Tidal Current Charts

[John C. Mannone]

Hello,

Does anyone know how I can get a tidal current chart for parts of New England?

An example of what I want appears in an article called "Using Tidal Current Tables", Jim Sexton, July 31, 2000 on Sailnet:
http://www.sailnet.com/collections/seamanship/index.cfm?articleID=sexton0059

I have found online software, like WWW Tide and Current Predictor than gives tabulated or graphical output for Boston Light, but not for Charles River, Charlestown, MA (it gives only tides here). Unlike other free online software, it is not limited to the present day (or week). Data can be generated for many places for anytime beteen 1970 and 2025. I am interested in specific seasonal information (three days after an April full moon for any year).

Oddly, Sailnet, who seems to produce this software (as evidenced by their copyright on the tidal current chart shown in the link), will not respond to questions by e-mail (nor will they respond by telephone or return calls!).

Data is nice, but a pretty nautical type chart is far more appealing.

Thanks in advance,
John

[Mike Field]

John, it's my understanding that US Pilot Charts give the sort of information you're after. Whether such charts are available for New England coastal waters you would have to find out. (The Navy's Oceanographic Office is the publisher.)

[John C. Mannone]

Hello Mike,

I search Google for US Pilot Charts and found http://chartmaker.ncd.noaa.gov/

The Coast Pilot on this site has a free download, including the New England area I am interested in. Book 1, Chapter 11 details navigational data and mentions average currents in various parts of Boston harbor. This is helpful.

However, I really like to have the attractive Tidal Current Charts as exemplified in the link to Sailnet above for the Charles River/Boston Harbor area for my final report.

Thanks for your help.
John

[Ian McColgin]

Either Eldridge's or Reed's can be obtained mail order from places like WasteMarine.

Eldridge's is more focused on New England tides and currents.

Reed's covers the whole east coast (other editions have other coasts and countries so be sure you get the right one) and has a lovely coast pilot, astronomy, etc. Much fun.

There's a companion volume to Reed's that has the stuff that does not go out of date - msl seamanship, rules of the road, communications, etc etc.

G'luck

[John C. Mannone]

Hi Ian,

Then is my understanding correct that to get those attractive charts showing the currents, I need to buy a hard copy of Eldridge's or Reed's work?

I was hoping for an electronic version I could access from the 'net. I did find previously Eldridge's software that may have produced the charts I want, but it is supported by PC only (I am a Mac man).

Thanks,
John

[Donn]

John...most online or electronic current predictors use data published by NOAA, and NOAA does not publish current data on Charles River. I just checked "Tides and Currents" and while they have tide predictions, there are no current predictions.

[ishmael]

Without looked at the charts or maps for what you are trying to figure out, Boston harbor and environs has changed substantially since the 18th century. I would think modern tidal current charts questionable in this regard, but don't know. Perhaps old Admiralty charts would be more in line? Hmm, how to get ahold of those? I'm sure, in an archive SOMEWHERE, is a currents chart of Boston harbor at various states of tide, circa 1775. Maybe post a question to ACB about it?

What, exactly, is your project?

[ 12-16-2002, 01:31 PM: Message edited by: ishmael ]

[John C. Mannone]

Hi Donn,

I too found that on the WWW Tide and Current Predictor (free online software). However, It will give it for Boston Light. I would happily settle for the Boston Harbor tidal current charts for the attractive graphic in my PowerPoint presentation.

It would be interesting to take these currents to be the same as at the mouth of the Charles River/Boston Harbor. Then, I think I may be able to calculate the current through the narrower "inlet" from the dimesions and depths of the harbor mouth and inlet channel.

I have some data already, the important thing is to get a pretty graphic.

Thanks for your input,
John

[Donn]

John...this wouldn't have anything to do with determining the currents faced by Paul Revere and his oarsmen, would it?

[John C. Mannone]

Hello,

Yes Donn, you've guessed it. This is the continuing saga of uncovering interesting physics and engineering surounding the events of Paul Revere's ride. I have reasonable information and calculations to make my point for historical value and any more could convict me of over analysis. However, the attention to detail and authenticity adds credibility, which in turn, increases the interest in my work. The exploration for these details also demonstrates the importance of resourcefulness to my students. Also, with a higher level of detail, more advanced calculations can be demonstrated to them. Forgive the pontification. Most of this is for fun.

