Tuesday, July 29, 2014

Lobster Pot...my foot.

Original reports that the whale watcher was caught in a lobster pot sounded so absurd that a quick check shows what really happened, which is again another good reason to study navigation.

After multiple descriptions of the location in the public media, the USCG finally reported the location as 13 mi East of Nahant, MA. As shown below.


Zoomed in to see the region below shows how this area is charted.



Now we come to our classroom preaching... always read the chart! Below is Note J.



And next comes the power of the Internet.  We just google 33CFR 150.940 and read what the regulation really says, just part of which is below.



Then we decided to take another look to the Boston District 1 USCG to see if they had any other news, which they did...

"...Initial attempts by the divers to clear the line from the propeller were unsuccessful. Original reports indicated a lobster pot line was caught in the propeller, but further analysis revealed it was a cable from Northeast Gateway's offshore facility which required additional dive resources and heavy duty equipment for removal.

Considering the dangers of an at-sea night operation, a passenger transfer was deemed unsafe until morning. Meanwhile, Boston Harbor Cruises provided additional water, food and blankets to the passengers."


In other words, someone else has now looked at a chart!

To repeat the message, if you do not want your passengers spending the night at sea when they planned on just a 3-hr outing, then read the chart.





Tuesday, June 3, 2014

Finding Longitude by Lunar Distance using the Stark Tables

by 
Steve Miller

The follow are instructions for using the Stark Tables for finding UT and Longitude with a work form that I have modified from the original Stark Tables form. This procedure requires a copy of the Stark Tables, which also have notes included related to this process.


After completing the [lettered steps] to get the data,
follow the numerical steps to fill out the form.


[A] Do an IC check using the Moon or Sun (3 on and 3 off). It has been found that using the Sun (with a special Baader Film filter) is preferable to using the horizon. The procedure for the IC sights and how to make the filter is explained in How to Use the Plastic Sextants.

[B] Time and measure a set of Lunar Distances (Try to do at least 6 sights).

[C] Do another IC check as above.

[D] Average your Lunar Distances and Times to obtain a single value of Time and Distance for your determination of your GMT and Longitude. A direct average can be used, or  better still use the Fit Slope method explained in detail in Hawaii by Sextant

[E] Obtaining the Body Altitudes

OPTION 1 - Take normal sights of both Bodies for their respective Hs if possible. This requires a  good sea horizon or an artificial horizon.

OPTION 2 - Compute the Hc for the Moon and Other Body. This is a bit circular as you must know your Lat and Lon, but you can use an approximate position.  These values are not super crucial. They are mainly used to compute a refraction correction.  For the computations you can use the standard trig functions, or use get the answer online from starpath.com/usno or starpath.com/calc.



Fill in the main data for the sight: Date, Times (Local and GMT), Bodies, IC, Near or Far sides of the bodies used, HP for the Moon, and p (additional altitude correction if Venus or Mars). These go in the blue boxes 1 through 19 at the top of the Form. Box 1 is for your Sight Identification.

Note that the parallax of Venus or Mars, when applicable, is called "p" on page 259 of the Nautical Almanac, but it is called "Additional altitude correction" in Table A2. In our form it is labeled "p."

Figure 1

Using OPTION 1 from above. Using Stark's Table 1,  lookup values for your Bodies and enter them in boxes 20, 21 for your Other Body and 26, & 27 for the Moon. 

Unless otherwise noted, when entering the Stark Tables using a value that is in-between two values listed in the tables, always take the answer corresponding to the higher one. Do not interpolate.


Figure 2



Add your Bodies Hs degrees (box 14) and box 20 values to obtain Sa degrees and enter in box 30 repeat with boxes 15 & 21 to obtain Sa minutes and enter in box 31. Then add the Moon's Hs degrees (box 16) and box 26 values to obtain Ma degrees and enter in box 32,  repeat with boxes 17 & 27 to obtain Ma minutes and enter in box 33.

