Shallow Water Anchoring

[ahnutts!]

Originally Posted by Pyrate

Personally, I'd go conservative. There's always the chance of some rock jutting out of the sand, unexpected waves or wakes, or a low pressure system far enough away to not affect the local weather, but affect the tides. And, being in Alaska, where today's tide went from -2.85' to 18.14', I just plan on anchoring in as deep water as I can while keeping a 5:1 ratio for the scope. Of course, we have few if any crowded anchorages up here.

Pyrate

agreed, and that 5:1 can leave you a long way from your last sounding at low water. I'm far more conservative if I drop the hook near high water than I am if its low when I arrive.

[David_Old_Jersey]

Although my boat can take the ground, plenty of places I wouldn't choose to dry out on (or bounce up and down on) - for much the same reasons as those with Fin Keelers.

A big consideration for me is how familiar (cocky? ) I am with the waters. Would range from a couple of feet (for lunch in stable conditions) to at least half a dozen. and preferably nearer 10 for a good night's kip. I expect even (if) travelling in more benign waters (at least tide wise) would be hard to shake off a conservative approach, for mental comfort if nothing else. And perhaps not simply being prudent - also Maths / Tidal Calculations not being my strongpoint

And all that leaving aside that much boat parking around these parts is less about how much water is under the keel and more about how far away it has gone.........as the crow flies and whether coming back the same day 40' Tidal range.

[rusky]

I'd go pretty shallow if I'm happy too. Have had less than 6' under us, knowing it was going to rise. However, if not exactly sure - then calculate for about 3' under keel at low.

If things could get nasty, then I would anchor safely in deeper water - as everyone else has said. I certainly disagree with the 'sheep' mentality of following everyone else in an anchorage. I go for a drive about my swing before I drop the anchor, just to see if the echo shows anything odd!

[GordMay]

Originally Posted by Pete7

You have a choice, either some maths involving time and the rule of twelves ...

The Tidal Rule of 12 states that, in the first hour after low tide the water level will rise by one twelfth of the range, in the second hour two twelfths, and so on according to the sequence - 1:2:3:3:2:1.

Hence the greatest Tidal Flow (strongest current) will occur during the middle* 2 hours of the run.

There are about 12 Hours, 25 minutes between high tides, so tidal flow reverses every 6 Hours 12 ½ Minutes.

* The greatest tidal current occurs in the third & fourth hours of the approximately six-hour half-cycle.

See also “Tides & Tidal Prediction”
Here ➥ RYA navigation courses - Chapter 7 Tide prediction from charts and tables.

[capngeo]

Originally Posted by GordMay

The Tidal Rule of 12 states that, in the first hour after low tide the water level will rise by one twelfth of the range, in the second hour two twelfths, and so on according to the sequence - 1:2:3:3:2:1.

Hence the greatest Tidal Flow (strongest current) will occur during the middle* 2 hours of the run.

There are about 12 Hours, 25 minutes between high tides, so tidal flow reverses every 6 Hours 12 ½ Minutes.

* The greatest tidal current occurs in the third & fourth hours of the approximately six-hour half-cycle.

See also “Tides & Tidal Prediction”
Here ➥ RYA navigation courses - Chapter 7 Tide prediction from charts and tables.

Disregard all of the above on the West Coast of FL! The GOM has its own rules!

[David_Old_Jersey]

Originally Posted by GordMay

* The greatest tidal current occurs in the third & fourth hours of the approximately six-hour half-cycle.

The greatest speed of tidal rise occurs when you have left your wellington boots at the bottom of the ladder.

[GordMay]

Originally Posted by capngeo

Disregard all of the above on the West Coast of FL! The GOM has its own rules!

Indeed.

In parts of the northern Gulf of Mexico and Asia, tides have one high and one low water per tidal day (diurnal tides).

The map shows the geographic distribution of different tidal cycles.
Coastal areas experiencing diurnal tides are yellow.
Areas experiencing semidiurnal tides are red.
Regions with mixed semidiurnal tides are outlined in blue.

[Auspicious]

Originally Posted by GordMay

Hence the greatest Tidal Flow (strongest current) will occur during the middle* 2 hours of the run.

With all due respect that is not correct in most places. Generally slack water lags tidal extremes by a good bit.

In the US, look at NOAA Tides and Currents - Home and for any given location, look at times for high and low tides and at maximum and slack currents.

The rule of twelfths works fine for tidal height but isn't relevant to tidal currents.

[boatman61]

And how about this tidal chart to do your head in....
Tue 7
LW HW LW HW LW HW LW HW
01:00 02:46 05:40 10:42 13:29 15:00 18:03 23:13
1.6 m 1.9 m 0.7 m 2.3 m 1.7 m 1.9 m 0.5 m 2.1 m
This is in Poole UK...

