Tidal Cycle? Help me out here!

[jackdale]

Originally Posted by SimonV

Moon shmoon. It's simple if the tide is out, it will be back in about six hours. Simple.

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Only if it is semi diurnal.

[IdoraKeeper]

A very good subject indeed. Local tables for areas of swift current in areas of strong geographic affects are surprisingly accurate. I was once motivated to stay on a mooring in Obstruction Pass in order to attend a family event. The bottom under the mooring was jagged boulders.(certain disaster) Leaving the mooring before the expected minus low was a safety issue. We corrected our depth sounder reading for transducer location using a lead line and interpolated the time from the tide chart that we would need to leave the mooring to insure a mere 12 inches between the rocks and the keel. We left on time and the error was 6 inches....low. Whew!!!!That was close. Science is cool huh? Perhaps the moon was a little early? Perhaps the slosh was a little slow?

[IdoraKeeper]

A request for Jack...please explain the difference between Canadian chart datum and US chart datum. It can't hurt to remind folks of the difference when thinking about moorings, docks and anchorages.

[jackdale]

Originally Posted by IdoraKeeper

A request for Jack...please explain the difference between Canadian chart datum and US chart datum. It can't hurt to remind folks of the difference when thinking about moorings, docks and anchorages.

Canada is Lowest Normal Tide, US charts (at least in the PNW) use Mean Lower Low Water. The difference can be as much as 1.5 meters.

Let's just all go to LAT (Lowest Astronomical Tide)

CHS charts are generally also metric. A few are still fathoms and feet.

Read the Title Block.

[NewMoon]

Originally Posted by deblen

The sun gets involved also especially during spring & neap. If the sun's pull is aligned with the moons pull,you get a spring.Conversely.if sun's pull is opposite the moon's,you get a neap. This happens twice per month roughly.

Maybe it's just another way of saying the same thing, but here's my understanding:

The sun's gravity is also quite influential. When sun and moon are pulling in the same direction from earth (new moon) we get spring (largest) tides. Also, when they are pulling on opposite sides of the earth (full moon).

When sun and moon are pulling at 90 degrees from each other relative to earth (half moon), we get neap (smallest) tides.

On the Inside Passage, further in toward the mainland especially, spring tides can be as much as 24 feet. Not Bay of Fundy, for sure, but impressive.

[Hoosiersailor]

Originally Posted by a64pilot

Lord, you guys are way over my head, all I know about the cycle is it goes out, where does it go? I don't know, seems to always come back though

I just read the entire thread and I'm with you...

[Simonsays]

Originally Posted by Hoosiersailor

I just read the entire thread and I'm with you...

over here weather forecast usualy includes tidal predictions, at least for the North Sea, the Baltic side just goes "ti-what?" (they have a 30cm difference low to high)

[Mike1956]

Thanks for ALL replies, I am the OP.

Before I posted you will be aware I was sure I was correct. This information is no secret! I just needed reassurance before I fire another email off to the publisher of the training booklet.

Raindog nailed it in post 2.

Spring & Neap tides would occur randomly if they do not follow the Synodic/Lunar month. My understanding is that Lunar month is another name for Synodic month.

During my correspondence with the same publisher he advised me of something else I feel is probably incorrect but have been unable to determine, perhaps someone can help.....

I think we are all in agreement that the Synodic/Lunar month averages about 29.53 days. A google determines this never varies by more than about 7 hours.

This period is made up of 4 phases of the Moon and interestingly the individual phases can vary by much more. However I have doubts about the publishers claim that a phase of the Moon can be as little as 5 days and as much as 9 days.

I may be wrong, he may be right. I have so far been unable to determine this.

Any ideas.

Mike

[Reefmagnet]

Originally Posted by Mike1956

Thanks for ALL replies, I am the OP.

Before I posted you will be aware I was sure I was correct. This information is no secret! I just needed reassurance before I fire another email off to the publisher of the training booklet.