[7]  Using OPTION 2 from above.  Enter the your Hc values in boxes 14, 15, 16, & 17 on the Form

Figure 3

Enter Stark Tables with the Moon Hc to extract the WWP correction and enter in box 22 & 23, then got to the WWRef Table to get the parallax correction for both the Moon (enter value in box 28 & 29) and the other Body (enter value in box 24 & 25).

Add the values in boxes 16, 22, & 28 together for the Ma (Moon Apparent Altitude) degrees value  and enter in box 32  and  add the values in boxes 17, 23, & 29 together for the Ma (Moon Apparent Altitude) minutes value and enter it in box 33, then add the WWRef box 24 & 25 and HC box 14 & 15 together for the Sa (Body Apparent Altitude) enter results in boxes 30 & 31 (degrees & minutes in proper boxes)



Upon examining  the Ma (boxes 32 & 33) and Sa (boxes 30 & 31) values select the column where you can write the smaller value under the larger. If the Ma is less than Sa copy the value in box 32 into box 42 and copy the value in box 33 into box 43. Otherwise copy the value in box 30 into box 44 and the value in box 31 into box 45. Subtract the lesser value to get the Ma ~ Sa value either in boxes 46 & 47 or in boxes 48 & 49. You could now copy the Ma ~ Sa values thus found and recorded into boxes 73 (degrees) & 74 (minutes) in Section 2.

Figure 4


Enter Table 2 and extract the 2 values at the Ma (boxes 32 & 33) and HP (box 18) row/column intersection, insert these values on the Form in boxes 34 & 35, also extract and enter the values in the sidebar table for the additional HP remainder from the value in Table 2 in boxes 36 & 37.

Enter Table 3 for the same 2 values for the Other Body (Sun, Jupiter, Stars and Saturn columns are self explanatory, Venus and Mars have special columns based on their HP) Enter the Table 3 values in boxes 38 & 39.

Figure 5


Once these values are entered add the values in boxes 34, 36, & 38 together and enter in box 40. Next, add the values in boxes 35, 37, & 39 together and enter in box 41.

Round the value in box 40 to 1 decimal place and enter it under the Ma ~ Sa value obtained earlier (your rounded value will be entered in either box 50 or 51). You will be adding this value to Ma ~ Sa value if you Lunar Distance sight was to the Far side of the Moon or subtracting if it was to the Near side. This operation will result in the H ~ H value (depending on which column you are working in this H ~ H will be in box 52 & 53 or in box 54 & 55) .

You could now copy the H ~ H values thus found and recorded into boxes 84 (deg) & 85(min) in Section 2.

* * * 

You have now completed the Input section (Section 1) of the Form and are ready to compute your Cleared Lunar Distance (D) using the Green highlighted boxes 91 & 92 in the next section of the Form.

Add the values in boxes 34, 36, & 38 together and enter in box 40. Next, add the values in boxes 35, 37, & 39 together and enter in box 41.

Round the value in box 40 to 1 decimal place and enter it under the Ma ~ Sa value obtained earlier (your rounded value will be entered in either box 50 or 51). You will be adding this value to Ma ~ Sa value if you Lunar Distance sight was to the Far side of the Moon or subtracting if it was to the Near side. This operation will result in the H ~ H value (depending on which column you are working in this H ~ H will be in box 52 & 53 or in box 54 & 55).


You could now copy the H ~ H values thus found and recorded into boxes 84 (deg) & 85(min) in Section 2.

You have now completed the Input section (Section 1) of the Form and are ready to compute your Cleared Lunar Distance (D) — in the Green highlighted boxes 91 & 92 in the next section of the Form

Enter Table 4 to obtain the Moon's Augmented Semi-Diameter value using your Ma value (box 32 & 33)  and the Moon's HP (box 18) and enter that value in box 56 on the Form.

