All of which adds up to a feeling that if it isn’t talked about elsewhere, lets talk about it here.<\/p>\n
As can be seen in the examples, if give tenths of a hour, multiply the tenths by 60 to come up with minutes.\u00a0 If given minutes, divide the minutes by 60 to find tenths.\u00a0 If you can’t remember whether to multiply or divide, just pick one.\u00a0 If the answer doesn’t look right, do the other.<\/p>\n Set and Drift<\/strong> – technically, set and drift are limited to the impact on a vessel of the direction and speed of current in the area the vessel is passing through.\u00a0 Unfortunately, while underway current is nearly impossible to separate from the effects of leeway (wind) and helm error.\u00a0 Instead, all are typically lumped together as “set and drift” to show the net result.\u00a0 Set is direction and drift is speed.<\/p>\n How to correct a course to arrive at a specific destination<\/strong> – A new course line can now be drawn from the fix location to the location of the next turn.\u00a0 The new course to the next turn point is C 184.\u00a0 However, if you steer that course, you will end up .5 miles of target due to set and drift.\u00a0 To correct for set and drift, continue the set line thru the fix location for a distance equal to the drift.\u00a0 In this case, 0.5 miles.\u00a0 Draw a line from that point, to the DR location 1 hour later.\u00a0 The course line to correct for set and drift is 189\u00ba, and should be relayed to the helm as “C 171 PSC”.\u00a0 Steering that course should result in a course made good of 184\u00ba, allowing you to reach the next turn at approximately 1250.<\/p>\n Estimated position<\/strong> – every skippers tip should have a bit of controversy in it, right?\u00a0 If so, here it is.\u00a0 The term “estimated position” is used differently by different authors and publications.\u00a0 One such usage involves estimating a position based on a DR position and a single line of position (LOP), like a bearing to a charted object.\u00a0 The process involves drawing a line that runs perpendicular to the LOP, running through the DR position.\u00a0 The estimated position is the point the perpendicular line crosses the LOP.\u00a0 Sadly, I have had times where the error in this estimate was unacceptable.\u00a0 On one occasion, I was able to get a fix by using a bearing and the depth.\u00a0 I was\u00a0 1.3 miles of of the Farallons, but an EP calculated at the same time had me 2.1 miles out, simply because I was travelling .2 kts faster than my DR indicated.\u00a0 Others will argue, however, I don’t recommend the use of estimated position in that manner unless there is no other choice.\u00a0 Neither does The American Practical Navigator as it isn’t even in the book.<\/p>\n I hope you stuck with me this far.\u00a0 If so, you will be happy to know that I my opinion, you have survived the most challenging part of coastal piloting.<\/p>\n","protected":false},"excerpt":{"rendered":" If ever there was a “catch all” navigation tip, this is it.\u00a0 While determining a course to steer, the navigator needs to know and\/or take into consideration: The relationship of Distance, Speed, and Time Conversion from True to Magnetic to … Continue reading ![\"\"](\"https:\/\/tradewindssailing.com\/wordpress\/wp-content\/uploads\/2018\/07\/DST-2.jpg\")
<\/a>The Relationship of Distance, Speed, and Time<\/strong> – If any two are known, the third can be easily calculated using simple multiplication and division.\u00a0 The “D ST Triangle” to the right provides a great representation of the relationship of calculating distance, speed, and time.<\/p>\n<\/a>When using the triangle as an aid, D is always in nautical miles (e.g. 5.5 nm), S is always in knots (e.g. 6 kts), and time is always in hours (never minutes or hours and minutes).\u00a0 90 minutes is not 1:30.\u00a0 90 minutes would be represented as 1.5 hours.\u00a0 When doing D ST calculations, values to the nearest 1\/10 generally provide sufficient accuracy for the coastal pilot.\u00a0 In use, visualize the horizontal line under D as a division sign, and the vertical line between S and T as a multiplication sign.<\/p>\n<\/a>Conversion from True to Magnetic to Course to Steer<\/strong> – When converting directions, the TVMDC table shown here is invaluable.\u00a0 Conversions will become second nature, however, when first starting out, I recommend duplicating the table on a pad and using it to fill in and calculate direction.<\/p>\n\n
<\/a>D = deviation is specific to the vessel and compass being used.\u00a0 Because of magnetic influences on the vessel, no compass can be adjusted to be perfectly accurate.\u00a0 Instead, adjustment gets it as close as possible, and a record, called a deviation card or a compass card, is kept of the differences.\u00a0 Deviation will vary at each heading, so the card records the difference at set intervals around the compass, generally 22.5 degrees apart.\u00a0 Deviation in between the provided headings can easily be estimated.\u00a0 055\u00ba is approximately half way between 045\u00ba and 067.5\u00ba, therefor deviation would be 1\u00baE.<\/li>\n<\/a>All of this seems quite complex, however, if the TVMDC table is drawn and used, it all becomes very simple.\u00a0 The course over ground from just north of Southeast Farallon Island to the entrance to Drakes Bay is “C 012”.\u00a0 Variation according to the compass rose is 14E, which is subtracted to get\u00a0“C 358 M”.\u00a0 Deviation of 4W is added to get a course to steer of “C 002 PSC).\u00a0 All that’s left to arrive at Drakes Bay safely is to account for wind, current, helm error, typically grouped together as set and drift.<\/p>\n<\/a>Back to our example of a Tradewinds Advanced Coastal Cruising class.\u00a0 On day one, the vessel is piloted out to G “8” of the Main Ship Channel through the San Francisco Bar.\u00a0 At 0954 a turn to the south is executed.\u00a0 The desired course over ground is “C 180”.\u00a0 Applying variation of 14E results in “C 166 M”.\u00a0 5E of deviation at 157.5\u00ba per the chart above is used, resulting in a course to steer of “C 161 PSC”.\u00a0 DR positions are plotted for 1000, 1100, and 1200.\u00a0 At 1100, a fix was obtained (see Part 4 to follow) and it was determined the vessel is .6 miles away from the expected DR position, in a direction of 110\u00ba.\u00a0 Set therefore is 110\u00ba.\u00a0 Drift will require further work, because the .6 miles is over 1 hour and 6 minutes, not over 1 hour as required.\u00a0 Using the D ST triangle, distance is .6 divided by 1.1 hours equals 0.545, so set is approximately 0.5 knots.\u00a0 You can also find a DR 1 hour from the DR start time of 0954, draw and measure the set.<\/p>\n<\/a><\/p>\n<\/a>Instead, an estimated position is correcting a DR plot for known set and drift.\u00a0 In the above example, if I consistently have a set and drift of 110\u00ba\/0.5 heading south from buoy G “8”, then it would make sense to note it as part of my DR during later voyages.\u00a0 You will note in the illustration I did not reset my DR to the EP.\u00a0 Instead, I just noted it, and would include the information in developing my DR expansion circles.\u00a0 The DR should only be reset with a fix or a running fix.<\/p>\n