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Old 12-18-2012, 12:40 PM   #1
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Tides and Forward Progress

From another thread Marin's friends seem to think one's progress as affected by tidal currents does not average out. I think it's obvious that it can be no other way given enough time underway in tidal waters.

With the exception of river influence tidal currents spend 50% of their time going one way and 50% of their time going the other way and when their velocities aren't the same other tidal current will balance that out so if one traveled over time their progress would average out so they would be running with the current half the time and bucking it half of the time.

However People's notions about it dosn't seem to mach up. I think it's like rainy days. Sunny days come and go but if it rains on the weekend people will be whining about the rain big time. And they won't be inclined to forget it for quite some time. When we run w the current we think or say "oh thats nice" but when we buck it for hours the effect of the current has much more emotional impact. And when the trip is over and a year has past we tend to think the current was against us much more than with us. I submit that that is true no more than an old wives tale.
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Old 12-18-2012, 01:23 PM   #2
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In our area, with long deep channels, the tides change from the edges in towards the centre. The main body of flow in mid channel slows down and reverses at a slower rate than the shallow water at the edges. Ever see those long, thin foam lines parallel to the shore? That's the line of opposing currents which you can feel through the seat of a kayak when you cross it. Sometimes sticks will spin in circles in the foam.

Knowing this, you can milk the main flow mid channel for an extra bit, or scoot closer to shore if that's an advantage. So, in other words, it doesn't have to be 50/50
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Old 12-18-2012, 01:24 PM   #3
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Eric- There are the 'doers' and the 'cut and paste' posters.
Why do you want to waste your time with it?
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Old 12-18-2012, 01:32 PM   #4
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Anode,
Peraps there's something to learn. There's probably 100 guys here that will learn from MurrayM's input. Frequently I move around in a channel to find the best water and my looking at this question in simplistic terms isn't reality. Just like the gas engine thing I like to explore. But you're completely free of all this aren't you?
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Old 12-18-2012, 01:38 PM   #5
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You're 100% correct. Using Marin's logic the Sound would soon be empty and the bay's all dry.

While I would say that running against a river outflow would affect you're progress, it still is invalid because most of us RETURN from our journeys eventually, so any extra fuel would be recovered on the home trip.

Another thing would be the prevailing winds. They alter the times I like to head out. Sometimes I'd rather buck a current than deal with deal with quartering seas.

It's all about choices.
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Old 12-18-2012, 01:40 PM   #6
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Knowing this, you can milk the main flow mid channel for an extra bit, or scoot closer to shore if that's an advantage. So, in other words, it doesn't have to be 50/50
I do this a lot if know the tide is changing under me and I'm gonna get caught in it. Also, cutting the inside of a corner will put you across the tide longer.
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Old 12-18-2012, 01:41 PM   #7
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When I put the camper on the truck and head out on a trip it is always uphill. That's just life.
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Old 12-18-2012, 01:58 PM   #8
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When I put the camper on the truck and head out on a trip it is always uphill. That's just life.
That's because it's downhill into both Smithers and Prince Rupert

Howdy neighbour, from Kitimat
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Old 12-18-2012, 03:08 PM   #9
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I believe Marin is right and I'll attempt to prove it mathematically.

Imagine cruising a 2 NM course, round trip at 6 kts with no current. Each leg will take 20 minutes (2/6=.333x60=:20) for a :40 round trip.

Now cruise the same course with 2 kts of current with you and against you. Leg 1 is into the current yielding a 4 kt speed over 2 miles and it takes you 30 minutes. (2/4=.5x60=:30)
Leg 2 is completed at 8 kts with the push from the current and takes you 15 minutes.(2/8=.25x60=:15) Adding leg 1 and 2 is :30+:15=:45.

The same course traveled with the 2 kt current took 5 minutes longer. The reason is that when the current is helping you, you have less time to benefit from it. When it slowing you down, the effect of the current hits you over a longer span of time.