Next, go to Table 5 to obtain the Sun's Augmented Semi-Diameter value using your Sa value (box 30 & 31) and enter that value in box 57 on the Form.

Add box 56 and 57 together and enter the result in box 58. If you are dealing with Low Altitudes for the Sun or Moon enter the appropriate value from Table 6 in box 59.

Subtract the value in box 59 from the value in box 58 and enter the result in box 60. Round this result to 1 decimal place and enter it in the box that matches the column for Near (box 65) or Far (box 66) as appropriate for your sight. 

Figure 6

Enter your IC in box 61 (off) or 62 (on) and Instrument error in box 63 or 64 and combine with your Lunar Sextant Distance (Ds in boxes 67 & 68) and box 65 added as well resulting in your Da value (off - boxes 69 & 70 (copy to boxes 71 & 72) or on - boxes 71 & 72 with box 66 subtracted - see arrow on Form as a reminder)

Next bring down your Ma ~ Sa value, if you have not done so already, and subtract the Ma ~ Sa value in box 73 & 74 from the Da in box 71 & 72 entering the result in boxes 75 & 76.
Then add Da (boxes 71 & 72) and the  Ma ~ Sa (boxes 73 & 74) values together entering the result in boxes 77 & 78.
 
Figure 7

The next operation requires entering the K Table and extracting the K value for the Da/ Ma ~ Sa (boxes 75 & 76) difference and entering that K value in box 79 and the Da/ Ma ~ Sa (boxes 77 & 78) addition and entering that K lookup value in box 80. Add these two K values (boxes 79 & 80) together entering the result in box 81 and then divide that result (box 81) in half and enter that value in box 82.

Bring down the Q value (box 41) from Section 1 of the Form and copy it to box 83, next, add boxes 82 and 83 together and enter the result in box 84. Round the value in box 84 to 5 decimal places.


Figure 8

Enter the K Table again and extract the K value for the H ~ H (boxes 84 & 85) that you copied from Section 1 of the Form


Figure 9

Compare the Value in box 86 with the value in box 84a. If box 84a is less than box 86 copy the value in box 84a into box 87 and subtract box 87 from 86 and enter the result in box 88, otherwise copy the value in box 86 into box 89 and subtract box 89 from box 84a and enter the result into box 90.
Lookup this 'resulting value' (box 88 or 89 value) in the body of the Gaussians Table and extract the 3 digit value at the top of a column where your 'resulting value' is found and extract the 2 digit number next to your 'resulting value'. 

Place these digits as a 5 digit number (first the 3 digits, then the 2 digits) in the empty box under the number you originally moved to the other column(if you entered the Gaussian table with box 88 enter the gaussian value in box 89 OR if you entered the Gaussian table with the box 89 value enter the gaussian value in box 87. (put a line through your 'resulting value' so you do not mistakenly use that value in the next step.

If your gaussian value was placed in box 89, subtract box 89 from box 84a and enter result in box 90, otherwise subtract box 87 from box 86 and enter the result in box 88.

Now enter the K Table again, but this time this latest result (box 88 or 89) will be found in the body of the K Table. You will be determining the degrees from the page and the minutes and tenths from the row and column in the K Table.
This resulting Value as found in the K Table is your Cleared Lunar Distance (D) and is now entered in Green highlighted boxes 91(deg) and 92 (min/tenths).

You have now computed your Cleared Lunar Distance (D) boxes 91 (deg) & 92 (min) and have completed Section 2.



Next, You will determine the Computed Lunar Distance for the Hour PRECEDING and Hour FOLOWING the TIME of your Lunar Distance Sight in order to determine the Computed Lunar Distances for those hours.

