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
Now look at today’s tidal period;
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
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
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
This article was written by Grant Headifen, Educational Director for NauticEd Online Sailing School
Imagine if you could just hang out at the yacht club every day – how much you’d learn from everyone. That’d be cool. Well … now you can!
It’s a very cool piece of technology we just installed on the NauticEd site. It’s called DisQus and the concept is based on crowd intelligence. It shows how the power of the Internet can beat out a boring ol’ book. Thousands of websites have already introduced it and it’s ideally suited for you and NauticEd.
On every page through out all of the NauticEd sailing courses you can now discuss (Disqus) the topic at hand and read what others are saying about the topic. For example, lets say you know a few things about how to dock a boat using spring lines but are a bit confused about backing into a slip. Right in the course you can add your springing off knowledge and ask all other students their opinions on reversing. When any one comments and adds to those comments you’ll be sent an email (if you want). You can add pictures and diagrams if you want. Our part is to use the crowd intelligence to improve our sailing course material for everyone.
You can even invite facebook friends to join in on the conversation and help out.
Crowd-Intelligence with DisQus and NauticEd Sailing School
How cool is this? Now you’re tapping into the knowledge of thousands of other NauticEd students – wow that’s a big yacht club with a lot of combined experience. You’re not on your own any more. It’s not just us and our authors pontificating about sailing – it’s a real open discussion and conversation in real time.
But like any party or social – you can’t just stuff your mouth with cake and listen – you’ve got to add your two cents. And you can’t be rude because people are watching and the bouncers will bounce you out. So come on join in – ask questions and post your knowledge.
To kick off, I’ve gone in and asked a few questions and posted a few comments in each course topic. I invite you to join me and start new conversations. Like who gives way – the paddle board or the sailboat? Do you know the answer?
Login and give us your opinion to the Rules of the Nautical Road topic embedded in our Rules course.
And to celebrate the launch of crowd intelligence via DisQus, we’ll award a free sailing course of choice to a student randomly picked from everyone who participates in the conversations over the next week (through May 25th) . Hint, the more you talk the more we’ll notice.
Last weekend we met up with our friends Chris and Christine Ellsay in Nelson New Zealand. Chris and Chris, with their three kids aged 10, 8 and 6 are sailing around the world and it was refreshing to hear them say – “I don’t know how long we’ll take”. They’re 3 years into it and have made it from the great lakes in Eastern Canada to New Zealand so far. The route has been via the Caribbean, Venezuela, Columbia, Panama, Galapagos Islands, Marquesas Islands, French Polynesia, Tonga and now Kiwiland. (We missed them by a week when we were in Tonga with the NauticEd Graduation Trip in September last year.)
Holding out in New Zealand for the summer while the tropical cyclones pass overhead in the pacific islands, they say they’re returning to the Pacific, starting with Fiji in Late April 2011. Then they’ll decide if to hang another year in the pacific or head off to the top of Ausy through the Indian ocean in 2011 or 2012.
I interviewed Chris and Chris (and the kids) on their experience with a catamaran rather than a monohull for sailing around the world. Their opinion after 10,000 miles is that they would not have done it any other way. The comfort and space was the resounding feedback.
Here’s a short video introducing Stray Kitty a World Cruising Life Style, and Abel Tasman National Park In New Zealand.
Here’s a few pics of Stray Kitty, their 42 foot PDQ Antares 2002 Catamaran.
Stray Kitty in the Nelson Marina
The foredeck at anchor is a great place for a few gins after a hard day sail.
Foredeck of Stray Kitty 42 ft Catamaran
The Kids are being home schooled by Christine and by the sounds of it – they were way ahead of where they should be – good job Christine!
Kids sailing around the world - pretty cool kids
These three kids (my one is the 2 1/2 year old 2nd to right ) are pretty amazing – they fear nothing, do their school work, do as they are told, release the lines on command, know which electrical switches to flick on at the right time – in fact I think they’d make it back to land if mum and dad fell overboard. They’re pretty cool kids and are a delight to spend time with.