That's this engineer's take on it anyway.
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Old 12-18-2012, 03:12 PM   #10
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On the east side of Princess Royal Island, if you time it right, you can travel with the current on the flood, keep going in the same direction, and keep going with the current on the falling tide.

We got there on a calm day on a rising tide and there was a weird band of jumpy little wavelets from shore to shore. On the chart there was a notation that read, "Tides Meet".
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Old 12-18-2012, 03:34 PM   #11
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Here ya go....pick your location

Tidal Current Tables
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Old 12-18-2012, 03:49 PM   #12
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That's this engineer's take on it anyway.
WHAT?

OK... when would take a 2 NM trip... and go so slow that THE TIDES are a problem for you. (you're making trawlers look bad around the world)

We originally were talking about maximum range and accounting for the tides on long (Inside Passage types of runs)

Here's another for you (engineering problem)

I get up in the morning and notice that if I wait 2 hours to get started... I can catch the Slack tide ahead of the flood... the flood itself... and then the slack before the Ebb tide starts.

I can take advantage of all three, run for 6 hours and never have a problem. (plus have an extra two hours to enjoy my coffee)

Each day, there are two high tides and two low tides (about 12 hours, 25 minutes apart). Therefore... at worst, I'd have to wait 4 hours (& 12 minutes) to miss whichever I wanted to avoid and take advantage of the other.
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Old 12-18-2012, 04:18 PM   #13
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You're 100% correct. Using Marin's logic the Sound would soon be empty and the bay's all dry.
Of course it would. You guys need to learn to read better. I never said the overall flows don't cancel each other out. Of course they do. My dog knows that.

I said that in running a boat through the maze of islands and channels along this coast, the current effects on the boat will not always cancel themselves out. Anyone who's actually run a boat around here-- and I'm beginning to think some of you haven't or if you have you've not been paying attention to what's going on around you-- knows that when you follow a winding course through the islands you will encounter all manner of local currents, some helping, some hindering your forward progress even though the overall flow is ebbing or flooding.

And, depending on your route, you can encounter more adverse current than helpful or neutral current, or it can be the other way around. This happens to us every time we go to destinations in the islands. The fact most of you don't seem to realize this is pretty amazing, frankly. Everyone we boat with is well aware of it.

The same is true going up the Passage. So you can't assume that all the local current effects on your way north or south will cancel themselves out. According to the people I know who've done it, as opposed to speculating about it from the couch in front of the TV, the current effects on their boats do not equal out on these trips and some of them have the log and fuel consumption records that show this.

One person here who seems to have a grasp of this local current concept is Murray. What he's experienced with varying current strengths and probably directions in different channels is what I'm talking about.

How the current will effect you depends on the route you take and what the current does locally along that route. And these effects will not always cancel themselves out.

My dog knows that, too.
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Old 12-18-2012, 04:26 PM   #14
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The same is true going up the Passage. So you can't assume that all the local current effects on your way north or south will cancel themselves out.
But you also cannot assume that it's harder to get UP the Passage, than it is coming back.

Traveling long enough, and choosing your departures and arrivals will easily null out with just a little forethought. I can get there without EVER bucking a tide if I chose, but I also understand how tides, currents, ebbs and flows can work to my advantage.

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My dog knows that, too.
My dog isn't allowed to advise me on matters of navigation.
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Old 12-18-2012, 04:40 PM   #15
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But you also cannot assume that it's harder to get UP the Passage, than it is coming back.
I never said that. I'm at the bottom of it so to me a trip along the Passage is "up." If I lived in Price Rupert a trip would be "down" and I would have used that term. This all started when I said in another thread that our boat barely had the range to get up the Passage to Prince Rupert, but because of the current effects and winds we probably wouldn't get anywhere near that far. But then someone said the current's not a factor because it cancels itself out and that, in this case, is wrong for the reasons I've stated.