The next steps involve entering the Nautical Almanac and extracting the GHA and Declination for the Moon and the Other Body for the Hour PRECEEDING and FOLOWING the TIME of your Lunar Distance Sight. If the Other Body is the Sun or one of the Planets the GHA degrees and minutes are entered in the center column boxes 105 through 110. The Declinations are entered in the left column in boxes 111 through 119 (Preceding) and 149 through 157 (Following). Please make sure that you note the sign (N/S) in boxes 111 and 115 (Preceding) and 149 and 153 (Following). If your Other Body is a Star the computations required for determining the GHA of the Star are provided in the right hand column boxes 93 through 101 (Preceding) and 131 through 139 (Following). 


Figure 10
Once you have entered the GHA and Declination of your Bodies in the appropriate boxes, enter the difference between the GHAs in boxes 109 & 110 (Preceding) and 147 & 148 (Following)  and the difference between the Declinations in boxes 118 & 119 (Preceding) and 156 & 157 (Following). NOTE: make sure that you take into account the SIGN of the Declination when determining the difference - i.e. Same = subtract, Contrary = add.

The next step involves entering the K Table with the value of the GHA difference boxes 109 & 110 (Preceding) or 147 & 148 (Following) and entering that K value in box 120 (Preceding) or box 158 (Following).

Figure 11
Then, enter the log Dec Table with the Declination values and extract the log Dec for each Declination and enter those log Dec values in boxes 121 & 122 (Preceding) and 159 & 160 (Following). 

Add the values in boxes 120, 121 & 122 (Preceding), enter result in box 123 and in boxes 158, 159 & 160 (Following), enter result in box 161.

Enter the K Table once more with the difference between the Declinations Boxes 118 & 119 (Preceding) and boxes 156 & 157 (Following) and extract the K value and enter it in box 124 (Preceding) and box 162 (Following) in the center column of the Form as indicated.


The next step is to compare the values in boxes 123 & 124 (Preceding) and boxes 161 &  162 (Following). The accompanying Form shows how to handle the TWO POSSIBILITIES  resulting from the comparison. Either or both possibilities can occur in your solution.
POSSIBILITY 1

Figure 12
In the PRECEEDING area the value in box 123 is lesser than the value in box 124, thus we will copy the value in box 123 into box 125. Then, subtract the value in box 125 from the value in box 124 and enter the result in box 127. You will now enter the Gaussian Table with the value in box 127 in the body of the Gaussians Table and extract the 3 digit value at the top of a column where your 'value' is found and extract the 2 digit number next to your 'value'. 

Place these digits as a 5 digit number (first the 3 digits, then the 2 digits) in box 126 (put a line through your 'value' in box 127 so you do not mistakenly use that value later. Next, subtract the value in box 126 from the value in box 123 and enter the result in box 128. 

Now enter the K Table again, with box 128 value which will be found in the body of the K Table. You will be determining the degrees from the page and the minutes and tenths from the row and column in the K Table. This resulting Value as found in the K Table is the Computed Lunar Distance (D1) and is now entered in Green highlighted boxes 129 (degrees) and 130 (minutes/tenths).

POSSIBILITY  2

Figure 13

In the Following  area the value in box 162 is lesser than the value in box 161, thus we will copy the value in box 162 into box 164. Then, subtract the value in box 164 from the value in box 161 and enter the result in box 166. You will now enter the Gaussian Table with the value in box 166 in the body of the Gaussians Table and extract the 3 digit value at the top of a column where your 'value' is found and extract the 2 digit number next to your 'value'. 

Place these digits as a 5 digit number (first the 3 digits, then the 2 digits) in box 163 (put a line through your 'value' in box 166 so you do not mistakenly use that value later. Next, subtract the value in box 163 from the value in box 162 and enter the result in box 165. 

Now enter the K Table again, with box 165 value which will be found in the body of the K Table. You will be determining the degrees from the page and the minutes and tenths from the row and column in the K Table. This resulting Value as found in the K Table is the Computed Lunar Distance (D2) and is now entered in Green highlighted boxes 167 (degrees) and 168 (minutes/tenths).

NOTE.  Either possibility can occur in the PRECEEDING hour or FOLLOWING hour areas in your solution and not necessarily in the order or place that I have shown them.