Plenty of room inside the catamaran for school work
The Catamaran has heaps of room inside and it’s easy for the kids to do their school work underway because the boat stays flat when sailing.
Stray Kitty is sailing the traditional route around the world following the trade winds. Chris reported that much of their sailing has been downwind and so here he is showing me his much used bowsprit for flying their Gennaker. Oh and by the way – notice the incredible bay that we stayed overnight in – in the back ground in Abel Tasman National Park at the top of the South Island.
The Catamaran Bowsprit
There is plenty of safety gear on board and Chris and Chris are doing it right. Notice all the the MOB gear at the stern of the boat ready to be instantly deployed should anyone go overboard.
The boat has on-board a generator, two alternators and solar panels for powering all the electrical requirements of the boat. The total solar production capability is about 500 watts. Chris says for every thing to maintain with out the use of the generator or alternators – he’d like to have about 1000 watts of solar capacity so they do have to kick on the generator every now and then.
Solar Panels on the hardtop of Stray Kitty
Chris also discussed with me his Internet connections via SSB and his weather information gathering capability. Here he has downloaded a GRIB which is a map forecast of the sailing area we were in. The expected forecast was for 35 knots and they got it right. Out sailing we saw it peak to 36 knots on the wind meter. Made for some fun sailing.
Downloading the Weather GRIB
And the kids loved the bumpy ride that day as you can see here.
High waves making the trampoline a fun place to be
And here’s us busting through the 1-2 meter swell.
Crashing through the waves sailing the catamaran
Over the 4 days we spent with these true ocean sailors, we had a blast (beyond the 36 knotter). We scored some amazing shots of the Able Tasman National park in New Zealand which will be on the next blog. Stray Kitty will be making the passage up to Auckland via the east coast in a few days but first they’ll have to wait for right weather conditions to cross one of the world’s renown rough water ways, the Cook Straight which lies between the North and South Islands. High winds and current can make this one a bit tricky.
We’re pretty jealous of Stray Kitty. One of Chris’ sayings over the weekend was the “we regret in life more things that we don’t do than what we actually do” and this was one of the big reasons they sold their business and set out across the oceans and wow they had some good stories to match.
If you’re thinking about sailing around the world then we’d certainly recommend our more serious NauticEd sailing school sailing lessons associated with the Captain’s Rank, those are Safety at Sea, Storm Tactics, Weather, Sail Trim as well as – if you think a Catamaran might be the way to go – take the Catamaran Sailing Confidence Clinic.
It was great to see this family making fun light work of sailing around the world. It’s certainly got me thinking – any one else?
Torrent Bay - Abel Tasman National Park New Zealand
Today, NauticEd Online Sailing School announced its release of the NauticEd Captain’s Rank. This coincides with the posting of the NauticEd Safety at Sea Clinic which is the final required course to attain the Sailing Certification Rank. The NauticEd Captain’s Rank focuses entirely on sailboat operations both near shore and offshore and is directed specifically towards the recreational sailboater.
Until now, many recreational sailboaters have been gaining a commercial boating license to attain the educational equivalence of Captain but with out the intention of operating commercially. Now with NauticEd, students can gain a Captain’s Sailing Certification with out jumping through the significant hoops associated with a commercial operator’s license.
This is very exciting for the sailing industry says Grant Headifen, Educational Director for NauticEd. ‘It means that we can have more educated boaters on the water and the investment cost in the education is well within reach of every sailboater. We’ve lowered the barriers and made the experience fun and interactive with multimedia learning. Now, if anyone wants to learn to sail, gain a sailing certification or just increase their sailing education, doing it online makes it more accessible and thus more likely to be done”. The Educational investment in the Captain’s Rank is less than $US300.
Headifen estimates it will take the average student 60 hours of study over time to complete the theory courses and online tests associated with the NauticEd Captain’s Rank. The NauticEd online Courses required to gain the rank cover a wide breadth of topics listed as follows:
Skipper
Maneuvering Under Power
Coastal Navigation
Bareboat Charter
Sail Trim
Storm Tactics
Weather
Safety at Sea
Captain's Rank bundle of Sailing Courses
In addition, a NauticEd Captain must have logged a minimum amount of real sea time which is denoted by a level associated with the Rank as follows:
Captain Level III - 50 days of sea time;
Captain Level IV – 100 days of sea time
Captain Level V - 200 days of sea time.
