Tides and Coastal Navigation

Posted by Director of Education on October 5, 2015 under About NauticEd, Bareboat Charter, Coastal Navigation, Crew, Skipper, Storm Tactics, weather | Comments are off for this article

ATTN: The NauticEd Coastal Navigation Course has been upgraded and updated. See below.

If you like that we update things for free, LIKE us over there —->

Snap Test:

(1) At Eastport, Maine. What was the max spring high tide height after the eclipsed super full moon on September 28th 2015. What was the min spring low tide and was it below the datum? What will be the height of the tide at noon today – Oct 5th?

(2) You live in SanFrancisco. You’ve got friends in town and you want to take them sailing today. What are the best times to take them out of the Bay under the Golden Gate bridge and back?

(3) How often does a spring tide occur and how could you predict it?

(4) Can the water level ever get below the chart datum? Why so or why not?

ANS: posted below – see where we got these plots (in less than 10 seconds)

eastporttidessep28

Eastport tides oct5

ANS: Goto http://tidesandcurrents.noaa.gov/waterlevels.html?id=8410140&units=standard&bdate=20150928&edate=20151005&timezone=GMT&datum=MLLW&interval=6&action=

(1) The Max was 22.424 ft at 1700 GMT = noon EST 2 days after the full moon. The min was minus 1.818 feet (below the datum) at 23:30 GMT (6:30pm EST).

(2) The prediction is 3.675 above the datum

(3) Best go at low water slack time as the water will be flowing back into the bay after you dawdle around outside the bridge. This is right at about noon. See http://tidesandcurrents.noaa.gov/waterlevels.html?id=9414290&units=standard&bdate=20151005&edate=20151006&timezone=LST&datum=MLLW&interval=6&action=

(4) The USA sets the datum at MLLW which is the mean of the spring low tides over the 19 year cycle lunar solar. UK and the rest of the world set the datum at LAT which is the lowest astronomical tide meaning it should be the lowest it could ever get over the 19 year cycle. Thus often using MLLW the water level can drop below the datum. Using LAT it is less likely but can still happen.

The above images were taken from http://tidesandcurrents.noaa.gov/stations.html?type=Water+Levels and the WorldTides 2015 iOS App respectively.

These questions are a breeze when you know what you are doing and the data answers are at your finger tips on your phone or on the Internet within seconds, if you know what you are doing.

One of the really cool things about eLearning software is that you can upgrade a course on demand – you can do a big update or a little one and the update goes instantly to your students. You don’t have to wait until the inventory is sold out and you don’t have to leave schools holding old inventory to be thrown out.

Last week we did a huge upgrade to the Coastal Navigation course. Mainly because we added in lots of new technology about tides and currents but we also added better explanations of plotting courses using animations.

Understanding of tides and currents have come a long way and websites have been automated to include instant data and tide predictions. Older courses and textbooks make you rely on looking up charts (on paper) – but why would you do that on a daily basis when the exact data is at your finger-tips. Off course, you must understand the fundamentals and we teach that but now we also give you access and knowledge on how to use apps and websites for instant data. It’s what a modern sailing course should do!

Students who have taken our older course now have the benefit of the new course at no cost. Just sign in to NauticEd now and go. You can retake the tests and get up to date on latest coastal navigation techniques and understanding.

Take the NauticEd Coastal Navigation course now for $39.

Learn the theory of course plotting, how to do it and make it second nature, how to measure distances, predict ETAs, account for current flow in course plotting, calculate current flow rate and direction, determine water depth relating to tide, best times for harbor entry, understand GPS, using parallel rulers, bretton plotters, buoys-markers-ATONS (aids to navigation), lights etc etc. Lots and lots of real examples and plotting challenges. You use a real chart. At the end of this course you will have completed the World’s most up to date Coastal Navigation Course and will fly through any other required course like the USCG Commerical Captains License navigation course.

Get free updates for life. Access the course for life. Take the test as many times as you want.

Coastal Navigation Course

 

Take the NauticEd Coastal Navigation course now for $39.

Oh and the other cool thing we did was to add in the requirement to have passed either the FREE Navigation Rules course or  the Navigation Rules Module in the Skipper (or RYA Day Skipper) course. This ensures everyone taking this course is up to date on Navigation Rules. It was the responsible thing to do. We did this by adding this piece of code to our software.

IF FREE Navigation Rules Course = Passed
OR IF Skipper Course OR RYA DAY Skipper Course = Passed
AND IF Coastal Navigation Course = Passed

THEN Add Coastal Navigation to the Certificate and the Resume

Sailing Preparations Phuket Thailand – Tides

Posted by Director of Education on March 12, 2015 under Bareboat Charter, Coastal Navigation, Crew, Skipper | Comments are off for this article

It’s one day out before we leave for Phuket, Thailand. We’re going to be bareboat chartering a Cat 4600 from the Moorings. Today I’m doing some final prep work. I have downloaded the World Tides 2015 App for $2.99. When I arrive and during the trip I want to be on top of the tides (pun intended).