[GordMay]

Originally Posted by Auspicious

With all due respect that is not correct in most places. Generally slack water lags tidal extremes by a good bit.

In the US, look at NOAA Tides and Currents - Home and for any given location, look at times for high and low tides and at maximum and slack currents.

The rule of twelfths works fine for tidal height but isn't relevant to tidal currents.

Yes, indeed. The Rule of Twelfths is about the rise and fall of the water levels at various stages of the tide while the Rule of Thirds and 50/90 Rule deal with current.

These rules are rough approximations only, and should be applied with caution. Officially produced tide tables should be used in preference whenever possible.

The rules assume that all tides behave in a regular manner,which is untrue of some geographical locations. The rules also assume that the period between high and low tides is six hours, but which is an underestimate and can vary anyway.

* Rule of 50/90: instantaneous speed of the current observed at each of the six hours in the tidal period ON THE HOUR.
* Rule of Thirds: cumulative distance the current travels DURING THE ENTIRE LENGTH OF EACH OF THE HOURS in the six-hour tidal period.
* Rule of Twelfths: cumulative change in the height of the water DURING THE ENTIRE LENGTH OF EACH OF THE HOURS in the tidal period.


The Rule of Thirds states that relative to the total maximum current speed, the current jumps 50% the first hour, 90% the second hour, 100% the third hour. The current then decelerates to slack in the same order.

Hence the ubiquitous 1:2:3:3:2:1 abbreviation of the rule which stands for 1/3 of the max current rate in the first hour, 2/3 in the second hour, 3/3 during hours three and four, and then down to 2/3 in the fifth hour and 1/3 during the last hour of the cycle.

The 50/90 Rule gives you the speed of the current at the end of each hour. Starting from slack, the current will flow at 50% of its maximum speed at the end of the first hour, 90% at the end of the second hour and full 100% or maximum speed at the end of the third hour. It will then slow down in the same steps: 90% at the end of the fourth hour, 50% fifth and back to slack at the end of the 6-hour period. The full Rule of 50/90 should be stated as 0/50/90/100/90/50/0.

“Bowditch” on Tides & Currents
➥ http://www.irbs.com/bowditch/pdf/chapt09.pdf

[Bash]

Originally Posted by Don Lucas

I was expecting people to say that they would anchor in water less than their draft at low tide and soft bottom sometimes.

Not if I can avoid it. Silt scours the bottom paint. Those of us who don't haul our boats out every winter usually try to make a paint job last 2-3 years. That won't happen if you keep anchoring shallow.

[Auspicious]

Originally Posted by GordMay

These rules are rough approximations only, and should be applied with caution. Officially produced tide tables should be used in preference whenever possible.

Those rules of thumb do qualify as rough approximations in those tidal areas with open access to the ocean. I think the Solent qualifies, although there is lag even there.

On major bodies of water with which I am familiar (Chesapeake Bay, Delaware Bay, Long Island Sound, San Fransisco Bay, most of the major inlets in North Carolina) the tidal and current sequences are out of sync by an hour or more. In the Bahamas I only pay attention to tide when aground, otherwise the current tables are much more useful.

[Uncle Buck]

Originally Posted by beetle

Too right - I've awakened in the mud in more than a few marinas, and don't fit into Clipper Cove to boot. On the other hand, Beetle goes to weather like a banshee!

My favorite local places are Drakes Bay and Half Moon Bay. Up at Drakes the anchorage starts at 80' and it's a stable even slope to the surfline so it's dial-a-depth by just motoring in until your happy and drop the hook. At Half Moon I avoid the area by the outer-harbor pier, elsewise it's good. It's nice to be in the eastern Pacific and have some depth. Thin water no good!

- rob/beetle

I think you may want to take a close look at your charts.....I am writing this while on the hook in Clipper....I draw 7'6 and anchor close to the beach. Last night was a minus 1.3' low.....and still had 10' of water. First time here and it's one of the deeper anchorages I have been to on the Bay (in terms of how close you can get to the beach).

[Mark Johnson]

With our Center Board up we draw 3' 4". We regularly anchor in 4', up a river or in the Chesapeake... IF we have a soft mud bottom, and know the tidal range. In the Bahamas and firm sand, 5 or 6'.

Mark

[Pyrate]

Even a soft bottom can rub off bottom paint. And the stress of putting weight on a keel that is designed to hang, not support the boat above it, is not a good thing in the long run. Just because you can run slightly dry in an anchorage doesn't mean you should. Especially because it just takes one other boat putting out a big wake to slam your boat repeatedly into the ground if your keel's giving lobsters a back rub. And we all know that there are those people out there that only complain about others' wakes without thinking of their own. Now, if you had a boat that was designed to be grounded (sideboards or centerboard, relatively flat bottom, removable or retractable rudder), then I wouldn't have any problem letting her rest gently in a soft spot. But with my current boat? Heck no!

Pyrate