Time is logged on NauticEd’s online sailing logbook and can be accessed via iPhone and Android apps or on an internet connected computer.
NauticEd which stands for Nautic Education offers 2 lower level Sailing Certifications; Skipper and Bareboat Charter Master. These are achieved by passing fewer courses than listed above. NauticEd also offers other online courses such as a Catamaran Sailing Confidence, Celestial Navigation, and a Crew Course.
To learn more sailing tips from NauticEd Sailing School visit our website.
Hooowa – said Al Pacino in the movie Sent of a Woman.
Skipper Sailing Course Bundle
We recently launched a new Sailing Courses and Clincs Page. The first thing you’ll notice is that we have Bundled the Courses and Clinics into their respective Ranks. So now you can buy the Skipper Bundle which contains the
Skipper Course
Maneuvering Under Power Clinic
And the Bareboat Charter Master Bundle which contains the
Skipper Course
Maneuvering Under Power Clinic
Coastal Navigation Clinic
Bareboat Charter Clinic
The investment in the Sailing Lesson bundles now saves you a ton. The Skipper Bundle comes in at $95 and the Bareboat Charter Master Bundle comes in at $161. That is a significant savings over buying eaah sailing course A La Carte.
Bareboat Charter Master Bundle
We’re also finishing up the Safety at Sea Clinic now and so that will make the final Captain Bundle ready. This will contain
Skipper Course
Maneuvering Under Power Clinic
Coastal Navigation Clinic
Bareboat Charter Clinic
Sail Trim
Weather
Storm Tactics
Safety at Sea
The investment in the Captain Bundle will be $293.
AND – we did something that is really cool – we wrote the software so that you automatically get credit for sailing courses and clinics that you have already purchased. So the smart ones will figure out now how to beat the system (and we’re ok with that) that you can start getting the Clinics for $33 instead of $39.
Visit the new NauticEd Sailing Courses and Clinics page now and learn how you can get your yacht license.
Whether you’re learning to sail or are an advanced sailor, this tip is a really good one. It’s basic stuff for any competent crew but you’ll see the value pretty quickly, especially if you don’t like going up the mast.
Recently on our NauticEd flotilla to Tonga I semi embarrassingly re-learned a valuable lesson and that’s the value of a stopper knot.
A stopper knot of course is a knot at the end of a halyard or jib sheet that will prevent the line from pulling through fairleads etc and always keeps your line accessible in its proper place. It’s tied in such a manner that you can get it undone – as opposed to the definition of a knife knot which means you need a knife to undo it. And on a sailboat the old saying of “if you can’t tie a knot tie a lot” doesn’t really work).
Well with all the excitement of Tonga and the flotilla and sailing a really nice 47 foot Catamaran chartered from the Moorings in Vava’U Tonga, I neglected to do the walk around to ensure every appropriate line had a stopper knot at the end.
The result? No so good. At the end of the first day I pulled down the sail and tookt he halyard connected to the head of the mainsail and wraped it under a cleat at the base of the mast. I do this as standard operating procedure so that the sail can not get lifted back up during the night should a big wind come through. Well unfortunately, the halyard was designed to be long enough to accommodate this extra precaution and the end of the halyard disappeared up into the mast. Opps!
I just stood there looking at it going “ok that’s not good” and wondering how I was going to keep it a secret. Not possible. My call back to the Moorings Base was heard by the other captains in the flotilla and so I was instantly awarded the first blue duck of the sailing trip.
A blue duck award is a sailing faux pas certificate that is handed out to captains who make such silly mistakes and has become a tradition on the flotillas I run. It’s kinda like a yellow card in soccer but not that bad.
So fortunately the Moorings Base people were extremely understanding and sent out a chase boat. We all stood around wondering what trick they have up their sleeves to fix this interesting problem.
