Dead Reck­on­ing Robert Reeder

Passage Maker - - Contents -

Suc­cess­ful nav­i­ga­tion is re­ally just an­swer­ing three sim­ple ques­tions: Where am I? Where am I go­ing? How do I get there from here? Even when all of our nav­i­ga­tion tools are op­er­a­tional, it is pru­dent to have three in­de­pen­dent sources of nav­i­ga­tion. For coastal nav­i­ga­tion on a clear day, I would prob­a­bly use Dead reck­on­ing, vis­ual and radar nav­i­ga­tion; if fog took away my vi­su­als, I’d prob­a­bly add in GPS and a chart plot­ter. Dead reck­on­ing will al­ways be the cor­ner­stone of my nav­i­ga­tion un­der any con­di­tion be­cause re­moved of all other nav­i­ga­tional tools, dead reck­on­ing can never be taken away from me.

Dead reck­on­ing is the cor­ner­stone of all Euro­pean-style nav­i­ga­tion, and there are many dif­fer­ent meth­ods to go about it. All of the meth­ods ex­trap­o­late our cur­rent po­si­tion from a known po­si­tion in the past, or else pre­dict our fu­ture po­si­tion from a known po­si­tion in the present. All

of it is based on the course and speed we have made or in­tend to make since the known po­si­tion.

This pre­lim­i­nary known po­si­tion, based on in­for­ma­tion ex­ter­nal to our boat, is called a “fix.” A fix may be de­rived from vis­ual bear­ings, radar ranges, ba­thym­e­try (as dis­cussed in pre­vi­ous ar­ti­cles), GPS, and al­ti­tudes of celestial bod­ies; ba­si­cally any source of po­si­tion in­for­ma­tion that does not orig­i­nate in­side our ves­sel. For this ex­er­cise, our fix is a range and bear­ing to a fixed ob­ject close aboard. I use the Ch­e­sa­peake Light for this ex­am­ple, lay­ing one-tenth of a nau­ti­cal mile abeam to port, while on course 015° True.

We be­gin our dead reck­on­ing by plot­ting the fix di­rectly on our chart. It is crit­i­cal that we la­bel the time of this fix. We must choose a work­able in­ter­val with which to plot our fixes, whether on pa­per or on an elec­tronic chart plot­ter. In open

ocean, the in­ter­val may be only once per hour, but in pi­lotage wa­ters it should be ev­ery three or six min­utes. A good rule of thumb when in close prox­im­ity to shoal wa­ter is for your in­ter­val be­tween ex­ter­nal fixes to be less than half the dis­tance to shoal wa­ter. For this ex­er­cise, we’ll use a 30-minute in­ter­val.

From our fix we draw the course line on our chart, us­ing the com­pass rose and par­al­lel rulers. What­ever my head­ing source, if needed, I con­vert it from Mag­netic to True be­fore plot­ting it, and ex­tend it out to some amount of time into the fu­ture. For this, we con­tinue on course 015° True at a speed of 12.2 knots, so our

dead reck­on­ing in­ter­vals of 30 min­utes will be 6.1 nau­ti­cal miles. We will ex­tend out twice that dis­tance in or­der to see our dead reck­on­ing po­si­tion in one hour.

We want to use Speed Through the Wa­ter for our dead reck­on­ing rather than GPS-de­rived Speed Over Ground, be­cause the lat­ter is a con­stantly chang­ing vari­able.

Note that the stan­dard sym­bol for our vis­ual fix is a dot with a cir­cle around it; the stan­dard sym­bol for a dead reck­on­ing po­si­tion is a dot with a semi­cir­cle, or bird’s-eye; the idea be­ing that a dead reck­on­ing is about half as re­li­able as a vis­ual fix (see ref­er­ence above).

At time 1500 we ob­tain an ex­ter­nal fix of 37° 04.8’N, 075° 38.1’W. For sim­pli­fi­ca­tion, I’m us­ing GPS. We plot this on the chart and com­pare to our

dead reck­on­ing. Note that I do not know or care what the lat­i­tude or lon­gi­tude of my dead reck­on­ing is, I only know where they hap­pen to be on the chart. For the past hour the cur­rents have pushed me in a di­rec­tion of 155° True, at a speed of 1.3 knots. This is my “set and drift,” with set be­ing the di­rec­tion and drift be­ing the speed at which I was pushed. In this case, I was pushed 1.3 nau­ti­cal miles in ex­actly one hour, so my drift is 1.3 knots; how­ever, for any time in­ter­val other than one hour, you will need to cor­rect for the ac­tual time run. For ex­am­ple, if I had run 30 min­utes and had been pushed 0.7 nau­ti­cal miles over that time, my drift would be 1.4 knots.

Some older books and newer books writ­ten for sailors, will sep­a­rate out set and drift as ap­ply­ing to cur­rents and lee­way as ap­ply­ing to wind, into two sep­a­rate cat­e­gories. For power­boats, we re­ally don’t care what is caus­ing our off­set from our dead reck­on­ing, we just have to know what it is and adjust ac­cord­ingly.

I now up­date my dead reck­on­ing to my time 1500 GPS fix, and con­tinue.

At time 1530 we change course to 060° True, and up­date our dead reck­on­ing ac­cord­ingly. Bet­ter find that next chart.


Left: The Ch­e­sa­peake Light sits at the open­ing of Ch­e­sa­peake Bay. Op­po­site: The Ch­e­sa­peake Light on NOAA chart 12221, which we use to es­tab­lish our first fix for this ex­er­cise.

Above: Us­ing our par­al­lel rulers, we tran­si­tion our head­ing from the com­pass rose to our fix to lay out the bear­ing line of our DR. Be­low: We ex­tend our DR out based on time and speed.

Top Left: Af­ter up­dat­ing our fix we no­tice our set and drift from our DR. Top Right: De­pend­ing on our type of fix, we mark them dif­fer­ently on our chart so we know the ac­cu­racy and method of our plot­ted points. Be­low: When we change course at time 1500, we plot a new DR.

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