I’d heard legendary stories of this before but was never too sure of the reality of it. However, this weekend I actually experienced it. In our saturday regatta race the wind forecast was for 3 knots rising to 8 knots in the afternoon. Not so good for the sweet spot performance of our Beneteau First 45 who likes higher winds. We started the race first across the line in “A” fleet at 1pm. Winds were still howling (not) around 3 but down the harbor we could see the wind line coming. Smiles on faces.
The tide was in full ebb running out of the east facing harbor at about 3 knots. While much of the fleet elected for the center of the harbor where the current was higher, our tactician had observed the wind had filled in more on the south side of the harbor. So we took advantage of the higher wind with slightly less tide flow nearer the edge. This tactic helped us keep up with the faster fleet boats like the TP52’s.
Current Profile in the Waitemata Harbour
Exiting the harbor, the first mark is far to the north and many boats headed directly for the mark on a starboard close reach. Watching the winds die back down, our tactician knew better. We continued to head further out of the harbor entrance before tacking over. Why?
Regatta Race Course
Most regatta races are won by local knowledge. Here, the tide flow exits the harbor and splits in two around Rangitoto Island. Half heads north and half heads east. Our first mark was north east and with the tide still ripping out and dying wind we didn’t want to get caught in a situation were we couldn’t overcome the tide.
As the wind died, the tide took over. We were making about .25 knots headway and 1.5 knots sideways. Furthermore, the race committee recognized that with the dying wind they should end the race at the first mark. That meant that the fleet had to pass between the first mark and the committee boat set 100 yards west of the first mark. Those that had not gained enough ground east were simply “floating” past west of the finish line with no ability to do anything except DROP ANCHOR.
Our float course
Our tactics had paid off – we floated sideways through the race finish line to take a first place.
It was a very amusing sight – dozens of sailboats ANCHORED to the west of the committee boat – RACING. LOL.
Anchored during a regatta race
So what’s the take away lesson here?
Almost certainly if we’d been cruising, we would have turned on the engines and motored to the destination. During a race like this we won by understanding the tide, tidal current and matched that with the peak times. Understanding tides ARE important, they can overpower your ability to sail out of trouble should you be in a light wind condition or heavy current condition. Trying to sail back into San Francisco bay one time was impossible against the tide even with 12 knots of wind. Had we known a little more back then and pre planned the trip we would not have ventured outside the gate.
The NauticEd Sailing Courses cover all aspects of sailing. In particular, the NauticEd Coastal Navigation Course covers Tides and Current, and how to predict them.
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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