Hoisting up the mast
Well no trick really, we winched one of them up the mast who pushed the halyard back down inside the mast, then the other poked a piece of bent wire into the hole in the mast from when the halyard is supposed to come out. He hooked the wire in behind the halyard. However strangely enough the hole is too small for a doubled over halyard to come out and so the top o’ the mast guy had to slowly pull up the halyard until the bottom o’ the mast guy could see the end. Then with the wire still hooked behind the halyard, he pulled it out the hole in the mast. It was a pretty intricate operation and one that would certainly be impossible in rough seas. Just imagine that, hoisting someone up the mast in a storm and intricately retrieving the halyard all because you didn’t check for a stopper knot! I’m hoping this blog report will help you not have to do that.
Fetching the Halyard from the Mast
Moral of the story? Check your stopper knots and if I operated a charter company (as I used to do) I would sew a small stitch to lock in a stopper knot into essential lines. When you do this however make sure that it’s not a line where you will need to get the knot out quickly in an emergency situation.
BIG NOTE: Don’t put stopper knots in spinnaker sheets and guys. These lines need to be dumped out fast in the likely event of a broach.
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
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
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
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
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
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.
So 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.
During the Americas’ Cup campaign in New Zealand in 2003, I saw one of the best explanations of this on a TV interview with the Greg Butterworth, the Tactician for the Alingi Team.
Most of us sort of understand the concept and we’ve been left with the answer of “Well – weather helm is better because it’s safer.” But few explanations go into how it gives your boat a sailing advantage.
The definition of weather helm and lee helm is simple and it is easy to remember which is which. If you have a tiller, weather helm is when you have to pull the tiller to weather (toward the wind) in order to keep the boat going in a straight line. Lee helm is when you push the tiller to lee (downwind) in order to keep the boat going in a straight line. We’ve probably all felt this slight pressure required on the tiller when underway.
Your boat can be tuned to give weather helm or lee helm. Rake the mast forward and you move the center of effort of the wind forward which causes your boat to want to turn downwind. Rake the mast back and you move the center of effort of the wind back causing your boat to want to go upwind to weather.
When your boat gets rounded up – you just experienced massive weather helm. No matter how much you pull the tiller to weather, you can’t stop the boat going to weather. Dumping the main sail moves the center of effort forward thus reducing the weather helm.
The basic perception of weather helm being safer comes from this effect: if you let go of the tiller, it will automatically go to center because of the water flowing over the rudder and because the rudder is pivoted at its leading edge. Now there is no rudder force to counter the desire of the boat to turn up wind to weather so the boat does exactly that. It turns to weather and rounds up slowing the boat down and reducing forces on the rig. Conversely, lee helm means that if you let the tiller go the boat will turn away from the wind, heel over more increase forces on the rig.
So from a safety point, weather helm is good. BUT there is another advantage that we’re not generally taught. Holding the tiller to weather means that there is a slight pressure on the rudder to windward. This actually MOVES THE BOAT TO WINDWARD as it slices through the water. And we all know what that means, race advantage!
The Weather Helm Advantage
The illustration shows how the water pressure from weather helm creates a sideways force on the rudder tending to push the boat to weather.
Now Greg Butterworth went on to explain that there are other cool things you can do. One is to put a little trailing edge swinging control surface on the keel.
The illustration below shows this effect too. For us pilots, this is much like a trim tab on a wing of a small airplane. The trim tab creates the ability to adjust the lift at that point on the aircraft and thus create a balance of forces. The issue to remember here is that you’d need to trim the tab the other way when you tack over.
A control surface on the keel
So there you have it. While we’ve all been understanding the lifting effects of the wind over the sail, the other fluid that we’ve ignored is the water under the boat and how we can gain lift from it too.
Next time you’re out sailing on a nice steady 10 knot breeze, come up on a close haul, trim the sails perfectly so that all your tell tails are flying smoothly. Then notice what pressure you’ve got on the helm. Note that if you’ve got a wheel, weather helm will be a tendency to apply downwind turning pressure on the wheel (which is the same as pulling a tiller upwind right?). Ideally you should have slight weather helm. If not, you should probably not jump right in and start raking your mast back. Talk to a mast tuning specialist in your area first.
