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Go Back   SailNet Community > Out There > Destinations > Chesapeake / Central US east coast > Chesapeake Bay
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  #21  
Old 01-28-2013
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Re: Currents in the C&D Canal

I disagree with the suggestion that NOAA's tidal current prediction are based strictly on modeling. I always believed that their multi-harmonic models were tweaked based on the hundreds of sensors that they have. And until someone provides proof to the contrary, that's what I'll continue to believe.

If there are discrepancies between Eldridge, Garmin, NOAA, or others, I assume that it's because they update their models based on different weighting factors. It's the classic case of separating "trends" from "noise," and it will cause differing predictions, much like the weather. Over time you may decide to have more confidence in one than the other.

Recommended reading: Tidal Current Predictions and Data

C&D Canal is a perfect example of hydraulic current.

The Delaware river exhibits something between progressive wave and standing wave, depending on location. Up near Philly where I sail most of the time, slack typically lags high and low tide by about an hour. The progressive wave model explains why you can have a flood current even after high tide has passed. If you've ever stood in waist deep water at the beach and felt a wave pass you, you've observed a progressive wave. Right after the wave passes, the water level drops, but the surface continues to push you in toward shore, while the strong undertoe starts to pull you out. Similar things happen in rivers - high tide is a wave that moves up the river, but its wavelength is so long that you can't really see it aside from observing the changing tide height along pilings, seawall, etc.
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  #22  
Old 01-28-2013
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Re: Currents in the C&D Canal

BTW - the differences persist up and down the Delaware Bay, well outside the unique influences of the C&D canal.

I've posted a note with Garmins tech support team and will let folks know what they say. I suspect it'll be related to either their source data or their underlying model.

In any event, this is a classic example of why the prudent navigator will look to many sources, not just one. So carry a copy of Eldridge, print out NOAA, use Garmin and compare to what you see with your own eyes.
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  #23  
Old 01-28-2013
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Re: Currents in the C&D Canal

Quote:
Originally Posted by BubbleheadMd View Post
Timely discussion. I'm looking to sail around DelMarVa this year, and have been wondering how to time my passage through the C&D canal.

I'm looking to make a counter-clockwise trip, so I'll be hitting the C&D from the East, motoring West. James W. has been a big help with planning.

My engine runs very well, but I'm not interested in stressing it out. I hope I don't have to run at WOT for long periods in order to time the currents.
Bubble,

If you leave Cape May/ Cape Henlopen 2 hours before maximum ebb you will make it through the Canal to at least the Northeast/ Sassafras River riding the tide all the way. When are you thinking of doing this? ( date) I can give you more specific data.,
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  #24  
Old 01-28-2013
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Re: Currents in the C&D Canal

Quote:
Originally Posted by paperbird View Post
Just to add to the confusion a little.

I'm working on the planning for a possible June 2013 transit to NE including the C&D/Del Bay transit. The currents shown in Garmin BlueChart mobile and on the NOAA current tables don't match up.

Here's an example

June 1, 2013 for Reedy Pt, DE
Max flood Slack Max Ebb
Garmin 03:11 06:39 11:10
NOAA 04:49 07:35 11:49

Any ideas on what might cause the differences?


My reading from my Raymarine and confirmed by navionics on my I Pad are

From the Arnold Point shoal ( 10 miles s of Reedy on June 1 are)

slack maxebb slack max flood slack maxebb
6:02AM 10.12AM 12:30PM 3:15PM 6:43PM 10:43PM

From Finn Point 6 miles north of Reedy on June 1 are

slack maxebb slack max flood slack maxebb
6:17 AM 10:37AM 1:03PM 3:35PM 7:02PM 11:16PM


Confirms the Garmin Readings

If this was me taking the trip I would do th following. On May 31, the day before you will have the current through the Canal; in your favor from 9 AM till about 3:30 inthe afternoon. Go through the Canal and anchor behind Reedy and enjoy dinner till about 5 AM the next day. Or just continue through and get to Cape May harbor at 2 AM.

Setting your trip to be the fastest will be by using whatever technique to get to Reedy at 1 hour before the slack to ebb tide on the Delaware there which on June 1 looks like about 5:30 AM start time from Reedy.. Your best bet is to set your trip up by going through the Canal and anchoring at Reedy and leaving from there, 1 hour before slack tide before Ebb. This will enable you to reach Cape May Point before the current reverses. You will travel around 8 depending upon wind and sea conditions. Going South you get about 5.5 hours between Slack and the next incomming. Slack water at the Brandywine Shoal is about 10:00-10:15, (37 miles) Then another another 16 miles to Cape May Harbor. trip will take between 7-8 hours from reedy Anchorage to Cape may Harbo. If you leave at 5:30 AM you be in Cape May by 1:30 PM

I see you ahve a 42 footer so I am assuming power is no big deal.