You have now completed Section 3 and determined the Computed Lunar Distance for the Hour PRECEDING (D1) and Hour FOLOWING (D2) the TIME of your Lunar Distance Sight. Next you will determine the Computed GMT for your Cleared Lunar Distance in Section 4.

Compare your D value in boxes 91 & 92 with the D1 value in boxes 129 & 130 and enter the difference in box 169. Usually it will only be necessary to use the minutes/tenths values in boxes 92 & 130 as the degrees are generally the same. 


Figure 14

Next, compare your D1 value in boxes 129 & 130 with the D2 value in boxes 167 & 168 and enter the difference in box 170. Usually it will only be necessary to use the minutes/tenths values in boxes 130 & 168 as the degrees are generally the same. 

Now enter Table 7 with the values in boxes 169 & 170 and enter the respective values found into boxes 171 & 172.

Subtract the value in box 172 from the value in box 171 and enter the result in box 173.
Go to the Table 8 body with the value in box 173 and find the minutes and seconds corresponding to that value. Enter the resulting values in box 178 (minutes) and box 179 (seconds). Copy the value in box 103 (Preceding Hour) into box 177.


You now have the Computed GMT in boxes 177 (hr), 178 (min), & 179 (sec). Copy your GMT from box 7 into boxes 174 (hr), 175 (min), & 176 (sec). Once you have done this enter the difference between boxes 176 & 179 into box 180. Note: you may have to get the difference between the minutes found in boxes 175 & 178. If you do convert the result into seconds and add this result to the value in box 180.


The value in box 180 is your Error in Time. By dividing the value in box 180 by 4 (derived from 15' of arc movement in 60 seconds of time) and entering the result in box 183 you have determined your Longitude Error. Applying this error to your DR Longitude in box 4, you will calculate your Computed Longitude. Enter this value in box 184.

 * * *

You have now completed Section 4 and have determined your Computed GMT and corresponding Longitude for your Lunar Distance Sight.

CONGRATULATIONS!

GOOD LUCK WITH YOUR LUNAR DISTANCE SIGHTS.


Editor's note: A copy of the full Miller-Modified Stark Work Form is shown below, annotated with the box numbers referred to in the text along with some notes, colored boxes, and some arrows indicating where to place certain data values. You can here download the form in Excel format.



Figures 15 and 16






Tuesday, May 20, 2014

Tricky Terms in Navigation

Good navigation calls for clear communications. We would like to think, then, that the terms we use have precise meanings. Most do, but there are common exceptions that we should know about. This is especially important when learning navigation. We go over here a few important terms that require special attention, either because their meaning changes with context or the definition is more subtle than might be guessed. Significance is hard to rate, so we default to alphabetical order.

Aspect. This describes the heading of another vessel from your perspective. It is a crucial concept in collision avoidance, especially at night as it directly reflects what lights you should see. It is defined as the relative bearing (R) of your vessel as seen from the other vessel. It is measured from 0° to 180° and labeled Red when we are on the port side of the vessel or Green when we are on the starboard side. Thus a vessel with aspect Red 270 means you are looking at its port side. Green 45 means you are looking broad onto its starboard bow. (I did promise tricky—045R is the definition of broad on the starboard bow.)  

Allision. When two vessels underway run into each other it is called a collision. When a vessel runs into a dock (assumed above the water) then that is an allision. Allision is damage causing impact between a vessel underway and something not moving, such as an anchored vessel. This would seem just Admiralty Court jargon—there is, for example, an official “Oregon Rule” that presumes the fault lies with the moving vessel—but there are more subtle implications to this concept that I have always considered fundamental to basic navigation.

