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NauticEd International Sailing Education is the proud title sponsor for the May 12th 2016, Oregon Offshore International Yacht Race. Two of NauticEd’s practical sailing schools, Island Sailing Club and Vancouver Sailing Club are a significant part of this title sponsorship and many of their students are participating.
The race, in its 40th year is 193 miles long and begins off the coast of Astoria, Oregon and finishes in the harbor at Victoria, British Columbia.
As part of the education sponsorship, NauticEd is giving away 6 Captain’s Sailing Education Packages to 6 lucky participants. This represents over a $2000 donation to the cause of keeping people save on the water with advanced sailing education. View the contents of the Captains package below. This represents extensive and vital education for all sailors wanting to sail more than 20 miles off shore or over long distances.
All participants are encouraged to create a new account with NauticEd whereby they will receive 2 FREE NauticEd courses, Navigation Rules and Basic Sail Trim and a FREE sailor’s electronic logbook.
Students of Island Sailing Club and Vancouver Sailing Club are encouraged to join in on the race.
WINNERS: If you are a winner of one of the 6 Captain Education Packs, sign up for a free account at www.NauticEd.org/signin then send us an email. Once we verify with the Committee your prize, we will drop the 12 sailing courses into your curriculum. Congratulations!
ALL OTHERS: Set up a free account at NauticEd here Sign in to NauticEd you will automatically be given two free courses and a free sailor’s electronic logbook. You’re Welcome!
Here is a great question from a student with our answer below.
Any chance you can email me the barber hauler article direct?
I’m at sea, on a out new-to-us 40′ Leopard Cat on a 1200nm journey and have been really struggling to get the headsail to set right while running downwind. I currently have it barber hauled using the lazy sheet and a midships cleat, but it is far from eloquent.
The issue had been driving me nuts and I’ve really struggled to stop the headsail from back winding. It feels like a velocity header, but it is not. It feels like the headsail is too far out, but I’m sure it’s not. It feels like the headsail wants to gybe, but I’m at 130-140 awa [apparent wind angle]. Admittedly the sea is a factor – 3-5m swell, but it really feels like I’m just doing something basic wrong… But I can work it out.
The problem is when running (150+ twa / 25tws). The only way i can stop it is to come up, but I’m sure i should be able to run!! So frustrating!
Anyway, if you can email me the barber hauler article, I’ll have a read and see what I can see.
I’ve also cc’d my friend Nathan who is a sailing coach based in Auckland.
A Barber Hauler is more for close to beam reaching and helping to shape the gap between the main and the jib. When trying to run from 130 awa and more you’re experiencing shadowing by the main. As you pointed out, as you roll the awa will change vastly because of the velocity of the roll. But also the apparent wind will change as the boat speeds up and slows down with the waves. You can use a pole to get the jib out further and to hold it in place so it does not collapse. But ultimately to keep the sails full and gain the best speed of the boat you will need to fore reach. Fore reaching is sailing the boat to the waves to keep the sails full and the same awa. As you climb a wave the boat slows down, the wind shifts aft so you need to turn up. As you surf the wave and the boat speed picks up you need to bear away. Also as the wave passes you and you slip back off the top of the wave the mast will roll to windward shifting the away forward – turn up. As you go through the trough and the mast rolls downwind the awa shifts backward – turn up.
Surfing a wave or rolling backwards off the top as the wave passes you underneath – turn downwind
Climbing up a wave or rolling forward in the trough as the trough passes you underneath – turn upwind
Ultimately from the boat polar plot that we talk a lot about in the Electronic Navigation course – your best speed to a downwind destination is going to be around 130 degrees awa. In doing this you are keeping the sails full and not shadowed – however again as you point out this is a challenge in waves. Sailing at 150 deg although seems faster because you are heading more directly to your destination, your boat speed is suffering. Also keeping the sails full will make for a more comfortable ride.
Advice is to keep the sails full at around 130 apparent wind angle and to forereach the waves.
I’ll try to put up an animation soon on fore reaching.
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 —->
(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)
(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.
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.
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.
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
We think this is the world’s best sailing App and for good reason.
NEW APP WAS UPDATED ON SEPT 20th 2015
First off, it is free (that’s good) and second off with that you get NauticEd’s free course on Navigation Rules. Pretty soon we’ll also add NauticEd’s FREE Basic Sail Trim Course.
