The Rule of Twelfths for Tide Estimation and Navigation

[Steadman Uhlich]

The following is posted as a general introduction to a topic I think most navigators and ocean sailors should know or be aware of when sailing the tidal areas of any ocean. It should be taken as a "rule of thumb" and it is always preferable to use more accurate local tidal charts for navigation.

I hope CF Members and readers find this helpful. If you have additional info on this topic, feel free to add to the discussion. I don't claim to be an expert on this topic, but am simply sharing what I find useful to know.
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The Rule of Twelfths

Tides ranges (how high, how low) and how often they occur, vary around the world.

Local ocean bottom bathymetry (shape/contours) has an influence too, so the tides experienced in a small cove may be different from those seen in a large bay. Make sure you check to know the local conditions or what tidal range is expected for a specific location.

Some places have one high tide per day, other places have two high tides per day.

The following applies to the East Coast of the United States, which has two highs and two lows each day or in places with a semi-diurnal tide zone (i.e. A location that has two low tides and two high tides).
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The Rule of Twelfths

Since the average interval between high and low tide is just over six hours, a sailor can divide the cycle into six segments of one hour each. On average the tide rises or falls approximately according to the following:
1st hour - 1⁄12
2nd hour - 2⁄12
3rd hour - 3⁄12
4th hour - 3⁄12
5th hour - 2⁄12
6th hour - 1⁄12

In places with only a single high and single low (a diurnal tide zone), you can double the time to 2 hours each segment.
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From Wikipedia:
"The rule states that over the first period the quantity increases by 1/12. Then in the second period by 2/12, in the third by 3/12, in the fourth by 3/12, fifth by 2/12 and at the end of the sixth period reaches its maximum with an increase of 1/12. The steps are 1:2:3:3:2:1 giving a total change of 12/12. Over the next six intervals the quantity reduces in a similar manner by 1, 2, 3, 3, 2, 1 twelfths."

The rule is a rough approximation only and should be applied with great caution when used for navigational purposes. Officially produced tide tables should be used in preference whenever possible.

The rule assumes that all tides behave in a regular manner, this is not true of some geographical locations, such as Poole Harbour[4] or the Solent[5] where there are "double" high waters or Weymouth Bay[4] where there is a double low water.

The rule assumes that the period between high and low tides is six hours but this is an underestimate and can vary anyway."

The illustration below is from Wikipedia (Creative Commons).

[daletournier]

I keep this in mind regularly, it also surprises me how few I meet know it. I was explaining it to some friends while diving just the other day.

[thinwater]

The navigator should also be aware that in many bays, sustained winds can drain or pile-up the water with several times the intensity of long-term tide predictions. Even tide tables often miss these local conditions by hours and feet. I'm not talking about classic storm surge, just the effect of a sustained breeze. I know in the upper Chesapeake Bay tides drive a small fluctuations, not much more than a foot, but sustained 20-knots winds can change the tides by 3-4 feet either way. The summer Bay is generally 1-3 feet deeper than the winter Bay, because the prevailing winds reverse.

[Paul L]

In the atolls of the South Pacific the currents in the cuts into the atolls are generally tidal. Strong seas breaking over the windward side often over ride any tidal changes, leaving the cut with a continuous ebb or a very diminished and delayed flood.

[Benz]

Since the rule doesn't tell you what the tide range is in any particular spot, and you have to refer to tide tables anyway to see that, I see it only as a nifty thing to know--I can't guess what practical use it has.
Anyone know where it would be useful to know except to be aware that the current will flow fastest at half tide? I guess if I had to get over a shallow bar and needed x amount of feet, but honestly, I'd hate to cut it close enough that that sort of hair-splitting was necessary.
There is another diabolical tide feature that's bit me several times: the odd corners of the planet where both the ebb and the flood set the same direction, and you have to watch objects on shore for a few minutes to see what it's doing just then.

[Tillsbury]

Different cruising grounds need different skills. I used to use the rule of twelfths all the time on the east coast of the uk, where three feet under the keel counts as deep water. Tidal height is everything in your planning. In the Solent and English Channel, the depths are less important and you’re often concentrating on tidal streams, particularly around headlands and in the Channel Islands, where it can run 10knts.

