ASA 105, Skill 11, Ded Reckoning (DR)

 

11.Plot a dead reckoning position on a chart using speed, time and course to steer.

Deduced Reckoning or Ded Reckoning or DR ("dead" is a common misnomer) is based only on two factors:  speed and direction.  No outside influences are considered.

For example:  If you proceed due North at 6 knots in one hour your DR position is 6 nautical miles due North of the starting point.

Use Chart 1210Tr for this exercise.  

Make a DR plot of three legs from Cleveland Ledge Light to Lone Rock at Quick's Hole, transit Quick's Hole, and finish at the entrance buoy for Menemsha Harbor, Bell "1."

Assume a speed over ground of 6 knots.

Mark of course	Heading to next	Distance to next	Time to next
R "14" Fl R 2.5 sec	215 deg T	12.2 nm	2 hrs 2 min
BR Qk Fl G	167 deg T	2 nm	20 min
G "1" Fl G 4 sec Bell	143 deg T	5.3 nm	53 min
"1" Bell		19.5 nm	3h 15 m in transit

Your DR plan looks something like this.  Knowing one's departure time one can estimate their time of arrival absent any outside influences.

We can convert to PSC using an assumed Deviation Table.

T	215 T	167 T	143 T
V	15 W	15 W	15 W
M	230 M	182 M	158 M
D	2 E	11 E	10 E
C	228 PSC	171 PSC	148 PSC
AW or SE	AW or SE	AW or SE	AW or SE

Most navigation plans begin with a DR plot.  For example, Bermuda is 650 nm SW of Newport, RI.  At 6 knots it is 108 hours or 4 1/2 days distant.  A rough guess, at best.  Three different current streams the heading.  The weather will change during the transit.  These all affect the actual course or track sailed.