Dead Reckoning

captrigney said:

........The Straight Dope: Is "dead reckoning" short for "deduced reckoning"?.....

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I have some issue with the statement "First, a definition is in order. Dead reckoning is the process of estimating the position of an airplane or ship based solely on speed and direction of travel and time elapsed since the last known position (or fix). So all you need to figure out approximately where you are is an airspeed indicator or log or other measure of speed, a clock or watch, and a compass."

I was taught DR was the initial 'plotting' of a course. You grab a chart, mark off your current position and your intended destination. Draw a straight line between the two and you should be able to determine your course and diatance. Once underway, you can calculate your ETE for any given speed. DR does not take into account things like wind speed and current. That is DR - plain and simple. With DR, your course and heading are the same - which rarely happens in the real world unless on a small lake with no wind or current
Once you start accounting for 'set and drift', you are no longer DRing. You are navigating by known parameters.
Anyway, this is what I was taught in Sea School in the land that time forgot.

TonyB you are quite correct in your take on what a "DR" and its use. The other side, taking fixes to prove your closeness to the intended course and destination requires the deduction. Now to another issue.
TVMDC: Jeffnick used the more common, but bear with me for a minute. I taught CPS courses in Ottawa for years and still do. However during the session on compasses and the issues of bringing that reading to the chart I used the old "True Virgins Make Dull Company" and "Can Dead Men Vote Twice". All is the class seemed to have gotten the point, but at the back of the class sat two ladies, both quite good looking, one a blonde, she had her hand up. So I asked her if she had any difficulty. No was her answer, but she felt that "Tired of Male Derogatory Comments" would be a better way to remember TVMDC.
A number of years ago I had the oppertunity of going from Cape Town SA to Tilbury UK. Each hour the bridge officer confirmed our position from three GPS units and using speed/time did it manually on a paper chart with the course laid out and DRs at the supposed hour location.

Doesn't anyone believe SPY when he says what you are talking about is called "pilotage?"

Good grief, look it up. Dead reckoning is based on heading, speed, and time, not landmarks.

FlyWright said:

I do carry paper charts for the areas I travel and I always track my position on the chart plotters and confirm it visually. In the delta, the fog can form quickly and in pockets, so those old forms of navigation on the serpentine rivers and shallow sloughs can be inadequate.

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This is in essence what we do and is why we always follow routes we have put on the plotters and have the radar on even when the visibility is excellent.

While we don't have the frequency of localized fog that seems to be characteristic of SFO Bay, we can get it during certain parts of the year. At those times it's not uncommon to be running along in bright, clear weather and come around an island to be confronted with heavy, low fog in the next channel we have to traverse.

We both feel that by using the plotters and radar on an all-the-time basis it makes it a lot easier to transition to the "instruments" when we need to. But we use the paper charts (and now the iPad with the really good Navimatics chart application on it) to confirm our position and what's around us regardless of the visibility.

sunchaser said:

PS I have about 150 charts for the PNW alone and storage is a huge issue, any suggestions?

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Well, we don't have 150 charts. Maybe twenty or thirty for the waters from the San Juans up through Desolation Sound which is as far as we've taken the GB so far. (We go farther north with the trailerable Arima).

We keep our charts rolled up, usually two or three to a roll, and stored on top of the closet in the aft cabin. I keep meaning to make a rack for them--- two vertical, teak "bulkheads" spaced apart with holes for the map rolls-- but so far have not gotten around to it. One could also make the same sort of thing for vertical storage of charts in a closet or wherever as long as there was the necessary headroom available to pull them out of the rack.

For everyday use we have the big chartbooks for the San Juans, Gulf Islands, Sunshine Coast, and Desolation Sound. They sit on a modified MapTech chartboard next to the helm console (photo).

Yes, it has always been dead reckoning for mine...
I have always assumed it related to the fundamental accuracy required in the various course, speed and elapsed time parameters, to be absolutely spot on to be correct - which of course they never can be - or you were dead wrong, in effect. Maybe the concept has the same origin as the old saying one gave to a marksman, he's a "dead eye dick', because for dead reckoning to be accurate, the speed, course and time needed to be 'dead' accurate. Of course inaccuracy of instruements like clocks, compasses, and tidal movement make the above impossible, so one could therefore extract the inference that if you rely on dead reckoning for more than a brief spell, you might end up 'dead'.

True Virgins Make Dull Company

Can Dead Men Vote Twice ?

IN Illinois its the law!

Someone else mentioned those two mnemonics FF. What do they stand for? I suspect we have different ones for the occasion.

Peter B said:

Someone else mentioned those two mnemonics FF. What do they stand for? I suspect we have different ones for the occasion.

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T=True
V=variation
M=Magnetic
D=Deviation
C=Compass

And vice/versa

The 'Add Whiskey' mnemonic is to remind operators to ADD Westerly correction and SUBTRACT Easterly when correcting for magnetic deviation. This is needed when converting from true to magnetic or magnetic to compass.

I was a Antisubmarine Warfare Tech in the Navy and flew / operated a piece of gear early in my career called a 'Nine Term Compensator' and your whole flight was spent trying to keep magnetic deviation nulled out. It was a real chore at times.

It became automatic a few years later and my life got a lot simpler.

FF said:

IN Illinois its the law!

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What's this one?

SomeSailor said:

The 'Add Whiskey' mnemonic is to remind operators to ADD Westerly correction and SUBTRACT Easterly when correcting for magnetic deviation. This is needed when converting from true to magnetic or magnetic to compass.

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East is least and West is best.

Dead men voting? Ok, it's all coming back to me now.

I think Dead Reckoning was an ancient form of navigation once practiced by long-ago deceased mariners? I believe the navigator used a specially matched set of chart darts which, when thrown at a chart hung on a convenient bulkhead, indicated the approximate position somewhere within the triangle described by the darts.

I'm pretty sure that is what it was . . . . . . . or maybe it's that 'Deduced' thing someone was talking about earlier . . . . .

I will have on board all necessary paper charts to navigate in any point of the Brazilian coast or any other country as I get there. Electronics is good but it does not hurt to back it up on paper every now and then....

For those who think these phrases are just more TF nonsense...

Can Dead Men Vote Twice

Compass heading +/- compass Deviation = Magnetic heading +/- magnetic Variation = True heading

Let's not forget ANDS. More of an issue in an airplane than in a trawler where speeds are relatively constant.

Accelerate North Decelerate South

(as one accelerates, the wet compass swings toward the north)

yeah...but who just did a 6-10 hr voyage in the pea soup fog with just a compass and a chart and a stop watch?

Cool to talk about it...it's another to do it.

Google "Honda point disaster". September 8, 1923 was the largest peace-time loss for the US Navy while DR south from San Francisco to San Diego. The fleet turned east when they thought they were clear of Point Conception. However, they were still quite some distance north and wrecked on Point Honda.

Moonstruck said:

East is least and West is best.

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You got it Don, when I was doing my yachtmasters, it was taught as..."variation east, compass least", (ie deducted)..."variation west, compass best", (ie added).

Peter B said:

You got it Don, when I was doing my yachtmasters, it was taught as..."variation east, compass least", (ie deducted)..."variation west, compass best", (ie added).

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"Add Whiskey" is a variation of that.