In 2020 NOAA announced that this was the last year they were going to authorize an annual set of tables for tides or for currents. The tables were called:
Tide Tables
2020 East Coast of North and South America Including Greenland
2020 Europe and West Coast of Africa Including the Mediterranean Sea
2020 Central and Western Pacific Ocean and Indian Ocean
2020 West Coast of North and South America Including the Hawaiian Islands
Tidal Current Tables
2020 Atlantic Coast of North America
2020 Pacific Coast of North America and Asia.
Prior to 2021, these were "the official sources." All other third-party printed or electronic presentations of US tide and current data, readily found along the waterways and cybersphere, were derived from these, sometimes mixing up actual locations or confusing standard times and daylight times.
These are what we called "The Tide Tables" or "The Current Tables" that were either required or recommended to be on all vessels. These tables included daily data for numerous Reference Stations and then a Table 2 that included corrections to be applied to thousands of Secondary Stations.
That ended in 2021. And despite the fact that some third party companies still print these tables including the Table 2 data that they reproduce from the 2020 tables, the data are not valid. Hundreds of those secondary stations have been discontinued and values for many others have changed.
But more to the point at hand, up till just recently, the USCG license exams still tested on the Table 2 procedures using the old Table 2 data, which has been totally wrong for nearly 5 years now. Many schools around the country still teach this method as well.
The USCG has now corrected that and their new exams treat tides and currents in the modern, correct manner, which is outlined below. This greatly simplifies this important part of navigation. We wrote several notes on this in the past:
You can review these for background and in the second one for step by step procedures for most efficient access to the new data, including how to make your own set of annual tables
Another aspect of the simplicity (progress) is that tide and current questions are now essentially the same for entry level OUPV license exams as they are for unlimited ocean master.
Here is an example.
The diagrams included are:
The solution to #36 is fast. Go to the time on the graph and read the speed, then note that the harmonic directions are given in the figure titles.
That is the right answer to the test question, but not at all the guaranteed answer on the water. These currents are treated as pure reversing, with two directions only, but in practice they rotate, flowing with some strength in an ellipse of directions. The direction given is just the average direction around the time of peak flow at the long axis ends of the ellipse.
Question #35 asks about rotary currents, which is interesting in that the discontinued annual current tables did have a Table 5 listing details of rotary currents along the coasts. But like Table 2, much of the Table 5 data were not considered unreliable, so they have been discontinued. If we want coastal tidal currents, we should use the OFS model forecasts, which are very good. Nevertheless, we can answer this question by looking up any coastal station along the East Coast and seeing how long it takes between successive floods or ebbs, and it will be about 12 hr.
The tide problem, #37, is just as direct. We are between two stations that have different heights but we are only asked for time, and that is the same for both. If they had ask for height the answer would, presumably, be (0.51+0.29)/2 = 0.4 ft.
(Tide heights are pretty uniform over large areas of open water, so the tide values are more likely to be correct out on the water than the current values — assuming the atmospheric pressure is about normal, and the wind has not been strong over the past 12h, and there is no unusual river run off, all of which can throw the water level predictions off a foot or two.)
Likewise for the other end of the license exam spectrum, unlimited master.
This the same as #36, but we have to adjust the time. Starting at 0130 we must travel 15 nmi at 10 kt which takes 1.5h so we get there at 0300.
Question #6 for unlimited master is the same as the #37 for OUPV.
______________________
So the summary is the USCG exams now follow the existing procedures for tides and currents, which is tremendously easier and faster than it was before. It is a pity that the invalid Table 2 data (for tides and currents) are still being published by third party printers, but we should just know this, and move on.
Our main resource for all tide and current data is now www.tidesandcurrents.noaa.gov. Please refer to the article above (What do we do now?) for the exact steps for the most efficient use of the NOAA site. The best procedure is not intuitive. What might seem an intuitive approach can lead to other types of data that you likely do not want. We want the types of data shown in these USCG exam diagrams. In that article there is also a video showing the steps.
The article also shows how to make annual tables for any station. It takes just 4 pages per year, per station. We do not need the historic books that covered all of North America. We need just the stations covering the tidal waters we navigate.
Our book Inland and Coastal Navigationcovers the use of the NOAA website, and our Navigation WorkBook 18465Tr has practice exercises. In practice a new challenge arises in finding the nearest station you care about (illustrated in the links above), or you can use a program like qtVlm or OpenCPN that used tested harmonic data from NOAA and they show where all the stations are.
Here is a video summary of this article, which includes a demo of our recommended approach to the NOAA data.
* * * Thanks to Seattle Maritime Academy instructor Robert Reeder for alerting us to these USCG exam updates.
No comments:
Post a Comment