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Go Back   SailNet Community > Skills and Seamanship > Seamanship & Navigation
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Seamanship & Navigation Forum devoted to seamanship and navigation topics, including paper and electronic charting tools.


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  #11  
Old 07-14-2008
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Think about a large bay filled/emptied through a small opening. When the tide outside is lowest, the water is draining out of the bay and the tidal height at the opening is at the low. But as the outside tide rises, the tidal level in the opening rises, but the flow is still going out because the outside tide level is still lower than the bay. So you have the situation that the current is still ebbing, but the tide height is rising.
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Old 08-05-2008
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Rule of Twelveths: Clarification

Thanks sailingdog. This is helpful. I have one clarifying question.

In order to get to a specific safe harbor in Buzzards Bay by sunset (7pm) on Saturday, August 9th, 2008, I was trying to see if we could start at the N.East end of the Cape Cod Canal at 1500 this coming Saturday (aug 9th).
Using Eldridge charts, and the Rule of Twelveths, here is my thought process...

Time of Slack Water: 1001 (and 1628)
Max current is at 1301: 3.5 knots and flooding (heading east (going against us))
Interval between slack water and desired time (of 1500) is: 1500-1001= 5 hrs.
Current velocity and direction at 1500 is 2/12 of 3.5 = 0.633knots and flooding / heading east (i.e. going against us).

Based on my calculations, this seems really do'able. However, it just doesn't feel like the current strength should be this weak given the max strength is 3.8 knots, so I'm betting my thought process is skewed.

hmm . . . Is the result of the rules of 12ths a percentage? In other words, is there one more step and the current at 1500 is actually 63% of 3.8knots (not 0.63 knots)? That may make more sense at 2.4 knots flooding at 1500 (i.e. going against us). This does not seem do'able with a small inboard engine.

What is the result of the Rule of Twelveths? A percentage of the total knots? Percentage of the total tidal range? (I think it's both). Or is 0.63 the knots? I think 0.63 is the percentage (and then just multiply by the max current or the tidal range depending on what you're looking for), but I'd like to confrim that.

Thanks!
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Old 08-05-2008
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I'd have to look at my reed's but it's on the boat and I'm not. I don't have the info with me.
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Old 08-05-2008
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A minor detail is that you should be using the Tidal Current Tables and not the Tide Tables. Tidal Current and the Tides are many times out of phase due to the topography of the land masses in the area.
And when you have learned how to do the Current Tables by hand you will have a better understanding of the computer model of those time periods and the areas you are concerned about.
Bowditch, Dutton/s and the Tables themselves will instruct you on how to use them.
Also Local storms and rainfall up stream will affect the tables in their perdictions.
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Old 08-05-2008
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just did the run from cape may,nj to city island,ny via the east river - the math told us that at 5k it was about a 24 hour run - left at about 1400 on friday and actually got to sandy hook early - which was not good but a massive tstorm hit us twice so took a toll on our timing and put us through the narrows on time - past governors exactly on time and under the brooklyn at about 15-1600 or so not sure as i am single handing and not a lot of time to check exact time - just knew i was well within the window - what a ride - hells gate is aptly named - nothing serious but one does need to pay attention and make sure you have a firm hand on throttle as you need to maintain steering but not a lot of throttle as at times doing over 10k in a 40' sailboat - constant 7-8 k at lower rpms - out to city island and on hook between rat island and barrons marina at 1830
we timed it right but did the preplanning long before we left and set up a plan to anchor if we missed the window -- preplanning is the key - and setting up alternative if the plan goes awary
there were 2 boats by the way (the we) as the other boat has done this trip many times and i was lucky to tag along and learn
chuck and svsoulmates
ki4sry
on the hook city island,ny
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My rule about the Cape Cod canal is to regard it as a door. The door is closed whenever the current is running against you. Check the time of current change under the RR bridge (in Eldridge) and don't try to cut things close - you won't make it! The problem is that the current change in the canal occurs very rapidly and quickly ramps up to near max strength. The "Rule of Twelfths" does not apply here.
I recall one time (going south and west) that I was late in getting through. By the time I got to the entrance channel for Onset, I had to stand past the rip-rap pile marking the entrance by probably 100 feet in order to make the turn and get 50 feet on the other side where the current was essentially zero. And at the entrance to Onset, you are essentially out of the canal. Back under the bridges it runs a lot stronger!
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BTW, if you're doing the Cape Cod Canal, pray that the wind isn't out of the southwest... generally, with a southwest wind, you can get some very interesting standing waves forming near the canal's mouth.
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You know what the first rule of sailing is? ...Love. You can learn all the math in the 'verse, but you take
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—Cpt. Mal Reynolds, Serenity (edited)

