The Rule of Twelfths for Tide Estimation and Navigation

[Adelie]

Originally Posted by 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.

When you are anchoring in the PacNorthWest you need it. They have semi-diurnal tides that range from 7’ to 20’ depending on where you are in the Puget Sound.

You need a decent estimate of current tidal state so you can determine the minimum depth of water you want to anchor in for future low tides and the minimum amount of rode to beer to maintain minimum scope for future high tides.

If you have 175’ of rode, the boat draws 5’ with 3’ bow height, you want a minimum 5:1 scope & 2’ clearance under the keel and there is a 20’ tidal range then there is a depth range of 5’ you can anchor in right now and not have later problems. If you estimate current tidal state to nearest 1’ then you can anchor in a 3’ range to account for uncertainty in estimating current tidal state. That assumes that the wind is never going to blow you towards shore or the seabed is very flat across the whole swing area.

If you are coming into an anchorage 2hr after high, there will be 2 lows in the next 10hr or so you need to account for assuming you are leaving tomorrow morning.

On June13 in Olympia,WA tides will have a 20’ range at springs. In San Diego spring tidal range is 9’. In Hilo it will be a bit under 4’.

When you are dealing with smaller ranges rule of 12ths isn’t that important. With bigger tidal ranges you need some way to estimate current tidal state short of waiting for the next high or low.

[Paul L]

Originally Posted by Adelie

When you are anchoring in the PacNorthWest you need it. They have semi-diurnal tides that range from 7’ to 20’ depending on where you are in the Puget Sound.

......

Or you could use any of the many tide prediction apps, like XTide or one on your Chartplotter for anywhere in the PNW.

[Adelie]

Well that’s nice if you have any of those things but what if all’s you have is the tide tables? How do you cope when the electronics fail? The rule of 12ths is one of the ways.

I learned to sail in Seattle 3yr before the first handheld GPS came out (Magellan Nav-1000, $3,000 MSRP).

[Paul L]

Originally Posted by Adelie

Well that’s nice if you have any of those things but what if all’s you have is the tide tables? How do you cope when the electronics fail? The rule of 12ths is one of the ways.

I learned to sail in Seattle 3yr before the first handheld GPS came out (Magellan Nav-1000, $3,000 MSRP).

I owned one of those early Magellan bricks. It's nice to understand the basics, then use the tools that are easily available today. I haven't used my parallel rules in the last 30,000 miles.

[Jim Cate]

FWIW, one of those original Magellans came with this boat when we bought her 15 years ago... and it still works! Takes a loooong time to get its first fix, but patience is rewarded with a position!

I doubt if any currently produced GPS will last that long.

Jim

[NevisDog]

Originally Posted by Steadman Uhlich

Gord, Thanks for adding what you do to CF, you make it a better place ...

Steady on Steadman, don't embarrass poor Gord. But yea, you're right, makes all the difference, with never a rude word.

[Alan Mighty]

I'll add to the the Rule of Twelfths, the Rule of 7, and Rule of Thirds for tide height, if I may:

1. When cruising in the Francophone parts of the world, I've come across the Règle des Sixièmes or Rule of Sixths. It's for tidal stream in a mixed or semi-diurnal tidal regime. The Rule is 0, 3, 5, 6, 5, 3, 0. Goes like this:

slack water, no flow;
one hour later, 3/6 of maximum tidal flow;
two hours later, 5/6 of maximum flow;
three hours, 6/6;
four hours, 5/6;
five hours, 3/6;
six hours, slack.

2. I think the Rule of Twelfths originated in those economies that used Imperial Measure, with 12 inches to the length one of the King's feet, and/or were used to exchanging 12 pennies for 1 shilling. In the metric world, where most folk apparently only have 10 digits on their two hands, there's a Rule of 10 for the height of the tide in mixed and semi-diurnal tidal regimes. The Rule is 10, 15, 25, 25, 15, 10. Goes like this:

in the first hour after Low Water, the tide rises 1/10 or 10%;
in the second hour, 15%;
in the third hour, 25%;
in the fourth hour, 25%,
in the fifth hour, 15%;
in the final hour, 10%.

3. I've also met a simplified version of the Rule of Sixths for those (such as myself) who want an approximation of the strength of the tidal stream in mixed or semi-diurnal tidal regimes but don't have big mobs of mental processing power. It's another Rule of Thirds. The Rule is 0,1,2,3,3,2,1,0. Goes like this:

slack water, no flow;
after one hour, 1/3 of maximum tidal current;
after two hours, 2/3;
after three hours, 3/3;
after four hours, 2/3;
after five hours, 1/3;
after six, slack water again, little or no flow.

[Benz]

This is all very instructive, but my problem is I'm often cruising in areas where I have no tidal data available--whether range or any other creature. I think that knowing the tide range is necessary to the Rule's usefulness.
But to Adelie's post: why not simply anchor where the chart testifies that you will have plenty of depth at MLLW? I make it a rule never to anchor in areas where the soundings show less depth than I need for anchoring.

