect. Additionally we did not simulate the 7.5 lunar periods before the cycle repeats nor did we show differing declination of the orbits or the elliptical shape of the orbits … and a few other things.

How the Tides Work

How the Tides Work

In Reality

So now let’s take out some of the theoretical arguments and start adding back in landmasses and differing ocean depths. The effect is called Bathymetry. These affects greatly affect the timing, tidal heights and periodic frequency of the tides at different places on the earth. Regardless of this, the tides still occur with periodic consistency in each location according to the above discussion of the moon and sun movements. This last sentence, matched with empirical data over time, means that tidal predictions are very accurate, both height wise and time wise.

Lets say that again; Tides heights and times are highly predictable!

Most places on the planet experience two high tides every 24 hours and 50 minutes as discussed above. This is called a semidiurnal tide. However due to bathymetry, some places experience one high tide every 24 hours and 50 minutes. This is called a diurnal tide and is experienced in the South China Sea, the Gulf of Mexico along with a few other places. Another significant departure from the theory is the experiencing of two differing height of tides. This is called a mixed tide and is experienced in quite a few places including the west coast of the United States.

Keep in mind that 100% of the reason for departure from a semidiurnal tide in these locations is solely due to Bathymetry. Bathymetry is also the reason for departure from the high tide being at exactly the same time as the moon at its highest apex in the sky overhead (or diametrically opposite the planet). As an example, in Norfolk Virginia USA, high tide is approximately 2 ½ hours before the moon passes overhead. Typically on the planet, a spring tide occurs 2 days after a full or new moon.

Bathymetry also explains why tidal height in some places is far greater than the theoretical predictions above. As water moves and slushes around the planet it does so with lunar (and semi lunar) frequency setting up a crowding effect of the water in certain places.

Regarding all the above discussion about high tides, for every high tide there is a corresponding low tide. As sailors, we’re pretty concerned about low tides and low low tides due to grounding issues. For mixed semidiurnal and semidiurnal tides, low tides occur every 12 hours and 25.2 minutes. Low low tides occur at spring tide every 14 days about 2 days after a full moon or new moon. And lowest low tides occur every 7.5 lunar cycles when there is a perigee spring tide situation.

The Tidal Map

Putting it all together, and considering the periodic consistency, bathymetry and real empirical data, NASA has developed a tidal map of the planet. The map is quite revelational and will help you understand tides even further.

Tidal Map of the Earth

Tidal Map of the Earth

Grey parts are landmasses. The colors on the map represent the amplitude (height) of the tide. Note the locations marked in red. These are well known really-really high tide areas. Some in excess of 40 feet (11m). Also if you’ve ever sailed in the Caribbean or Mediterranean, you’ll have noticed a distinct lack of any tide. These are marked in Blue.

Of particular interest for me is the west coast of my home country, New Zealand. Near Auckland, two estuaries, one from the west coast and one from the east coast are separated by about 500 meters of land only. Yet the tidal difference is hours apart and the two tide heights are vastly different. Similarly take a look at the area around Panama. Extreme high tides exist on the west coast and almost none on the east coast. The UK also has some very interesting tidal results particularly in the Solent area.

The highest tides in the world can be found in Canada at the Bay of Fundy, which separates New Brunswick from Nova Scotia. At some times of the year the difference between high and low tide in this Bay is 16.3 meters (53.5 feet), taller than a three-story building. Anchorage, Alaska, comes in at a close second with tidal ranges up to 12.2 meters (40 feet).

The white lines on the NASA map above represent one hour difference in time for a high (or low) tide. Note how the lines appear to be randomly placed around the planet with out any particular reason for their placement. Again this is due to bathymetry. Of note however is that over time, the lines are don’t move. For example the blue region in the middle of the south pacific where the white lines converge always experiences little to no tide. The high tide travels around this point in a clockwise direction. The amphidromic point where the lines meet defines the meeting point of high and low tide giving rise to a consistent zero tide effect. In fact, at most amphidromic points as such in the southern hemisphere, the tide turns clockwise due to the coriolis effect of the earth spinning. In the northern hemisphere at most amphidromic points the tide turns anticlockwise (counterclockwise).

What does this really mean? It means that if you consider the amphidromic point near Hawaii and the radical lines from it incident on the west coast of the USA, then the high tide must move north along the coast over time. IE the high tide in Los Angeles will be after the high tide in San Diego (San Diego being south of Los Angeles). Of significant value when understanding this is that if you can read a table showing the high tide in one place you can predict the time of the high tide in another location.  Look at the southern part of the south east coast of South America. The high tide occurs all along that region at almost the same time but over approximately the same distance along the east coast of New Zealand, that experiences 5 hours of high tide time difference. Interesting!