Now, back to one of the issues. I want to calculate the bearing, rowing time, etc. for Betley and Richardson rowing Revere across the Charles under young flood conditions. These are some of the main reasons for knowing or deducing the rowboat design, operation and steering, and currents. Seasonal currents in Boston Harbor seem to be around 1.5 kts. For now, I assumed the rowboat encountered a 2-knot direct cross current through the Boston/Charlestown inlet. There approximate relative bearing was approximately 40 degrees towards the east.

The immediate Boston area has substantially changed since 1775. I found several good historical maps which show nautical information. The American Memory map collections 1500-2002 will show it. Follow the links from <http://memory.loc.gov/> to Collection Finder, then search on Boston Harbor. However, the main channel hasn't changed (I think).

I am familiar with Fischer's work and others and have been corresponding with several museums in New England about my efforts. Apparently, I am treading on some virgin territory. Your help is greatfully appreciated and will be acknowledged.

Thanks,
John

[rodcross]

Tidal currents are not that difficult to calculate. Although many of the current charts are based on observations, many are calculations based on a rule of thumb that is given in Reeds to enable you to determine water movement if you have the tide information.

Wouldn't you have to know the combined horsepower of 'Betley and Richardson' (Guys I've never heard of.) in order to make it historically accurate?

I mean, if Betley was big and Richardson was little, wouldn't they have gone in circles?

[ 12-16-2002, 02:49 PM: Message edited by: rodcross ]

[John C. Mannone]

Hi Rod,

The two oarsmen that rowed Revere across the Charles can be found in several references like "Last Refuge of Scoundrels: A Revolutionary Novel" by Paul Lussier.

http://www.twbookmark.com/books/75/0446523429/

These men were Paul's friends and associates. At least, Joshua Bentley was a boatbuilder. He learned his trade from his father who was a shipwright. I know even less about Thomas Richardson, but in the least, he was an experienced waterman.

If their rowing is synchronized, the bigger of the two will not cause their trajectory to curve unintentinally. Their strength (combined power) is limited by the boat design. I understand the approximate formula for the hull speed (theoretical top speed) of a displacement hull in knots to be 1.2 times the square root of the waterline length in feet (I have not verified this yet). A 14' length displacement hull rowboat cannot exceed 4.5 kts, no matter how hard you row.

It is true that it would be easier to row with two rowers instead of one. Based on what I learned from some canoers, If one rower expends a certain amount of energy to go, say 3 kts; then adding a second rower so that the rowing power is doubled, will only increase the speed to about 4 kts. This seems to make sense to me because of the added weight and the rapid increase in friction with increase in speed.

I can use the top speed possible for a bounding calculation (minimum crossing time). I suspect that as much as they wanted to just hull-out (pun intended) and get across as quickly as possible, they probably had to "throttle" back. The covert nature of their mission and danger of their imminent capture required stealth and perhaps slower rowing speeds.

John

[Donn]

John...a couple of questions. Did they land directly across the river from their launch site? Did they each use two oars, or did they sit side by side and man one oar each?

[Bruce Hooke]

If nothing else, you can bet that these two rowers where in much better shape, and much better rowers, than your average 21st Century "Yachtsman" who only rows if he absolutely cannot manage to start the motor on his Zodiac... :D

[John C. Mannone]

Hi Donn,

the landing point is fairly well established at Charlestown Battery just outside of the city. The departure point is a little vague; simplyfrom a wharf in North Boston (probably close to the Ferry Lane). The compass rose on the old nautical charts suggest a possible range of courses between 345 to 005 degrees with due north being most probable (I don't yet know if this is true course or magnetic course). In any case, they pretty much went straight across the channel.

It was a small rowboat. I believe the two rowers sat in tandem, each with a pair of oars, while Revere sat on the stern sheet. I am not sure how factual this is. Therefore, the different rowing configurations suggested should be examined.