Raindog nailed it in post 2.

Spring & Neap tides would occur randomly if they do not follow the Synodic/Lunar month. My understanding is that Lunar month is another name for Synodic month.

During my correspondence with the same publisher he advised me of something else I feel is probably incorrect but have been unable to determine, perhaps someone can help.....

I think we are all in agreement that the Synodic/Lunar month averages about 29.53 days. A google determines this never varies by more than about 7 hours.

This period is made up of 4 phases of the Moon and interestingly the individual phases can vary by much more. However I have doubts about the publishers claim that a phase of the Moon can be as little as 5 days and as much as 9 days.

I may be wrong, he may be right. I have so far been unable to determine this.

Any ideas.

Mike

Sounds like hogwash to me. There are some variations caused by orbital eccentricity and such, but a glance at a phase calender shows phases occur roughly around the 7 to 8 day mark (well, for 2016 at least)

http://www.calendar-12.com/moon_phases/2016

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[Mike1956]

I do accept that the phases of the Moon vary quite a bit. Assuming the average phase is 25% of the Synodic month +/- 7.3 days, the variation can certainly be 30 hours or more but I doubt as much as the publisher suggests. Just have been unable to put my finger on it!

Mike

[Jdege]

What is usually left out of the grade-school explanations of tides is that how the gravity of moon pulls the oceans to make peaks under the moon and opposite to the moon, and troughs at 90 degrees on either side is true only in the southern ocean, where the oceans are free to flow 360 degrees around the globe.

Everywhere else, what we experience as tides are enormous low-frequency waves that propagate northward.

In the roaring forties, high tide is beneath and opposite the moon. Everywhere else, the timing of high tide is dependent upon how long it takes for the wave to propagate northward. At any given longitude, high tide is later the farther north you go.

[TeddyDiver]

Originally Posted by Mike1956

I do accept that the phases of the Moon vary quite a bit. Assuming the average phase is 25% of the Synodic month +/- 7.3 days, the variation can certainly be 30 hours or more but I doubt as much as the publisher suggests. Just have been unable to put my finger on it!

Mike

Well if the publisher is using "wrong" month they surely get more variation too

[deblen]

Originally Posted by NewMoon

Maybe it's just another way of saying the same thing, but here's my understanding:

The sun's gravity is also quite influential. When sun and moon are pulling in the same direction from earth (new moon) we get spring (largest) tides. Also, when they are pulling on opposite sides of the earth (full moon).

When sun and moon are pulling at 90 degrees from each other relative to earth (half moon), we get neap (smallest) tides.

On the Inside Passage, further in toward the mainland especially, spring tides can be as much as 24 feet. Not Bay of Fundy, for sure, but impressive.

You are correct & I was wrong in my post #12.
I said a neap occurs when sun & moon are pulling against one another.
Actually,as you say,a neap is when sun & moon's pull are at 90 deg. to one another. We call them dead tides locally as opposed to full tides(springs).
There are 2 full tides per month(at full & new moon) & 2 neaps per month. In practice,this means a week of spring,week of neap, week of spring & week of neap.

Link to Saint John,N.B. tide tables-SJ is about 1/2 way up the bay.

http://www.waterlevels.gc.ca/eng/station?sid=65

Another important point when traveling in unfamiliar tidal waters is local knowledge.
Tide tables are helpful but asking the locals is wise.

Cheers/ Len

[Jdege]

Originally Posted by Jdege

Everywhere else, what we experience as tides are enormous low-frequency waves that propagate northward.

A followup:

Animations of models of the tides of the oceans

[ThresherMan]

The thing that has me baffled, and perhaps someone here knows, is how the tides have switched times. Here in SF, when I was a lad, the the extreme low tides in the early morning were followed by the 'king tides', around noon or so. Now that I have returned to the bay area, after 25 years, the king tides are early and followed by the extreme lows.

When did this happen?

Ed