It is doubtful that even the most complex mathematical models could predict all this. However combining the empirical data with the periodic frequency of the two celestial bodies gives us every thing we need.

Practical Application of Reading And Understanding Tides

Whether we’re experienced sailors or just starting out in our learn to sail quest, chances are you’ll be sailing in an area where tides are happening.

One of the greatest reasons you need to understand and deal with tide heights is when you are anchoring. Anchor at high tide and you might find your self stuck tipped sideways on the ocean floor a few hours later. Anchor at low tide and with a lee shore and you might be blown onto the rocks when your anchor scope becomes less than sufficient to allow the anchor to hold.

Tides are also significant when dealing with bridges. Cleverly, charts list bridge heights as height above high tide not low tide.

There is some language around the practical application of tides that we now need to learn.

  • Ebb tide:             Water going down towards low tide
  • Flood Tide:         Water coming up towards high tide
  • Slack Water:       Times when the water movement due to tidal effects are minimal. Note that because of the time it takes for bays and estuaries to drain out, slack water is not necessarily at high tide and low tide. Consult current tables to learn slack water times.
  • MHHW:             Mean higher high water level. Average height of high tides at spring high tide
  • MLLW:              Mean Lower low water level. Average height of low tides at spring low tide
  • Chart Datum:      Reports of water depth on charts are taken at MLLW
  • Bridge Heights:  Reported on charts as the height above MHHW
  • Tide Current:      Flow of water due to tides

Below is a tidal prediction chart for a buoy position in La Jolla,  San Diego,  California. Note a few things: (1) The prediction is extremely close to the observed. (2) That it occurs twice per day – Semi diurnal (3) The heights are different with in the same day – Mixed Semidiurnal. (4) The low tide drops below the MLLW.

Tide Data in San Diego

Tide Data in San Diego

Rather than carry around masses of graphs as above, convenient tide tables have been produced by the maritime agency in almost every country that borders an ocean. Visit you local chandlery and they’re bound to have a copy of the local tide table. Or search on the Internet. In the USA go to http://tidesandcurrents.noaa.gov/tidesXX – where XX represents the last two digits of the year you want to explore.  Tide table are abundantly available to us.

So whenever we want to know the time and height of the tide we can just consult the tide tables. And from the above, we now know and are very confident that, they will give us a very accurate prediction. Tables are arranged to give us tidal heights and times at most well known ports called Reference Stations. Time and height adjustments are then provided to get predictions for lesser known ports that are called Subordinate Stations.

To predict your tidal information from written tables find the closest Reference Station, then make the time and height adjustment from that Reference Station to your closest Subordinate Station. Because of the power of computing, on the Internet, many Subordinate Station calculations are already done for you. In this case you can just look up the tide heights and times directly at your location. Most of us don’t carry Internet connected laptops on our boats so the ability to read and understand the table is paramount.

For example, from a table you might read the time and height of the low tide on the afternoon of January 15th 2010 in Los Angeles. But if you are further north in El Segundo in Santa Monica Bay, the tide will be slightly different. From the NASA developed tide map above you at least know that high tide will occur later because of the anticlockwise turn of the amphridomic point near Hawaii, but by how much time and by how much amplitude? Thus we consult an adjustment table.

Below is a typical tide table for Los Angeles. Click the image to go to this page on the tidesandcurrnet.nooa.gov website.

Tide in Los Angeles

Tide in Los Angeles

You can read (probably with glasses) that the afternoon low tide occurs in Los Angeles port at 3:46pm and is 0.7 feet below the MLLW chart datum.

Correction tables will show that for the Subordinate Station of El Segundo, the correction from the Reference Station of Los Angeles will be to add 13 minutes for low tides and to add 13 minutes for high tides. And to multiply the low tide reported in Los Angels by 0.96 and the high tide by 0.96.

Correction of Tide for El Segundo

Correction of Tide for El Segundo

Thus the afternoon low tide in El Segundo will be at 3:59pm and will be 0.67 feet below MLLW.

The below table confirms that result. Click the image to go to this page on the tidesandcurrnet.nooa.gov website.

Correction of Tide for El SegundoSo there you have it. You completely understand tides and can now use the table to predict tide times and heights.

Of considerable further interest is the currents produced from tides. These must also be understood, especially as sailors, because in many cases your sailboat can not sail as fast as the tidal current. San Francisco Bay is a good example of this. Module 1 of the NauticEd Skipper Course discusses Tidal Currents and their prediction.

This article was written by Grant Headifen, Educational Director of NauticEd. NauticEd is an online sailing school providing sailing courses and sailing certifications for beginner to advanced sailors.

Learn to sail online with NauticEd.