I am waiting for Carl Zellner's analysis of the crossing that the Paul Revere Memorial Association has sent me by snail mail; it might shed more light.

Thanks again,
John

[Donn]

John...they may not have gone straight across. If they needed to sneak by the Evil Empire's vessel, they may have rowed up-current first, in order to slip back downstream more quietly.

[rodcross]

I admire people who take things seriously, no matter how arcane,

Can't we just accept that they got across?

[John C. Mannone]

Hi Rod,

I don't know if that was a compliment or an insult. I'll assume its the prior. Thanks.

With the exception of a few skeptics (not I), we all accept that Revere got across. My interest in the details is partly academic and mostly for the fun. Scientific analysis always brings this to me, even if it is speculative.

Some say that the rowboat passed close to east side of the Somerset, others say to the west; yet another feels it started and rowed well east. The question is not settled. In itself, historically it doesn't matter, they got passed it. However, the questions each scenario fosters are quite remarkable from an astronomical point of view. Though addressed to some extent in Sky & Telescope (to which Fischer's fine work alludes to), there are some questionable conclusions.

As you know, the tides, with its associated currents, are an astronomically explained phenomenon. These play an important role in trying to sort out the most likely scenario.

Now, I want to encourage you. This forum has been a blessing to me. It has provided me information I couldn't get elsewhere, has stimulated deeper thought, and has been sheer fun.

Everybody, please continue with your helpful comments and thought provoking questions.

Thanks,
John

[John C. Mannone]

Hi again Rod,

You may be right; in your earlier post:

" ...if Betley was big and Richardson was little, wouldn't they have gone in circles?"

I just finished reading the materials that Paul Revere Memorial Association just sent me. The Revere House Gazette (Issue no. 58, Spring 2000: "Paul Revere's Rowers", by Carl Zellner) shows a picture of a painting (by A. Lassell Ripley) commissioned by the Paul Revere Insurance Company in the 1960s. It shows a small rowboat (not much bigger than 12-13 feet, best as I can estimate) with the 3 patriots crossing the Charles River. It shows Paul Revere standing at the stern section (I think for dramatic effect). But, interestingly enough, Bentley and Richardson are rowing, each with one oar! Even though this painting is based on the artist's impression, it does support your question of one rower possibly over powering the other, with potentially poor consequences

Zellner suspects Bentley was "short and thick set". Very little is known about Richardson. It is noteworthy, that in April 1775, Revere was 40, Bentley was 48 and Richardson was 26.

Though the watermen were experienced, I imagine that this rowing configuration, if it is correct, added to the challenge of the night crossing and tidal currents.

John

[John C. Mannone]

Hello everybody,

Well, with the help of maineharbor.com, I found a source that displays tidal currents on a chart. This is a PC-only Nobeltec software- Visual Navigation Suite with a separate Advanced Weather Overlay (I run a Mac, plus the software is expensive).

It will generate present and future data only. I am interested in April 16, 2003 Boston Harbor data. Unfortunately, they won’t run it for me.

Is there anybody with this software who could do me a big favor and generate such a chart for me? It is for a lecture series detailing the astronomical impact on historically significant events, like Revere’s crossing of the Charles April 18, 1775.

Thank you so much,
John

[Donn]

John...I can't post a chart from my Tides and Currents package, but I can give you the hourly current data from the closest location I can find to Charles River (42*22.18N 71*03.38W)

Wednesday, April 16, 2003
Currents - East Boston, Pier 10, southeast of (10)
42*22.55N 71*02.80W

12:00am 0.2kt ebb
1:00am 0.3kt ebb
2:00am 0.3kt ebb
3:00am 0.3kt ebb
4:00am 0.3kt ebb
5:00am 0.2kt ebb
6:00am 0.0kt slack
7:00am 0.2kt flood
8:00am 0.2kt flood
9:00am 0.2kt flood
10:00am 0.1kt flood
11:00am 0.1kt ebb
12:00am 0.2kt ebb
1:00pm 0.3kt ebb
2:00pm 0.3kt ebb
3:00pm 0.4kt ebb
4:00pm 0.3kt ebb
5:00pm 0.3kt ebb
6:00pm 0.1kt ebb
7:00pm 0.1kt slack
8:00pm 0.2kt flood
9:00pm 0.2kt flood
10:00pm 0.2kt flood
11:00pm 0.1kt slack
12:00am 0.2kt ebb

Plug these values into a spreadsheet program, and you can create any kind of graph you like.

