Table of Contents

Class Prep

Class 1
Class 2
Class 3
Class 4
Class 5
Class 6
Class 7
Class 8
Class 9
Class 10

>>Topical Articles<<
Assumed Longitude
Casio fx-260 Solar II
Emergency Navigation
Making a Kamal
Noon Sight
Pub. 249 Vol. 1
Sextant Adjustment
Sextant Skills
Sight Averaging
Sight Planning,
  Error Ellipses,
  & Cocked Hats
Slide Rules
Standard Terminology
Star Chart
The Raft Book
Worksheet Logic

Class 10:  September 12, 2019

New/Changed Pages in This Web Since Last Week

Class Plan

  • If one is sailing from the Mediterranean to New York,
    • Cape St. Vincent, Portugal, to New York = ~2,937 nm
    • Cape St. Vincent, Portugal, to Halifax = ~2,441 nm. Hence...

      Halifax is the best place to make your initial landfall in N. America.


  • Confirm your position off Cape St. Vincent by compass bearing plus distance-off by sextant. Calculate distance-off either by Bowditch (American Practical Navigator), Volume 2, Table 15, or by direct calculation.

    If the Bowditch download site does not load, try again later. They seem to be under a more-or-less constant denial-of-service attack by hackers.

    Alternatively, you can go to and download the 2002 edition. The direct-calculation equation is found in section 2202.

  • Plot the initial stages of a hypothetical voyage from Cape St. Vincent, Portugal to Halifax, Canada.


         36° 54.6' N
           9° 12.5' W

  •                                       44° 38.8' N
                                          63° 34.4' W

  • Calculate your initial great circle course, either using an electronic calculator or a slide rule.
  • You depart at 10:00 AM on Sept 12, on the calculated course, at an average speed of 4.4 kn.
  • The next morning, during nautical twilight, you take sights of:
    • Rigel at 04:30:25 local time. No watch error. Height of Eye (HoE) = 6 feet.
    • Pollux at 04:32:12. No watch error. HoE = 6 feet.
  • Plot your trip up to now, including the celestial fix.
  • At 11:15 on Sept. 13, you plot a DR based on an average speed of 4.6 kn.
  • Take a sun sight. No watch error. Height of Eye (HoE) = 6 feet. Advance the LOP from Pollux for a running fix.
  • At 18:40 on Sept. 13, you plot a DR based on an average speed of 4.8 kn.
  • Take sights of:
    • The moon at 18:40:36 local time. Watch is 1 second fast. Index error = 1.5' on the arc. Height of Eye (HoE) = 6 feet.
      Reduce the moon sight using the Mark 1 Navigator's Slide Rule.
    • Jupiter at 18:43:02 local time. Watch is 1 second fast. Index error = 1.5' on the arc. Height of Eye (HoE) = 6 feet.
  • Recalculate your great circle route to Halifax.
  • Take a break from this detailed plot of your trip, and review the resources below for blue-water cruising.

  • In this exercise so far, we have ignored prevaling winds and currents...and just laid out the most direct course possible. That said...

    • Look at Pilot Chart for prevailing winds. Download North Atlantic Pilot Charts HERE. (227 MB)

    • Compare GC route with traditional trans-Atlantic sailing routes.

    • Download Pilot Charts for other parts of world HERE.

    • If planning for an offshore journey, consider getting Jimmy Cornell's World Cruising Routes HERE.

      Pilot charts have been evolved based on the experience of sailors over a period of centuries. However, global warming is producing changes in the long term patterns of wind and weather. Cornell's book is an attempt to take these changes into account.

      Though expensive, you might find Jimmy Cornell's World Cruising Super Pack a worthwhile investment before crossing an ocean.

    • In addition to pilot charts, you may want to consider a weather routing service such as this.

      Another alternative is a do-it-yourself weather routing tool like this.

    If you are using the do-it-yourself tool, HERE is the kind of output it gives based on a trip that departs Cape St. Vincent at 10 AM on Sept. 12.

    Because of the uncertainty in weather forecasts, it does not give a detailed projection for the whole trip. It assumes you will reconnect to the server via your on-board Internet connection, and refresh the route plan.

    • Prior to weather satellites and the Internet, Pilot sharts were THE way to do offshore planning.
    • Pilot charts are still useful if you have no on-board Internet connection. They give you the most statistically-likely course to succeed.
    • However, with on-board computing technology, you don't have to play the odds as they have been established, based on sailors' experiences over the past 400 years. Or rather, you can fine-tune the gambles you are willing to take.
      In this case, if the weather lives up to the forecast, you can make the journey in 13 days 15 hours.

Materials Required

  • Sight reduction worksheet and plotting sheet
  • Scientific calculator
  • Yellow highlighter


Homework Assignment