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My dog isn't allowed to advise me on matters of navigation.
You might want to talk to him about it. Particularly about local currents. Could be you might learn something.
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Old 12-18-2012, 05:00 PM   #16
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Here's another variation on our local currents; during heavy fall rains there can be so much fresh water flowing on top of salt water that the surface flow in long channels can actually be opposite of the incoming tide. You know it's raining hard when seagulls are taking fresh water baths in the middle of two mile wide channels.
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Old 12-18-2012, 05:00 PM   #17
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Sometimes you can have it both ways, depending very much on your cruise plan. When I was in Jersey City on the lower Hudson, our tide averaged about 3-4 knots. If I was just taking a day trip I would cruise upriver
towards Croton-Harmon on a rising tide or go the other way under the Verrazano and out towards Sandy Hook on a falling tide. The trick was to coincide your return with the tide swing. Not very practical if you are going to your favorite fishing spot, but it was neat to have a favorable tide both ways while just messing about.
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Old 12-18-2012, 05:03 PM   #18
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FlyWright is absolutely correct. And Marin won't even give him credit even when it's to his favor?????

FlyWright I wonder if this "time each way" factor (not theory) is significant enough to be noticeable in seat of the pants real life activities like Marin's friends that profess to know from experience. To know this it would seem to me it would have to affect overall cruising time very significantly.

For all practical purposes one is being slowed or sped along basically all the time but of course most of the time it's not enough tide to be aware of unless one is doing high tech navigation. And if this time each way factor (TEW) the wind will also play it's part in much the same way. I've always thought going both ways in calm water (whereas going 6knots is going 6knots) would/could not be the same or come out equal. Bucking the tide must have it's price. But I failed to transfer that over in this case or perhaps that's what drove me into this question.

So as far as what's right FlyWright has proven that bucking tides does not come out even or put another way ..... a wash.

So Anode you see ther'e are things to learn after all.

But I doubt that Marin's friends can know this without applying the same armchair theories that we did and then of course they wouldn't have needed to travel 2000 miles over water to know that. But perhaps someone can shor how that information should come to pass through normal navigational proceedures. Perhaps there's more to learn yet.
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Old 12-18-2012, 05:04 PM   #19
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There area few of us dock queens that have left the dock and because of slow deep draft we do pay attention to the current and tides.

Many electronic charts have tide/current calculator and simulations. After plotting the course I always go back and simulate the tide and current for narrow channels and marinas with a 5’9” draft. There are many channel and marinas we can not enter at low tide, and/or go thru as the current is swifter than the Eagle’s head way. I clock the Eagle at 14+ knots going through some narrow passages. Deception Pass is 5 kts. Tacoma Narrow is 4 knts, agate passes is 3 knts. In the San Juan there are several that are 6.0+ knts. When we go thru narrow/shallow/swift passages/channel I like to follow a sail boat. Sail boat make great bottom finders and they are slow.

The general rule when running a river/current is to stay to the out side as the depth is probable deeper as the current has gouged out. Might be Ok to cut the corner if you know the area and/or shallow draft. Anyway I do not cut corners.

Just north of Campbell River the tide changes, so if you time it right you can ride the incoming tide and then ride the out going tide. The current is quite swift through there, 7.0 knts and rather large whirl pool. Looks rather daunting looking down into a whirl pool several ft deep. Suck you right up!

I also simulate using the tide current on best time to depart and/or arrive. However, once out in the open Puget Sound the current tide is not that much, 0.5 to 1.0. So knowing tide and current charts can be very important especially with a slow deep draft boat.
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Old 12-18-2012, 05:11 PM   #20
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But then someone said the current's not a factor because it cancels itself out and that, in this case, is wrong for the reasons I've stated.
Just because you stated it, doesn't make it so.

All things being equal... if you sailed around the world for a hundred years, day an night... to the same destinations and back, they would most definitely cancel out. That being said, ruling out departure times, river crossings and narrows, the same would apply. (contrary to your position)

It's moot because I can gain benefit of EVERY tide if I choose. And you... for the sake of being right... can buck the tide every time if you so choose.

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You might want to talk to him about it. Particularly about local currents. Could be you might learn something.
He's asleep on the couch. It's hard enough listening to you explain it. You seem to be a pretty smart guy... but certainly are hard-headed.
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