As you can see, there isn't much current there.

[John C. Mannone]

Hi Donn,

The data you got for me is just for one position which unfortunately has been significantly changed by the corps of engineers(?), so it would not reflect the tidal current situation of times past. The harbor proper may have less of a change(?).

Anyway, I have data for maximum flood at a few places (Marblehead Channel, Boston Harbor, Cape Cod Channel, etc) for the dates of interest. These are rather general locations. I guess I will have to settle for this and try to draw arrows and show current strengths for these very generalized locations. Maybe there are no more detailed data for the harbor.

Boston Harbor is coming up at about 1.5 kts during Spring flood. It was probably a bit higher through the constriction between Boston and Charlestown in the Charles River at the mouth of the harbor (I have used 2 knots in my earlier analysis).

Thanks for trying; I appreciate you,
John

[Alan D. Hyde]

John, I expect you may see a little different result if you compare the effect of two (experienced) watermen rowing while standing and facing forward, to the effect of two such men rowing while seated with their backs to the bow.

My guess is that Revere may have been rowed in the former and not in the latter manner.

But it's just a guess, based on a little local knowledge going back (in the family) to the 1630's...

Alan

P.S. A seated man (on a fixed, not a sliding, seat) cannot as much benefit from the strength of his legs.

[ 12-20-2002, 04:34 PM: Message edited by: Alan D. Hyde ]

[Donn]

John...NOAA has literally dozens of station ID's in "Boston Harbor." for station ID 1296, which is Charles River, this is what they say:

"Current is weak and variable. Unable to predict."

[John C. Mannone]

Hi Jeff,

Recently got back from shopping and a few hours at the Observatory on the plateau west of Knoxville, TN.

As I looked down the mountain, the river and the opposite terrain layed just like the etchings I saw of Charlestown viewed from NW Boston across Back Bay to the NE. I think I'm even dreaming about this stuff!

Anyway, yes, I would appreciate the scans.

As I mentioned earlier, Carl Zellner (I think he works for the Paul Revere Memorial Association) wrote an article about the rowers. He shows a picturein which the artist depicts a small rowboat configured as you suspect.

I still wonder about the rowing while standing and facing forward mentioned earlier. Except for the incoming tide, with all the pitching and winding, I like the possibility. It has been difficult getting more information on this with Google searches.

Thanks for your help,
John

[rodcross]

My Visual Navigator has a Tide Station at the Charles River, Charleston Bridge, 41 16.226N 72 39.584W which shows that April 16,2003 may be close to a spring tide because the high tide is 11' and a -1.2' (Thats 1.2 feet below normal ) low tide. Check your moon phase and check for a 'new moon' around that date. An eleven foot tide in that location would generate SERIOUS currants even if the recent earthworks sized the river down to a couple of hundred feet; Not merely 2 or 3 kts. That would mean water movement of 5 or 6 kts.

My current data is not reliable for that area as it needs to be updated.

You may want to give the Boston Harbormasters office a call. I'll bet they have data on every quarter mile section of the harbor. The may have a printed schedule they could send you.

[John C. Mannone]

Hi Rod,

The new moon occurs April 1 and the full moon on April 16, 2003. I am specifically interested in the latter (recall there was a full moon on April 15, 1775). Your tide analysis software is consistent with the one I use (see below).

I can get the tides for Charles River at Charlestown, but the closest currents on this software are at Boston Light. The Charles River has been greatly modified; face of the harbor in that area also. I think the changes started in the mid to late 1800s. So I will probably have to take harbor currents where there hasn't been much change since 1775.