The word allision is not in the Navigation Rules. In fact, the only reference in the Rules at all that refers to collisions with anything other than another vessel is in Rule 6 (b) ii, on the things we must take into account when choosing a safe speed when using radar: “the possibility that small vessels, ice and other floating objects may not be detected by radar at an adequate range.” [emphasis added]. Though never stated specifically, clearly the authors of the Rules intended this document to be the guide to not running into anything. And it remains true. If you know and obey the Rules, you will avoid not just collisions but also allisions, which can be even more embarrassing.

Course made good (CMG). This is our average course over a specific distance or time period relative to the fixed earth. It is the direction from an earlier position to a later position, regardless of the track between these two positions. It can be something we have already done, such as our track of past positions shown on echart plotter, or it can be something we plan in the future by anticipating the course we will achieve in the presence of current or leeway. It should not be confused with COG.

Course over ground (COG). This term is known to everyone who uses GPS. It originated as one of the first important derived values we learned from LORAN units. It has, however, been generalized in modern times to a point where it risks distracting from effective communication. It is best thought of as the instantaneous value of our CMG relative to the fixed earth that we read from GPS. Past or future courses are best described as CMG, not COG. The predictor line on our vessel icon in echarting points in the direction of our COG. The trail of dots behind the icon shows our CMG. When we solve a vector problem to account for current we are finding or using our CMG, not COG.

The same distinction should be made between speed over ground (SOG) and speed made good (SMG).

Declination. To a land navigator declination is the difference between magnetic north and true north. To a marine navigator this difference is called variation. In marine work, we reserve declination to mean the latitude on earth directly below a star or other celestial body. We further distinguish declination from latitude by placing the label N or S in front of declinations and after latitudes.

Dead reckoning (DR). This most fundamental of all navigation terms has two different definitions in modern times, both of which refer to a position determined for your vessel without the aid of any piloting data. (1) Position by log and compass alone and (2) your best estimate of your position taking into account everything you know about your boat, the wind, and the waters you sail. The former is found by plotting distance run on each logged course, with no further corrections; the latter accounts for current, leeway, helm bias and sea state.

This distinction is not crucial. A practical implementation is to plot the DR position by definition (1) and then apply all corrections you know about. The distinction lies only in what you call this final position, the estimated position or the DR position. We prefer the latter definition (DR is everything) as there is no real need for a second named position, and it is difficult to coordinate the plotting in a logical manner—correction for current and correction for leeway are plotted differently.
 
From a Dictionary of the English Language by Samuel Johnson, 1755.

Drift. Used alone, this means the speed of the current, which can be measured in knots or nautical miles per day. Wind drift is sometimes used to refer to the wind-driven current, but in other contexts, wind drift is used to describe leeway speed and sometimes used as a vector to include speed and direction. Spindrift, on the other hand, is the foam blown off the tops of waves. Its first appearance is a good Beaufort Scale benchmark for about 30 kts of wind.

Estimated position (EP). If one chooses to define a DR position as that found from compass and log alone, then anything you do to that position to improve it changes it to what is then called an estimated position. This is common training, though it does deviate from the historical meaning of DR and may add some ambiguity to the plotting. On the other hand, if DR is defined as including all you know about your navigation in the first place (short of piloting), then a DR position and an estimated position are the same.

The term estimated position requires more care when it is expanded to include piloting data, such as a single line of position (LOP) or a depth contour. If you have a single measured LOP, then in its broadest sense, one can define EP as your best estimate of your position taking everything into account, including this one LOP. This is indisputably a sound definition, and indeed the proper guide to position evaluation underway.

The required care comes into play whenever a specific prescription is given on how to do this. This type of EP, for example, is frequently defined as the point on the single LOP that is nearest to the “DR position” at the corresponding time—which immediately drives us back to the terminology. This use of “DR position” cannot mean DR by log and compass alone, because known corrections can take you away from the nearest point on the LOP. Thus this prescription must be worded: the estimated position is the point on the LOP nearest to the estimated position without the LOP.