Here is a screen shot directly from the App for the day we head out from the Moorings Base.

phuketTides

So looks like we are going to be dealing with some pretty hefty tides and we need to take care. Notice here also the pic of the moon showing it is new thus we should expect even higher and lower tides a few days later with a spring tide. Potentially we’re looking at 3 meters. That’s a lot. With such tides we also need to be thinking about currents. (No we are not heading out at 3am – that’s just in the middle of the screen shot. We’ll plan on an early departure as soon as our eyes open and 2 cups of fresh coffee and fresh fruit are in the tummy)

Particular attention needs to be taken when anchoring. And this may mean an adjustment of the anchor rode length during the night. With this App onboard it’s going to be pretty convenient. We’ll know exactly what phase of the tide we’re in at anchoring time and can plan accordingly around the depth. This makes me feel very confident that we’ll not have any issues with tides. Even little things like beaching the dinghy we’ll know how far up the beach to pull it. What about tying the dinghy to the pier? Don’t forget about that one?

Do you really understand Tides? Are you CONFIDENT AND COMPETENT? You should take our Coastal Navigation Course. We talk all about tides and predictions and what to do – and we talk about currents and how to calculate courses based on known currents etc etc.

Take the Coastal Navigation Course now also available as an iPad App

Coastal Navigation Course

Coastal Navigation Course

oh and I’ve also downloaded the Navionics Apps for the local area

and for weather and winds I’ll be using pocket GRIB app.

How to solve a tide height using the rule of twelves

Posted by Director of Education on February 17, 2015 under Bareboat Charter, Coastal Navigation, Crew, Skipper, weather | Comments are off for this article

If you like this post – please LIKE it on facebook or g+1 it thx – it really helps us grow

The Rule of Twelves Tide Plot Curve

Here is a tide curve plot for any tides that follow the rule of twelves (note that none do exactly but it can be a decent rough approximation for some semi-diurnal locations).

Tap on the image and you can download a PDF that you can print out, laminate and keep on your boat with erasable markers.

A rule of twelves tide plot

A rule of twelves tide plot

NOTE:

If you are going to use this you had better make sure that the tide at the location of interest actually approximates this curve. Too many sailing instructors and sailing associations teach that tides follow the rule of twelves. They DO NOT. It can be a decent approximation in some circumstances. You are far better off to use an actual plot for that location using real data.

 

Below are some good example problems with solutions to follow so that you understand exactly how to use a tide curve. The QR scan code will lead you to these problems as well. Thus, if you forget and you are on your boat, just scan the QR code with your mobile phone and the example problem will show. A QR code scanner is being embedded into the new NauticEd Sailing App .

http://www.nauticed.org/sailing-blog/can-you-solve-this-tidal-problem/

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

Take our Coastal Navigation sailing course to REALLY understand tides

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

If you like exercises like this we will post more – like it on facebook or g+1 it.

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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 the Rule of Twelves

Posted by Director of Education on September 27, 2014 under About NauticEd, Bareboat Charter, Celestial Navigation, Coastal Navigation, Crew, Skipper, weather | Comments are off for this article

If you like this post – please LIKE it on facebook and g+1 it – it really helps us grow. Thanks!

 Understanding Tides

Tides are often intimidating to sailors and so, many sailors tend to try not to sail in tidal areas or just ignore them hoping that if they stay deep enough there is little to worry about.

The point of this article is to de-intimidate you about tides by explaining the predictive nature and how to access information easily about the local conditions.

But first let’s start with an intimidating situation to set the worst case then we’ll take it up from there.

On a recent trip to the UK we visited Ilfracombe in the south west coast of England. This whole area and further up the coast towards Wales exhibits one of the highest tidal areas on the planet. Ilfracombe has a 9 meter tide – that’s 30 feet for imperial speakers. The point of the visit was to experience this, get the photo and write about it. What we didn’t plan on was a real life threatening situation – not really but sort of – close(ish) – kinda -could have gotten bad – well, at least wet. Read on.

Here is a pic with the tide mostly in in Ilfracombe.

 

The tide is in

The tide is in – boats happily floating

Here is the same place a few hours later.

 

The tide is out

The tide is out – boats sitting on the bottom high and dry

9 meters – 30 ft is huge. Ilfracombe as a normal diurnal tide meaning there are two high tides and two low tides every day. From high tide to low tide is around 6 hours and 20 minutes. The rate of water movement for all that to take place over 6.3 hours is hard to conceptualize. As we walked along a flat beach – it seemed like every lap of the waves caused the water to progress up the beach about 6 inches. To experience it is an awesome wonderment.

Throughout the day as we watched, rocks covered and uncovered and some turned into rather large islands from nothing.

Not only is the vertical movement of the tide a concern, but the currents produced from the moving water is of a potential greater concern. We went out fishing on a kayak and were constantly weary of our position moving relative to the land and of the swirling directions of the current flow. 300 meters out we were being pushed in one direction quite fast, 2-3 knots, but closer in we were drifting in the opposite direction at 1-2 knots. And most of the time it was hard to even get the line on the bottom due to the current. According to the extremely hospitable local we stayed with and took us fishing, every year many people have to be rescued from the current. Mostly just due to the under estimation of the conditions. I’m a pretty good and fast swimmer, it takes me about 20 minutes to knock out 1 km in the pool with no waves. That’s swimming at about 1.5 knots. Thus even I would be swept out faster than I could swim.