From old charts, I can see how these currents would have been increased (venturi effect) as the mouth of the harbor gets necked-down in the vicinity of the old ferry lane. My first back-of-the-envelope calculation shows an increase from, say 1.5, to 2.4 - 3.4 kts (depending on where you measure). Therefore, your suspiscion of a stout current may be quite correct. I'm not sure it would be as high as you suspect, but still a serious current. Until I refine the analysis, I have defaulted to a lower value of 2 kts; this would have been a realistic minimum current.

As an aside, I remember doing a dive in the Indian River Inlet in Delaware in 1976. We had forgotten it was Spring Tide time (I think the moon was new in this case). We started the dive 20 minutes late, but thought we had at least 1/2-hour to check out the marine life on the rocky wall. There was actually no time left in the "slack period" and the tide had already started going out to sea as we started our descent, but we didn't know this yet.

Something was terribly wrong, the expected 10-15 feet visibility was less than 1 foot and our descent seemed endless for what was supposed to be a maximum 27 ft dive. I decided to abort and surface. We were about to be swept into the channel! When we started our descent, we were immediately caught in an eddy, the bay silt already murking the water. The lack of visibility and the lateral motion gave the sensation of a descent.

The inlet necked-down at least twice over the 1/4-mile stretch we could see, from the bay to the ocean. The constriction was severe for the outgoing flow.

When we surfaced, none too soon, the channel buoys were nearly fully leaned on their side and the water was tumultuous mid-channel. This very serious current was 7 kts (based on what I had later read). There is not much of a chance against something like that. "Swimming" speed in scuba dress is only 0.5 kt; an increase to 1 kt requires triple expenditure of energy, which can't be maintained for long.

There is very little time the water is slack during this time of the year. The tidal flow has a quick turnaround time. Our ignorance nearly got us killed. Fortunately, I learned from our mistake.

That having been said, if Revere and company had encountered such a current of even 5 kts, I strongly suspect this would rule out the possibility of standing while rowing. However, such a strong current would strengthen my argument that the rowers would not try to pass the Somerset to the east and downstream of them in a 5 ktcurrent.

Anyway, here is some of the tide and current data (new and full moon) obtained using WWW Tide and Current Predictor (all times shown are EST) accessible from:
http://tbone.biol.sc.edu/tide/tideshow.cgi

APRIL 1 NEW MOON
Charlestown, Charles River, Massachusetts
42.3667° N, 71.0500° W
2003-04-01 11:07 AM 9.95 feet High Tide
2003-04-01 02:20 PM New Moon
2003-04-01 05:14 PM 0.32 feet Low Tide
2003-04-01 06:09 PM Sunset
2003-04-01 11:26 PM 9.88 feet High Tide
Boston Harbor, Massachusetts Current
42.3383° N, 70.9567° W
2003-04-01 08:09 AM 1.16 knots Max Flood
2003-04-01 10:55 AM -0.00 knots Slack, Ebb Begins
2003-04-01 02:20 PM New Moon
2003-04-01 02:57 PM -1.34 knots Max Ebb
2003-04-01 05:08 PM 0.00 knots Slack, Flood Begins
2003-04-01 06:09 PM Sunset
2003-04-01 08:28 PM 1.24 knots Max Flood

APRIL 16 FULL MOON
Charlestown, Charles River, Massachusetts
42.3667° N, 71.0500° W
2003-04-16 10:33 AM 11.26 feet High Tide
2003-04-16 02:36 PM Full Moon
2003-04-16 04:44 PM -1.15 feet Low Tide
2003-04-16 06:26 PM Sunset
2003-04-16 10:54 PM 11.81 feet High Tide
Boston Harbor, Massachusetts Current
42.3383° N, 70.9567° W
2003-04-16 07:15 AM 1.48 knots Max Flood
2003-04-16 10:12 AM -0.00 knots Slack, Ebb Begins
2003-04-16 01:44 PM -1.47 knots Max Ebb
2003-04-16 02:36 PM Full Moon
2003-04-16 04:44 PM 0.00 knots Slack, Flood Begins
2003-04-16 06:26 PM Sunset
2003-04-16 07:32 PM 1.59 knots Max Flood

I like your suggestion in contacting the Boston Harbormaster.

Thanks for your continued support,
John