There are two
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Last edited by chef2sail; 01-28-2013 at 07:04 PM.
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  #25  
Old 01-28-2013
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Re: Currents in the C&D Canal

I've reached almost exactly the same timetable you've laid out, Chef. Sneak out of work on Friday (work by phone) and meander up the Bay, through the C&D to behind Reedy Island. Then catch the dawn tide shift to ride it through the Delaware.

I've done the Delaware at night and it's not my favorite. Although it'll be a waxing 1/2 moon around the beginning of June.

Yeah - I'm not too worried about powering hard if we need to. In fact, I would be OK bucking the current for a slow ride through the Canal at night if need be to catch the ride down the Delaware at first light.

The huge unknown, of course, is the wind. And no way to know that until June comes. I'm plotting out the departure times for all the weekends in June figuring that one of them will offer a good weather window.
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  #26  
Old 01-28-2013
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Re: Currents in the C&D Canal

Quote:
Originally Posted by paperbird View Post
I've reached almost exactly the same timetable you've laid out, Chef. Sneak out of work on Friday (work by phone) and meander up the Bay, through the C&D to behind Reedy Island. Then catch the dawn tide shift to ride it through the Delaware.

I've done the Delaware at night and it's not my favorite. Although it'll be a waxing 1/2 moon around the beginning of June.

Yeah - I'm not too worried about powering hard if we need to. In fact, I would be OK bucking the current for a slow ride through the Canal at night if need be to catch the ride down the Delaware at first light.

The huge unknown, of course, is the wind. And no way to know that until June comes. I'm plotting out the departure times for all the weekends in June figuring that one of them will offer a good weather window.
Good show,

Weve done this trip about 40 times...The most boring part of heading north. Taking the morning tide usually means calm water in the AM down in the Bay before the onshore breeze starts cranking in and builds up the chop.

The canals not bad at night, but the Delaware is not my favorite as there can be floating objects

Where are you going in NE?. brokersailor is headed nborth in mid June I think. Where do you sail from. Are you stopping in Cape May?

We wont be heading up till mid August as we like no crowds and theres no wind here in the Chessie usually.

dave
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  #27  
Old 01-28-2013
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Re: Currents in the C&D Canal

Quote:
Originally Posted by jameswilson29 View Post
Even though I had 10-15 knots out of the ESE, I motorsailed the most of the way up the Delaware Bay and Delaware River to the C&D, so I could make the 8 knots over the ground necessary to ride the current for 14 hours. The difference was the boat never slowed down, so the P28 would hit 8.5 knots surfing down the front of the chop and not slow down to 5 knots on the back.

You can hear the engine running (and my rudder clicking) in the video, while I sail at maximum speed: Glorious day on the Delaware Bay... - YouTube
Does the sun ever shine down there? Every video I've seen has dark gray or black clouds and/or small craft advisories playing in the background.
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  #28  
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Re: Currents in the C&D Canal

Just can't get enough, huh? Tentative plans are early to mid-June. Don't know brokesailor - will have to look him up.

We live in Annapolis, but keep the boat at Herrington North. Have done the round trip once as a family. But also the North-then-West one way a couple times. Last time we stopped in Cape May for a night's sleep, but we had 3 weeks for the trip. This time we may be limited to 2 weeks so am considering not stopping. Our air draft (53') is cutting it close for the Cape May Canal anyway. If we have to go around, then I'm inclined to just keep going for Block.

Funny story - when we came through the canal at night, I was asleep below and my wife was on watch. She yelled that a big ship with a strange light configuration - 2 whites in a horizontal line - was heading right towards us really, really fast! Turned out to be a truck driving on the access road next to the canal. But a serious enough adrenaline shot that she was very alert the rest of her watch.
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  #29  
Old 01-28-2013
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Re: Currents in the C&D Canal

Quote:
Originally Posted by paperbird View Post
...I've posted a note with Garmins tech support team and will let folks know what they say. I suspect it'll be related to either their source data or their underlying model...
Please let us know what they say - it will be interesting to see what they are willing to tell you. I have the Bluechart mobile on an iPad at home, so I just looked at its current predictions. It's still so new that it hasn't become my go-to device (I still use OpenCPN for mostly everything), so I'm just learning.