So what actually happened was that one of our party climbed down onto some rocks to get a better place to fish. An hour later he turned around and was completely cut off from the land by the rising tide. Since it was about half tide the water level was rising quite quick and was pouring over a weir about 6 inches of flow height into a basin created by a walk way that was completely covered and thus stopping his escape. We saved his sole by dropping me off the two person kayak onto land and Martyn returning to go get him. Actually pretty funny and I was grateful it happened to create the story here but… this is the stuff that you hear about. My friend was simply not used to and did not expect such conditions from any experience he has had in the past. Night time and a few waves and cold water and it could have been lights out.

The Rule of Twelves

Slack water occurs at or close to high and low tide. At this time the water is hardly moving and so fishing and swimming can be safe (ish). The highest current times occur at or close to ½ tide. However there is not a linear scale. It’s on a sinusoidal scale and is approximated by the following rule of twelves.First, the rule of twelves is not entirely accurate but it’s not a bad way to think about a rising or falling tide.

The tide rise and fall approximates a sinusoidal curve i.e it starts out slow then increases the fastest at about half tide then slows down to a very slow finish.

See the following animation (best viewed with Chrome or Safari). This is a tide rising over a 6 hour period using the rule of twelves as a basis for tidal rise.

 

When you push the play button the rising tide goes slow and not much has happened in the first hour, But by 3 the tide is rising fast. In the last hour the tides is rising to it’s final completion slowly.

Thinking linearly you would say that the tide has to rise 30 feet over approx. 6 hours that means 5 feet per hour. But the animation shows 2.5 feet in the first hour. Between 3 and 4 it averages 7.5 feet per hour and the last hour is 2.5 feet. This then is how the rule of twelves helps you approximate a tide in your head.

With the Rule of Twelves you start out with 1/12th rise in the first hour. Then 2/12ths rise in the second hour then 3/12ths rise in the 4rd and 4th hour. Then back down to 2 /12ths in the 5th hour and 1/12th in the 6th hour.

This is an ok approximation to the sine curve. In addition, tides are only approximated by a sine curve. So realize when applying this rule, that it’s an approximation. But not too too bad actually.

In our Ilfracombe 30ft (9m) example here is what the rule of twelves looks like and assuming a 6 hour tide (which it is not there it is 6:20 between high and low tide)

End of hour

12ths incr total 12ths inc

tide height ft

1

1 1 2.5

2

2 3

7.5

3

3 6 15

4

3 9 22.5
5 2 11

27.5

6 1 12

30

Now in reality take a look at this below. This is a screen shot from the World Tides App for Ilfracombe on Sept 27 2014. It shows an 8.1 meter (26.6 ft) tide rise on that day. Low tide is at 2:20pm and high tide is at 8:40 pm. The low tide is 1 meter above the printed chart reported datum. Thus the rise in height that day will be 8.1 meters. (Chart Datum in the UK and most of the world is set at lowest attainable tide levels (LAT) in the USA it is set at Mean Low Low Tide (MLLW)).

 

Tide at Ilfracombe on Sep 27th 2014

Tide at Ilfracombe on Sep 27th 2014

I then plotted the rule of twelves prediction on top of this tide shape. You’ll notice a fairly decent difference.

Tide with rule of twelves plotted

Tide with rule of twelves plotted

Now in addition – note that the blue area rise and fall of the tide is as predicted and not actual. However, since tide data is so well known, you are well served by assuming the predicted tide will be very close to actual. More on that in a minute.

So really what does this all mean. It means carry with you your iPhone or Android and have it loaded with a tidal App.
Tidal App
Here’s the link for the iOS version:
World Tides 2014
And here is the Google play store search results for tides:
Android – Tides search

 

De-Intimidating Tides

With a tidal App you have accurate access to the tide heights at any time of the day in virtually any port in the world. You can see if the tide is coming in (flooding) or going out (ebbing). If you know your eta at a port then you can simply pull out your Tide App and see what height the tide will be.

In addition – most tide Apps DO NOT need a connection to the internet after it is initially downloaded. The data is stored on your device. How is this so? Tides are wonderfully predcitable and we have got them so figured out that the accuracy is usualy with in a few inches at any time. This is because for hundreds of years now we (humankind) have been measuring the height of the tide at points all over the world and comparing those measurements to the positions of the Sun and the Moon. Since the Sun and Moon exactly repeat their combined cycle every 19 years, the tides repeat themselves to the exact heights and cycles every 19 years. It took a lot of work from a lot of people BUT now we have it. Tide prediction is accurate and the data is at your finger tips 24/7.

Don’t be afraid of tides.

In our Coastal Navigation course we have a very detailed discussion of tides going through the theory of prediction, how to predict at subordinate stations and lots of discussion about how to handle current flow due to tides. It is the best Coastal Navigation course available.

Take our Coastal Navigation Course

Coastal Navigation Course
Coastal Navigation Course

 

 

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