In addition, any course that you have invested in with NauticEd automatically appears on your App. And to top that off, you can also take your tests for all your courses on the App offline. That’s a big wow!
There is zero reason not to download the App – and imagine if everyone did and took the FREE Navigation Rules Course. You could stop worrying about if the “other guy” heading at you knows the rules or not. So spread the word generously.
Bored in the doctor’s office? Take the Free Rules of the Nautical Road test!
If you think this is the greatest idea on the planet or at least just a very good one, please like us on facebook.
Posted by Grant Headifen, Global Director of Education – NauticEd.
One of the greatest things I love about my job is the ability to apply the latest technology to the sailing education industry – it is so exciting to be leading the world in this area.
And – today comes as a greatly awaited day for us to announce one of the bigger innovations in not only sailing education but in the entire community of eLearning itself.
I’d like to introduce Nano-Forums!!!!!!!!
Please watch this video and you’ll see why our Sailing Nano-Forum is so innovative and such a benefit to the sailing community at large – You’re Welcome! It represents a MASSIVE investment in technology over the past 6 months. Ummm like really REALLY massive but we think it’s worth it!
We think you will really enjoy it.
Oh and btw since this is new technology to the world and we invented it, we are coining the phrase NANO-FORUM right here right now!
What it ultimately means is that we all now can collectively crowd source information in targeted specific areas and re-use the crowds knowledge for educational drill down topic purposes in a way never been done before.
Just watch the video – you’ll get what we are talking about.
Please engage in the Nano-Forums through out our courses. Look for the SeaTalks button at the top right of every page of the course.
Start by taking the FREE Navigation Rules Course at:
It is pretty amazing – you can now see the content of all your sailing courses in your curriculum offline AND take the tests offline. Once you reconnect, the test results are sent up to the cloud. i.e. it does not matter where you take the test – on iOS or on online on your computer.
NauticEd Sailing App
It is pretty amazing – you can now see the content of all your sailing courses in your curriculum offline AND take the tests offline. Once you reconnect, the test results are sent up to the cloud. i.e. it does not matter where you take the test – on iOS or on online on your computer.
Right now it’s for iOS – an Android version is coming.
Also in the App, is the ability to add to your new style logbook (launched in 2014). So on the dock after a day of sailing, just right there – add the day and it will show up in your sailing resume.
A really amazing feature is that you now carry your sailing resume and certificate with you on your phone at all times and can email it in an instant to anyone.
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
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 .
If you like this post – please LIKE it on facebook and g+1 it – it really helps us grow. Thanks!
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 – boats happily floating
Here is the same place a few hours later.
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
total 12ths inc
tide height ft
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
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
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.
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
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.
Rounding up is so – so – so dangerous. Last weekend we were out sailing and a speed boat came ripping by at about 20 knots to the windward of us and only about 60 ft away. Yes they are stupid – yes they should know better and yes they should be shot – but that is not going to save your life. You have to take responsibility for yourself out there. Think about what could happen if a last minute gust hit you and you rounded up into that speed boat. You’re dead – and actually by maritime law it would be your fault.
So you’ve got to know how to handle a wind bullet and you’ve got to be at the ready. A last minute gust can kill you and your crew.
… and btw if you are a speed boater please pass to the leeward side of a sailboat … (leeward is the downwind side btw)
First a definition and explanation of Weather Helm:
Forces from the wind aft of the keel are balanced by the rudder.
Weather helm is when the boat wants to turn towards the wind and you have to hold the helm in a leeward turning position to maintain a straight course. i.e. if you let the helm go the boat would automatically turn towards the wind.
Usually a boat is trimmed so that you purposefully have a little weather helm. Why? Well if you are turning the rudder so that you are fighting the weather helm it means that the leading edge of the rudder is pointing upwind. i.e. the boat wants to turn upwind but you are counteracting this by turning the boat downwind. Turning the boat downwind means by definition the leading edge is pointing upwind. See the graphic.
All this means that as the water hits the rudder there is a component of the water force that pushes the rudder (and thus boat) in a windward direction i.e. it actually makes your boat climb upwind from the water force on the rudder. This is a desired outcome when sailing towards a windward destination. Few sailors know this.
I say all that to say this – your boat should naturally have a little weather helm.