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Boasun, sailingdog, and gc111, thanks for your replies.
Boasun, regarding your suggestion to read the Tidal Current Tables, not the Tide Tables, I also have questions about this. (I am not new to sailing, but new to cruising, and sooo interested to learn.)
Quote:
Originally Posted by Boasun View Post
A minor detail is that you should be using the Tidal Current Tables and not the Tide Tables. Tidal Current and the Tides are many times out of phase due to the topography of the land masses in the area.
...Using Eldridge, I used the Tidal Current Tables to scope out the timing with Woods Hole, Mass, and I really like the way this is presented in illustration format, in combination with the Tide Tables. So the first thing I did was look for the same thing for the Cape Cod Canal. However, this doesn't appear to be included in Eldridge, so then I went to the Tide Tables. In order to replicate (in general terms, not specific depending on the topography) kind of information shown in the Current Charts, my thought was that one uses the Rule of Twelveths for this. (Again note that I'm still very new to this, so I'm doing a lot of reading, researching, and forum'ing and trying to piece all of this together.) I may just have not seen the Tidal Current Table for the Cape Cod Canal. Where do I find this?


Though I used the CapeCodCanal as the example. . . . In general, regarding the Rule of Twelveths, sailingdog cleanly describes about how the Rule applies to tidal height. Does it also apply to current? (I think yes). If so, when I multiply 1/12 (for the first hour) times the maximum current [(1/12)*5 knots==0.416) is the result (0.416) (option#1) a percentage of the total maximum current? In other words, does one then need to take 0.416 and multiply that times 5 to get the actual current speed at 1 hour past the current change, which would be 2knots.

or option#2: "0.416" is the actual current speed; at the first hour the current speed is 0.4 knots?

I think it's option#1, but i'm not positive (again, newbie), so any feedback is welcome.
thanks!
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Quote:
Originally Posted by captxtina View Post
...Using Eldridge, I used the Tidal Current Tables to scope out the timing with Woods Hole, Mass, and I really like the way this is presented in illustration format, in combination with the Tide Tables. So the first thing I did was look for the same thing for the Cape Cod Canal. However, this doesn't appear to be included in Eldridge, so then I went to the Tide Tables. In order to replicate (in general terms, not specific depending on the topography) kind of information shown in the Current Charts, my thought was that one uses the Rule of Twelveths for this. (Again note that I'm still very new to this, so I'm doing a lot of reading, researching, and forum'ing and trying to piece all of this together.) I may just have not seen the Tidal Current Table for the Cape Cod Canal. Where do I find this?

Though I used the CapeCodCanal as the example. . . . In general, regarding the Rule of Twelveths, sailingdog cleanly describes about how the Rule applies to tidal height. Does it also apply to current? (I think yes). If so, when I multiply 1/12 (for the first hour) times the maximum current [(1/12)*5 knots==0.416) is the result (0.416) (option#1) a percentage of the total maximum current? In other words, does one then need to take 0.416 and multiply that times 5 to get the actual current speed at 1 hour past the current change, which would be 2knots.

or option#2: "0.416" is the actual current speed; at the first hour the current speed is 0.4 knots?

I think it's option#1, but i'm not positive (again, newbie), so any feedback is welcome.
thanks!
I believe the tidal currents are affected by the geography far more than the tidal heights are. Also, the Cape Cod Canal is affected by the tides in two locations, Cape Cod Bay on the east and Buzzards Bay on the west, so the rule of twelveths isn't going to be easy to apply in any form.
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You know what the first rule of sailing is? ...Love. You can learn all the math in the 'verse, but you take
a boat to the sea you don't love, she'll shake you off just as sure as the turning of the worlds. Love keeps
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—Cpt. Mal Reynolds, Serenity (edited)

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I don't believe that rule of twelveths applies to tidal currents. If you peruse table 4 of the current table you may see how it doesn't fit. And Table three is the normal correction table for Tidal currents but table four is for the ligher currents. When you are working for times near slack water.
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