Over here in the South Island, nz there is generally a ton of water, and tidal heights and streams are not such a big deal. On the other hand, I’m having to get used to sailing where deviation goes from 20 to 25 degrees from true, which is a novelty to me

[Jim Cate]

Since the rule doesn't tell you what the tide range is in any particular spot, and you have to refer to tide tables anyway to see that, I see it only as a nifty thing to know--I can't guess what practical use it has.

In much of the world, the tide tables show only when high and low water are predicted to be, and the height at each of those times. Thus, if you want to know the height at any other time, the rule of 12ths is damn useful.

And in all too many places, there is no local tidal station,so you need to interpolate between distant stations. In such spots good luck may be more important than the 12ths!

Jim

[GordMay]

See also, the Rule of 7 & Rule of Thirds:
➥ http://www.cruisersforum.com/forums/...harts-395.html

[Benz]

^^ And that's my whole point....when the data is sketchy enough to need the rule of 12ths, a good measure of luck and a wide safety margin are also necessary. Would anyone cross over a reef trusting in the rule of 12ths to keep six inches under the keel, not knowing how much any other factors of wind or inland rainfall may have affected the tide, or would they just wait an extra hour or two longer out of prudence and know they had the depth?
No doubt there are scenarios conceivable when one might want to cross a shoal at the earliest possible moment, but I doubt they come up often in the cruiser's life.

[Steadman Uhlich]

Originally Posted by GordMay

See also, the Rule of 7 & Rule of Thirds:
➥ http://www.cruisersforum.com/forums/...harts-395.html

Good post!

I did not see that before starting this newer thread.

Your 2003 post (wow! 15 years ago!) has a lot of good info worth viewing.

[CaptTom]

Here in the US Northeast, tides are a pretty big factor we deal with a lot. The rule of twelfths gives you a quick estimate of the overall scale and rate of change of whatever issue you're facing, from tidal current, to height of tide, to how much debris to expect to find washing down from the inland bays on the ebb.

Maybe you encounter a strong current. Is it getting stronger or weaker? For how long? Should I wait or get going now? All things you can quickly estimate just from a basic knowledge of how long since the last high or low. Which by the way can be estimated by the phase of the moon, if you're familiar with the local area.

[GordMay]

Well Steadman, your post garnered more attention to the subject, in a few HOURS; than did mine in 15 YEARS.
Of course, when I posted, we might have had as many as 200 members (or less).

[wrwakefield]

We do a lot of sea kayaking, and camp on the beach when on expeditions. We use the Rule of 12ths to extrapolate where the high water mark on a potential camping beach might be over the duration of our intended stay.

This helps us determine if we can pitch a tent on a particular beach, and if so, how far up we need to be, and/or when we need to break camp. [There are often scenarios where a beach is only tenable for a couple of days before your sleeping bags will get wet...]

We have fun marking our individual estimates to see how accurate we were... [usually within 6"- which is pretty good sighting up a long, inclined beach with ~20 ft tidal variations...]

As an aside, we also carry hanging tents for those situations where you just have no dry [or level] place to pitch a ground tent.

Just another use case...

Cheers! Bill

[Emouchet]

The rule of 1/12th allows also an easy plot with straight lines for the water level by the tide time, when using for the X axis a non linear scale:
- first and 6th hour of tide: 1 unit
- second and 5th hour of tide: 2 units
- third and 4th hour of tide: 3 units
No need to messed up with a sinusoïdale curve.

[Steadman Uhlich]

Originally Posted by GordMay

Well Steadman, your post garnered more attention to the subject, in a few HOURS; than did mine in 15 YEARS.
Of course, when I posted, we might have had as many as 200 members (or less).

Gord,
From the very first day I joined CF, I have had great respect for you and your contributions to this online community. You have been an exemplary member, and have influenced my activity, in positive ways, by your example.

Your original (2003) post is still visible (thanks to the internet) and still valuable. I hope this thread will lead more people to it.

Thanks for adding what you do to CF, you make it a better place because of your friendly welcoming and your contributions (posts) of knowledge. I am sure that the growth of CF has been due to the sharing of knowledge, but also because people like you make it a friendly, welcoming place to visit.