This is day 4 of 6 in your introduction to NauticEd International Sailing School. Watch the video and/or read the text below.
Today we’ll help you discover which courses are best suited for you personally.
We developed a series of 10 quick questions that will easily eliminate any confusion about which course you should be taking for your own personal sailing education.
Run the personal course recommendation tool now. You’ll save money by investing in only the courses right for you. Getting Started with Multimedia Training
The first courses we believe that all sailors should complete no matter what their experience level is the FREE NauticEd Rules of the Nautical Road and the FREE Basic Sail Trim Course. These courses are already loaded into your curriculum.
If you’re an experienced sailor you’ll see the value in a quick refresher course. If you’re new to sailing then you’ll learn some VITAL nautical rules and sail trim knowledge.
In either case, these courses are free and thus you’ll be able to see how taking a NauticEd clinic and the associated test will work. Both are a highly graphical and fun 20 minute courses.
If our free courses gained your confidence in us, then you may have already invested in either the full Skipper, Bareboat Charter Master or Captain’s bundle of courses. Most people do this eventually because it saves money and sets them on the right track to a proper Sailing Certification recognized by the world’s largest yacht charter companies. When you invest in a bundle you’re automatically given appropriate financial credit for any courses you’ve already taken.
Ranks and Courses
In email #2 we discussed the ranks Crew, Skipper, Bareboat Charter Master and Captain. Here’s how you work through these Ranks:
Right Now Your Rank is: [Database!Rank] – [Database!Level]
Gaining the Qualified Crew Rank
Qualified-Crew-card
You are awarded the Qualified Crew Rank when you pass either the Skipper Course or the Qualified Crew Member Course and are level I experience qualified.
QUALIFIED CREW MEMBER COURSE: Learn to sail and contribute as a crew member on a modern cruising sailboat. Learn the lines, sailing terminology, sail trim and rules of the road. Estimated time: 7 hours total. Investment: $37.50.
Gaining the Skipper Rank
Skipper Card
In addition to the two courses below, you must be at least Level I experience qualified.
The SKIPPER SAILING COURSE is a beginner to intermediate sailing course. It is a prerequisite to any certification and covers the fundamentals that every one must know. The total time needed to complete this course will be about 20 hours. Investment $67.00
The MANEUVERING UNDER POWER CLINIC: This is our most popular course. An absolutely essential maneuvering and docking course that will save you thousands in dents, bumps and scratches at the marina. Want to dock your boat like a pro every time? Want to impress? Take the most popular NauticEd Sailing School Course now. Estimated time: 3 hours total. Investment: $39.
The investment in the Skipper Bundle of courses is $95 instead of $106.50 a la carte.
Gaining the Bareboat Charter Master Rank
Bareboat Charter Master Card
In addition to the Skipper Rank and the three courses below, you must be at least Level III experience qualified.
BAREBOAT CHARTER CLINIC: Taking a sailing vacation? All hands on deck – this is the yacht charter sailing course for you and ALL of your crew. Make your charter sailing trip more enjoyable by getting ALL the bareboat charter tips you’ll need. Estimated time: 5 hours total. Investment: $39.
COASTAL NAVIGATION CLINIC: Learn to navigate your sailboat. If you plan on sailing away from your home base or are taking a sailing vacation, you need this course. NauticEd Sailing School makes navigating a sailboat – a breeze. Estimated time: 10 hours total. Investment: $39.
ELECTRONIC NAVIGATION CLINIC: This Electronic Navigation course is the world’s only true interactive course where you learn all the instruments you might have onboard a sailboat. The exercises are designed so that you actually interact with a simulated GPS chart plotter and get inside the workings to REALLY understand how to maximize the information being presented to you. With ease, you’ll implement navigation techniques like setting your autopilot to track a waypoint or tack perfectly on a layline. Estimated time: 6 hours total. Investment: $25 or FREE when you invest in the BBCM bundle.
