Calculating Distance Using Parallel Sailing

  What is Parallel Sailing?

    Parallel sailing means sailing along the same parallel of latitude. In parallel sailing, you maintain a true course of 90° East or 270° West. Moreover, the distance sailed would be the departure at the latitude (distance=departure).

Formula: 

    1. Cos lat = Departure/Dlong

Deriving the formula will give us:

    2. Departure= Cos lat x Dlong

    3. Dlong= Departure/Cos lat

Sample Problem:

1. The final and initial courses are given. What are the course and distance?

    Position A= 19° N, 070° E

    Position B= 19° N, 100° E

Solution:

Step 1: To find the distance, calculate the difference of longitude (Dlong) first.

    Long A= 070° E

    Long B= 100° E (Same sign subtract, different sign add.)

    Dlong = 030° E (Sign of Dlong is East because the direction of the ship is easterly.)

                    x 60 (To convert it to nautical miles)

    Dlong=  1800 NM

Step 2: Since we already have the value of Dlong, apply formula 2 to calculate distance.

    Departure= Cos lat x Dlong

    Departure= Cos 19° x 1800 NM

    Departure= 1701.93 NM

Final Answers:

Distance= 1701.93 NM

Course= 090° T (Since the direction of your ship is easterly, the course is 090° T.)

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