NOAA's multi-harmonic model is very sophisticated (I read somewhere that it uses over 12 harmonics), and the source code can be purchased from them for a fee. Despite what some may believe, it is based on what I believe to be the world's most extensive collection of historical data for US waters. That historical data is available openly, and companies like Garmin almost certainly use the same data in developing their own models.

Unfortunately, much of the tidal modeling that used to be freely available for use has been tied up by private companies (like Garmin) making intellectual property claims, so unfortunately there will never be a convergence to a "single best" prediction (unless you believe that nobody can top NOAA's brainpower, which unfortunately has become as much an ideological question as a scientific one). But it's almost certain that everyone uses the same NOAA data (though different companies may update their models for recent data more or less frequently). So the main difference is probably the model that's used to fit the data and make future predictions. There are some interesting comments on the difficulties of doing this here on the XTide support page.

I took a quick look at Garmin's current predictions on the iPad, and the waveforms look very sinusoidal, which is a signal that they use fewer harmonics in their predictions than others. So in general, it may be less precise than models with more harmonics, although there is a valid question of how precise anybody's prediction can be because the effects of rain, wind, and other less predictable effects would limit the precision of any model, no matter how sophisticated. At some point your prediction becomes far more precise than Mother Nature is willing to cooperate with.

FYI, OpenCPN has some very complex wave shapes in both their tide and current predictions, which is a sign that their predictions (based on the XTide algorithms, but with significant manual tweaking) incorporate a lot more harmonics than Garmin's. Once again, whether that actually leads to a better prediction is up for debate.

In addition to the XTide support site, there is also some very interesting background information on tide/current predictions on NOAA's site here. For instance, the lower harmonics are such a low frequency that you need 18.6 years of tidal data to make a reliable prediction. This is not something that you can do by flipping through a few pages of data and manually fitting a spline curve!

By the way, click here for a fascinating rant on how privatization and "un-public-domaining" of tidal data has made it virtually impossible for anyone to offer free tidal predictions in Europe.
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Last edited by TakeFive; 01-28-2013 at 11:40 PM.
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  #30  
Old 01-28-2013
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Re: Currents in the C&D Canal

Quote:
Originally Posted by takefive View Post
please let us know what they say - it will be interesting to see what they are willing to tell you. I have the bluechart mobile on an ipad at home, so i just looked at its current predictions. It's still so new that it hasn't become my go-to device (i still use opencpn for mostly everything), so i'm just learning.

Noaa's multi-harmonic model is very sophisticated (i read somewhere that it uses over 12 harmonics), and the source code can be purchased from them for a fee. Despite what some may believe, it is based on what i believe to be the world's most extensive collection of historical data for us waters. That historical data is available openly, and companies like garmin almost certainly use the same data in developing their own models.

Unfortunately, much of the tidal modeling that used to be freely available for use has been tied up by private companies (like garmin) making intellectual property claims, so unfortunately there will never be a convergence to a "single best" prediction (unless you believe that nobody can top noaa's brainpower, which unfortunately has become as much an ideological question as a scientific one). But it's almost certain that everyone uses the same noaa data (though different companies may update their models for recent data more or less frequently). So the main difference is probably the model that's used to fit the data and make future predictions. There are some interesting comments on the difficulties of doing this here on the xtide support page.

I took a quick look at garmin's current predictions on the ipad, and the waveforms look very sinusoidal, which is a signal that they use fewer harmonics in their predictions than others. So in general, it may be less precise than models with more harmonics, although there is a valid question of how precise anybody's prediction can be because the effects of rain, wind, and other less predictable effects would limit the precision of any model, no matter how sophisticated. At some point your prediction becomes far more precise than mother nature is willing to cooperate with.

fyi, opencpn has some very complex wave shapes in both their tide and current predictions, which is a sign that their predictions (based on the xtide algorithms, but with significant manual tweaking) incorporate a lot more harmonics than garmin's. Once again, whether that actually leads to a better prediction is up for debate.

In addition to the xtide support site, there is also some very interesting background information on tide/current predictions on noaa's site here. For instance, the lower harmonics are such a low frequency that you need 18.6 years of tidal data to make a reliable prediction. This is not something that you can do by flipping through a few pages of data and manually fitting a spline curve!
tmi
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