ANCHORING A SAILBOAT CLINIC: Whether you are sailing your own vessel in coastal waters or chartering in the Caribbean or beyond, knowing how to safely and effectively anchor is one of the most essential and liberating skills you can have. Knowing about anchors, rodes, anchorages and anchoring techniques is a prerequisite for enjoying an evening in a magically beautiful setting as well as getting a good night’s sleep while swinging from the hook. The goal of this course is to either help you get more confident using the gear you have, or to help you select new gear and understand how to deploy it correctly. We discuss available equipment and its performance. Estimated time: 4 hours. Investment: $17
In addition to the BBCM Rank and the four courses below, you must be at least Level III experience qualified.
WEATHER CLINIC: If you’re a real sailor then you need to understand and read the weather. It’s as simple as that! Written by the professionals at Clear Point Weather, this is the best weather sailing course available. Estimated time: 7 hours total. Investment: $39.
SAIL TRIM CLINIC: Learn the true art and finesse of trimming the sails. When to adjust the fairleads, the traveler, the downhaul, the outhaul, the Cunningham, the boom vang. When leaning to sail properly, you should know what all these fine adjustments do. Estimated time: 4 hours total. Investment: $39.
STORM TACTICS CLINIC: Even when day sailing, a storm can be upon us in minutes. Are you prepared with the knowledge now? This storm tactics sailing course will teach the essentials to keep you and your crew alive. Estimated time: 4 hours total. Investment: $39.
SAFETY AT SEA CLINIC: Most mariners don’t realize that we never even hear about the many crews aboard vessels that had their share of problems offshore. Situations were evaluated, repairs were completed, and they made landfall quietly and efficiently – this done as a normal course of passagemaking. These able sailors had the skills, materials, and a plan to cope – having merely to carry out the work to get back on course. They understand that overcoming obstacles is a normal part of blue water sailing. Estimated time: 14 hours total. Investment: $39.
The investment in the Captain Bundle of courses is $307 instead of $357.50 a la carte.
In addition to the above courses and clinics, we offer the following: CATAMARAN SAILING CONFIDENCE CLINIC: Converting over to a catamaran or chartering a catamaran for the first/second time? Learn the essential differences between sailing a monohull and a catamaran. This clinic will give you the confidence. It includes an interactive experiential online game to practice maneuvering in a marina. Estimated time: 3 hours total. Investment: $39.
INTRODUCTORY CELESTIAL NAVIGATION CLINIC: If you’re in any way intrigued with Celestial Navigation, this is the best and simplest celestial sailing course available. You’ll be able to do an actual noon shot and determine your position. Estimated time: 5 hours total. Investment: $39.
Tomorrow we’ll discuss practical sailing schools and how you can get a verified proficiency stamp added to your sailing certificate.
While it is generally accepted that mooring is safer than anchoring, there are still some considerations. The following photo from Waiake beach in Auckland, New Zealand is proof of this.
Whilst visiting New Zealand for the purpose of spreading the NauticEd word, we came across this early in the morning after an overnight mild storm.
After talking with the locals, the mooring chains on all the moorings in the bay had recently been replaced about 6 months ago, and so that was not likely the culprit. Upon closer inspection of the boat, the cleat and bollard had broken off the boat due to age and lack of maintenance on the tie off points on the foredeck.
The storm was caused by high winds produced from a low combined with a high as in the following map.
High winds from two weather systems
In the southern hemisphere, lows spin in a clockwise direction and highs spin anticlockwise. The two systems then combined here produced 35 knot to 40 knot north easterly winds. This direction is completely open to Waiake beach (on the north east coast of New Zealand) and thus the moored boats in the bay are vulnerable to these high winds.
What should be done?
Check tie off points on your boat for rot.
Check for leakage of water under fittings. Often times water leakage under the fiberglass can rot out the plywood. Creating hidden rot and weak points.
Tie off onto stronger points on the boat rather than weaker points.
Use multiple tie off points to spread the load
Dive the anchor point on the bottom
Check all chain and rode connections
Ensure rode is not able to be chaffed
Use stainless steel wire to lock closed any d-rings
Check chain for rust. Don’t buy cheap chain for a permanent mooring. You get what you pay for.
Feel free to add to this blog regarding mooring safety.
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