Even then we must be careful. When you measure an LOP by any means (compass bearing to a lighthouse or sextant sight of the sun) and this LOP does not cross through your corresponding DR position, you know only two things: one, you are on the LOP somewhere, and two, your DR is wrong. We can project that point onto the LOP and call it the estimated position—as you must do on any navigation exam!—but underway, we should remember this is largely wishful thinking. If the single LOP crosses right through the DR position, then you can add to your knowledge that the DR might be right.

Log. This term has several meanings, all related. Verb: Make an entry into the log book (“I logged our mark rounding.”). Verb: Travel a distance (“We logged 130 miles today.”). Noun: Another name for logbook. Noun. Device for measuring distance traveled through the water (knotmeter log, taffrail log, chip log, etc).

True wind. Every meteorologist in the world, and I would hope every navigator in the world, agrees on the definition of true wind. It is the wind speed and direction relative to the fixed earth. For some aspects of sailing performance analysis, however, it can be useful to know what the wind is relative to the water, which in turn can be moving. Periodically we see this later wind referred to as “true wind,” and that should be avoided. We should not even say “true wind relative to the water,” which only muddies the matter. Ben Ellison of pandbo.com has suggested calling the latter the “water wind,” which seems a good solution. Google the phrase “true wind versus water wind” to find extended discussion of this terminology.

Range. This is an important term in navigation with several distinct meanings. It can be used to refer to a specific distance between two points on a chart (“range and bearing from A to B”), also used as distance from vessel to radar target (range rings, etc), but it is also used to mean the maximum effective distance a light shows, or a radio or radar beam reaches (nominal range, luminous range, VHF range, etc). Likewise we refer to the maximum range we can achieve under power without refueling. And of course there can be a mountain range along the coast. Thus there are a whole range of extents using this term. 
 
Extend a given extent on a chart and you get what the British call a transit, namely the line on a chart between two landmarks or aids, which in US parlance is called a range. A navigational range is between two aids put in place for that purpose; a natural range is any two objects you choose for navigation, charted or not. All navigational ranges show the nearest mark or light lower than the farther one, and a similar convention on ship's masthead lights (forward lower than aft) has led to the nick name “range lights” for the two white masthead lights on a ship that tell us which way it is headed. By watching the space between them we can tell if and how it is turning.

Less often used is the verb to range along a coast, meaning to travel at a fixed distance off.

Sea mile. A nautical mile is defined as 1852 meters, exactly. A sea mile is a distance of 1' of latitude. We tend to use these interchangeably, which is rarely an issue... unless you are hiding treasures by GPS coordinates in both Alaska and the Galapagos, where the latter has a sea mile that is 50 ft shorter.

Set. Set is the true direction a current flows toward, but it is also used as a verb (to be set by the current), and also used to refer to the magnitude of the offset. “The set of the current is 200 T, which is causing me to be set off course. The GPS shows my set is 30ยบ to port.”

Tide. Vertical motion of the water is the tide; horizontal motion of the water is the current. We are better off to not ask what the tide is doing when what we want to know what the current is doing.

Velocity made good (VMG). This is a derived term that actually predates LORAN. It began as a sailing performance term, which means your speed projected onto the direction of the true wind, either upwind or downwind. It takes a simple processor chip to compute; no position data are needed. It is still used that way, and in a sense this is the preferred meaning of the term. But with the advent of LORAN and later GPS, this term began to be used as the projection of your SOG onto the direction of your desired course. (Recall speed is just a number, but velocity is a vector, meaning a number and a direction.) Thus we get some integrated instrument systems reporting both VMG Wind and VMG Waypoint, which is tidy enough, we just need to be careful when discussing VMG on the boat. Our main concern arises when we have instruments reporting just VMG. Then we have to look up what it means.

Waypoint closing velocity (WCV). This is the NMEA term for VMG to a waypoint. It would be nice if manufacturers converged on common terminology, but they do not; we do not see this one used very often, maybe because it's harder to say.

With special thanks to Starpath instructor Larry Brandt for valuable suggestions.