[Paul L]

Originally Posted by Benz

This is all very instructive, but my problem is I'm often cruising in areas where I have no tidal data available--whether range or any other creature. I think that knowing the tide range is necessary to the Rule's usefulness.
But to Adelie's post: why not simply anchor where the chart testifies that you will have plenty of depth at MLLW? I make it a rule never to anchor in areas where the soundings show less depth than I need for anchoring.

There are lots of places that have inadequate charts, i.e. charts with no soundings inside your chosen anchorage.

Where do you sail that the tide prediction programs don't have some coverage? I've been to some pretty remote areas and I can usually find some tide station that at least puts me in the ballpark.

[Steadman Uhlich]

Originally Posted by Adelie

Well that’s nice if you have any of those things but what if all’s you have is the tide tables? How do you cope when the electronics fail? The rule of 12ths is one of the ways.

I learned to sail in Seattle 3yr before the first handheld GPS came out (Magellan Nav-1000, $3,000 MSRP).

I lusted after one of those when it first came out.

Bought a sextant instead. Used it.

[Benz]

Originally Posted by Paul L

There are lots of places that have inadequate charts, i.e. charts with no soundings inside your chosen anchorage.

Where do you sail that the tide prediction programs don't have some coverage? I've been to some pretty remote areas and I can usually find some tide station that at least puts me in the ballpark.

It is not that the data does not necessarily exist: it is that I have no access to it, since I cruise without any sort of programs. So in home waters (US), I can use Eldridge or any of dozens of printable tide tables from the internet. But on the south coast of Panama, (where the tides can be 20+ feet) and there's no internet for weeks on end, and every nook and cranny you enter has a different range, I'm off soundings, as it were, for pinpoint tide accuracy.
The cruising guide sometimes has general data (average range, etc.) but to carry tables for the dozen-odd countries I visited on that cruise would take too much space.
But where do you cruise that there's no soundings? I have yet to be anywhere where a chart or a cruising guide didn't have adequate soundings (except some parts of Newfoundland where the government intentionally left soundings off of large portions of their charts. Luckily, older-issue Admiralty charts still had the data)

[tstano]

Although it may not be exact, we used this methodology on the AICW from Charleston to St. Mary's, Ga. last fall through all the thin water. Drafting 7 feet we never saw less than 9 under the keel. Some days were short hops then anchor. It took us a week but we never got close to getting stuck.

[Dockhead]

Originally Posted by 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.

The Rule of 12ths is considered basic knowledge around here, where you'd be hard pressed to find someone who didn't know it. But I think you're absolutely right -- it's not much used, at least not for working out tide height, in this day of electronic tide stations.

The Rule of 12ths is nothing more than a rule of thumb for a Sine Curve -- so it's still a useful concept to keep in your head -- the rate of change of tide height and the CURRENT will be strongest in the middle of the curve, and will taper off at the extremes.

[Paul L]

Originally Posted by Benz

It is not that the data does not necessarily exist: it is that I have no access to it, since I cruise without any sort of programs. So in home waters (US), I can use Eldridge or any of dozens of printable tide tables from the internet. But on the south coast of Panama, (where the tides can be 20+ feet) and there's no internet for weeks on end, and every nook and cranny you enter has a different range, I'm off soundings, as it were, for pinpoint tide accuracy.
The cruising guide sometimes has general data (average range, etc.) but to carry tables for the dozen-odd countries I visited on that cruise would take too much space.
But where do you cruise that there's no soundings? I have yet to be anywhere where a chart or a cruising guide didn't have adequate soundings (except some parts of Newfoundland where the government intentionally left soundings off of large portions of their charts. Luckily, older-issue Admiralty charts still had the data)

The offline tide programs work fine in Pacific Panama. We used XTide on a laptop when there as well as the tide function on our chartplotter.

West coast of the Queen Chartlottes and inside many south Pacific atolls had large swaths with no soundings, for example.

[StuM]

Originally Posted by Benz

... or any of dozens of printable tide tables from the internet. But on the south coast of Panama, (where the tides can be 20+ feet) and there's no internet for weeks on end, and every nook and cranny you enter has a different range, I'm off soundings, as it were, for pinpoint tide accuracy.
The cruising guide sometimes has general data (average range, etc.) but to carry tables for the dozen-odd countries I visited on that cruise would take too much space.
But where do you cruise that there's no soundings? I have yet to be anywhere where a chart or a cruising guide didn't have adequate soundings (except some parts of Newfoundland where the government intentionally left soundings off of large portions of their charts. Luckily, older-issue Admiralty charts still had the data)

If you've got a device on board for internet connectivity, that's presumably a laptop, tablet or smartphone. There are many world wide tide programs for all of those devices which will give you just as good a tidal prediction as the internet ones (which generally use the same harmonics and base stations)


Here's the one I use on my laptop wxTide: