Piloting 101

[Sea Hunt]

It is my understanding (admittedly very limited) that the reason to plot a True course on a chart and to designate it as a True course is because anyone who looks at your chart later will assume you plotted a True course. If the only thing written is "210", it will be presumed this was 210 degrees True.

I was told, whether this is right or wrong, that if you want to be really detailed on a chart you can identify True course, Magnetic course and Compass course (in addition to plot times, distance, speed, etc.), but if you are only going to identify one course on a chart it should be the True course corrected for Variation, Deviation, etc.

The following may not be appropriate for certain sensitive individuals. I was taught the best way to remember the difference between the various course plots is with the following:

True (True North)
Virgins (Variation)
Make (Magnetic North)
Dull (Deviation)
Company (Compass heading)
Add (Add)
Whiskey (Whiskey)

The last two items refer to whether the variation and deviation are "westerly" or "easterly". If the Variation is, say 5 degrees W, then you would Add 5 degrees to True North heading in performing your above vertical calculations going from top to bottom. If the deviation (or variation) was 3 degrees East, you would subtract this number of degrees from the vertical calculation above going from bottom to top.

I strongly urge no one to rely on the above until seasoned, experienced skippers on this board weigh in on the efficacy of the above

[Neil Gordon]

IF you are sailing a compass coarse with a magnetic compass why care about the difference between mag and true north. Just plot magnetic using the charts mag rose? And visa versa.

I suppose that's why they have a magnetic rose as well as a true one. Otherwise, they could just tell you what the variation is and leave it at that.

For me, the best reason in plotting true courses is that all the lat/lon lines on the chart are true. Also, modern chart tools don't require plotting from the rose; they line up on the lat/lon lines. It's why my practice is to plot true courses, keep the gps true and only make the adjustment for mag compass steering.

[bottomscraper]

IF you are sailing a compass coarse with a magnetic compass why care about the difference between mag and true north. Just plot magnetic using the charts mag rose? And visa versa.

Works fine for me! If you are doing celestial navigation I think you may be more interested in true north but for most of us doing coastal navigation working in only magnetic makes life easy. All GPS's that I have had my hands on allow you to set them for magnetic also. Most paper charts have the "pre-plotted" routes in magnetic. If I remember correctly most current tables are based on true rather than magnetic so you may need to convert them.

[Chris Reinke]

Dick - These little quizzes are wonderful and keep my skills sharp while the snow is building up.

One homework question I had been asked by my sailing instructor when I was a novice sailor went something like this:
You depart your dock and sail True 225° for 2:14:00 hours at 4.3kts . You then sail east for 14.3nm before turning true north and arrive back at your dock after 3:24:00. When you get back to the dock you find a bear sitting at the end of the dock. What color is the bear?

I think she was trying to demonstrate that we needed to focus on the relevant information and not get worried by the superfluous data.

Did you get the answer?

The only place on the planet where you could sail a southerly route, turn East and finally turn North and end up at the same point without a fourth leg would be if you departed from the North Pole. Therefore - the bear must be white.

[Neil Gordon]

The only place on the planet where you could sail a southerly route, turn East and finally turn North and end up at the same point without a fourth leg would be if you departed from the North Pole.

The way global warming is going, there will be a marina there before too long.

[RWilker]

True (True North)
Virgins (Variation)
Make (Magnetic North)
Dull (Deviation)
Company (Compass heading)
Add (Add)
Whiskey (Whiskey)

The last two items refer to whether the variation and deviation are "westerly" or "easterly". If the Variation is, say 5 degrees W, then you would Add 5 degrees to True North heading in performing your above vertical calculations going from top to bottom. If the deviation (or variation) was 3 degrees East, you would subtract this number of degrees from the vertical calculation above going from bottom to top.

To quote John Rousmaniere, "These mnemonic devices can be both helpful and confusing."
My understanding is that when calculating from compass toward true, ie going from the bottom to the top of the above list, that east deviation or variation should be added rather than subtracted. But my navigation is limited to only going to places that I can see.

[Sea Hunt]

RWilker:

Yes, you are correct. East variations and East deviations are subtracted going from top to bottom on the mnemonic I was taught.

I apologize. Thanks for the correction. Like I said. I am a rookie. If there are other errors in my suggestion, I would hope they would be pointed out to me (and others) as well.

Again, thank you for correcting this.

[rtbates]

You wrote

Works fine for me!

Works for me too.

[Dean Abramson]

Randy,

I like aways having in my mind what the variation is. Say I am in the fog, and suddenly realize I may not be where thought I was, and I want to change course, QUICKLY. I glance at the chart, where the parallels and meridians are in true, of course. I can instantly surmise that, say, turning due north will keep me out of trouble. Where we sail, I know by acquired instinct to head 17 degrees Magnetic. (Or 107M or 197M or 287M, in other cases.) I just find that in a pinch, particularly if I am tired or stressed and in bad conditions, it is sometimes nice to just quickly deal with True N, S, E, and W. The lat and lon lines on the chart are just so easy to lock in on fast.

The 17 degree westerly variation resides in my brain. In my sailing, I have so far only made it as far as 16 Degree Land. (My instincts there were still "good enough for government work.") Anyway, I personally find it helpful to remember the variation; the deviation on my boat is not enough to worry about. And fortunately there isn't much fog here in Maine. Not.

Dean