1. Lesson VI: Dead Reckoning
Nautical Navigation
Lesson VI: Dead Reckoning
Preserving America’s Sailing Legacy * Engaging Sailing’s Next Generation

2. Determine Location by Triangulation
Each of these bearings are considered a “Line of Position”
3 LOPs form a triangle and provide accurate location
We call this location a “fix”

3. Determine Location by Latitude & Longitude
38º 45’ 15” º 30’ 25”

4. Dead Reckoning
After a fix, we know where we are. But…
…where will we be in 15 minutes? In 20 minutes? In an hour?

5. Dead Reckoning
Dead Reckoning - the process of calculating where you expect to be based on your:
Last known position (fix)
Course
Speed
0800
0815
0830

6. Dead Reckoning
Estimated Position
Fix Position
0830
0845
0900

7. Dead Reckoning: Fix Interval
How do you know how much space to have in between?
Fix Interval
The predetermined time between fixes
This varies based of many factors
Why would you have a very short fix interval? Long?
Fix Interval = 15 minutes
0830
0845
0900

8. Dead Reckoning: Fix Interval
30 Minutes
Fix Interval =
0805
0810
0800
5 Minutes
0820
0825
0815
12 Minutes
1432
1444
1420

9. Dead Reckoning
Dead Reckoning - the process of navigating by advancing your expected position on a chart using your last accurate fix
Need 3 Variables
Course – What direction are you sailing?
Speed – How fast are you sailing? (in knots)
Time – How long before you take another fix?
0800
0806
0812

10. Distance ? = 6 kts * 1/3 hr (20 min)
Fix Interval Example #1
Let’s do an example:
Course – 090 degrees
Speed – 6 knots
Fix Interval – 20 minutes
Hint
Distance = Speed * Time
Hint
Distance ? = 6 kts * 1/3 hr (20 min)
0800
0820
0840
2 NM
2 NM

11. Distance ? = 10 kts * 1/10 hr (6 min)
Fix Interval Example #2
Let’s do another example:
Course – 270 degrees
Speed – 10 knots
Fix Interval – 6 minutes
Hint
Distance = Speed * Time
Formula
Distance ? = 10 kts * 1/10 hr (6 min)
0812
0806
0800
1 NM
1 NM

12. Fix Interval
How do you determine what time to set as your fix interval?
What’s the problem with this fix interval?
0830
0845
0900

13. Fix Interval
How do you determine what time to set as your fix interval?
You should be able to get 2 DRs before shallow water
0830
0835
0840

14. Fix Interval
What’s wrong with this?
It’s Unnecessary!
0830
0835
0840

15. Fix Interval
In open water, or when you’re in no danger, you can use a larger fix interval
0830
0840
0850

16. Exercise Bearings @ 0800 Sailboat Information Radio Tower (Fl R Lt)
090º
G“5” (Fl G 4s)
170º
“HP” (15ft 6M)
230º
Sailboat Information
Course
120º
Speed
6 knots
Fix Interval
20 minutes
First plot your position
Then estimate your future position based on your sailboat information

17. Exercise
After 20 minutes, you look at your GPS and see that you are exactly where you thought you would be at 0820
Sailboat Information
Change Course
180º
Speed
6 knots
Fix Interval
20 minutes
Estimate your future position based on your sailboat information
Which buoy do you expect to approach at 0900?