Here is what a wind gust does:
First, it immediately heels your boat over. Because of the heeling angle, less and less of the rudder area is effective in providing turning force to counter act the weather helm. See the animation below.
Second, in a gust, the wind force on the sails increases with the square of the velocity but the counteracting force from velocity of water over the rudder does not increase because in that instant the boat speed has not increased.
So you’re trying to dip twice to use the rudder – eventually there is none left. The helm will be all the way over and the wind force has completely overpowered the rudder = round up.
Third, by definition an increase in true wind speed across the water shifts the apparent wind angle on your boat so that it comes more from an aft angle. i.e. if you are on a close haul, the wind now more feels like a beam reach. This exacerbates the heeling force because your sails are now in too tight. The wind gust is pushing sideways on the sails rather than flowing smoothly around both sides of the sails. This is now a triple whammy on the rudder. Poor Rudder!
Why does an increase in wind increase weather helm?
A boat is trimmed with weather helm by raking the mast backwards. This shifts the force on the sails backwards. To see the effect now, push sideways on a pencil on your desk. If you push in the center, the pencil moves laterally sideways. If you push towards the eraser end the tip moves “upwind”. Increase the force, the tip mores upwind more. i.e. the more force towards the back of the boat, the more the boat wants to turn up into the wind = weather helm.
But remember – a small amount of weather helm actually helps you “climb” upwind using water force from the rudder.
Why does an increase in wind speed move the apparent wind angle more aft?
Best you take a look at our free sailing course on sail trim. There is an excellent explanation there.
Wind Gust Directions
Offshore the wind gust is more likely to be in a consistent direction as the existing wind. Close in to cliffs, the gust can be heading in many different directions.
Back to dealing with gusts.
First off – you can see them coming. They are a change in perceived water color because the light reflects different off the small ripples generated by the extra wind. Seriously – they are easy to spot. Haa haa during the daylight.
As the wind gust approaches you should be prepared in your mind and with your crew for the outcome. Don’t leave it until the gust hits to start battle stations. Remember, gusts can be dangerous. Unprepared crew members can be thrown around. Boiling water in the galley can be splashed. People tossed out of bunks. Gear can be thrown around into someone. Someone can be thrown against a bulk head. Someone can loose footing and go overboard and finally as we started, you can be rounded up into another boat.
A crew member should be stationed and attentive to the traveler. You should warn the crew member of the approach. As the boat begins to heel, the traveler crew member should begin to ease the traveler. With a big gust, the traveler may need to be dumped all the way to leeward. This spills the wind out of the mainsail.
The mainsheet crew member should be made aware that if the traveler dump does not work that the mainsheet should be eased. But make sure that both sails are not being dumped at the same time. Traveler first then mainsheet if needed.
Tune the crew into whether an ease will work or a complete dump is needed. A good crew member will be able to anticipate and adjust. Training is good!
As the helmsperson, you can turn up into the wind gust a little assuming it is a lift. A quick look at the masthead wind indicator can tell you that answer. The gust will hit the top of the mast before it hits the boat. Turning into the gust will alleviate the heeling a little and allow you to take instant advantage of a lift. But make sure that you don’t overturn.
The headsail (jib or genoa) is to be left alone in a gust. Since the force on the headsail is positioned forward of the keel, the headsail does not contribute to rounding the boat up into the wind. In fact it acts opposite it helps prevent rounding up because the force on that sail is far forward of the keel. i.e. push on that pencil on your desk again. The head sail does however contribute to heeling. But again, the heeling in a gust can be controlled by the mainsail traveler and sheet.
If you’re getting hit by a lot of gusts and the crew is working hard to control – consider reefing the mainsail. This has three effects:
It shifts the center of pressure of the sail forward so that the rounding up effect from aft pressure is reduced
It reduces sail area aloft which reduces the heeling moment
It reduces the sail area in total which reduces the heeling moment
Heeling will be reduced by reefing the headsail, from the above arguments then this helps the rudder effectiveness.
Don’t try to tough out a windy situation by not reefing. Your boat will actually sail faster if you’re not weaving all over the place each time you round up and your crew will have a better time.
A professional delivery Captain told me once that his motto when crossing the Atlantic was “if you are thinking about reefing you should have done it yesterday. If you are thinking about shaking out the reef, wait until tomorrow”.
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.
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