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NAU 102

Lesson 28

Current Sailing

The horizontal movement of the sea surface.

Current

Expanded Definition

All factors that cause a ship to depart from its intended course and DR.

Current Sailing

Factors

•Ocean Current•Tidal Current•Wind-driven Current •Wind•Seas

Current Sailing

Factors

•Compass Errors•Steering Errors•Engine Calibration Errors •Speed Log Errors•Fouled Bottom•Unusual Trim

Current Sailing

Current

The combined effect on a vessel of all the factors.

Current Sailing

The art of selecting courses and speeds to compensate for the effects of current in

order to arrive at your desired destination.

Current Sailing

Current Sailing

•Track (TR) – the intended direction of travel.

Important Terms

•Set – the direction towards which a current is flowing.

Speed of Advance (SOA) – the speed intended to be made good.

Current Sailing

•Drift – the speed of a current.

Important Terms

•Speed Made Good (SMG) – speed along the Course Made Good.

•Course Made Good (CMG) – the resultant direction from a point of departure to a

subsequent position.

Current Triangle

Course & Speed

CMG & SMG

Set &

Drif

t

Current TriangleCourse & Speed

CMG & SMG Set &

Drif

t

If you know any 2 sides, you can solve for the 3rd side.

Current Triangle

Course & Speed

CMG & SMG Set &

Drif

t

If you know the Course & SpeedAnd the Set & Drift

You can get the CMG & SMG

Estimated Position

Current Triangle

Course & Speed

CMG & SMG Set &

Drif

t

If you know the Course & Speed

You can solve for the Set & DriftAnd the CMG & SMG

Current Triangle

Course & Speed

CMG & SMG Set &

Drif

t

You can solve for the Course & SpeedAnd the Set & Drift

If you know the CMG & SMG

Current Triangles

Can be solved using:

Charts

Universal Plotting Sheets

Radar Plotting Sheets

Maneuvering Boards

Solving for Set & Drift

Problem Type #1 - Determining an Unknown Current

Given two positions (fixes) and your course and speed, solve for the current.At 0919 your position is LAT 37°00.0'N,

LONG 75°30.0'W. You are on course 270°T at 8.7 knots. At 1000 your position is LAT

36°59.5'N, LONG 75°37.0'W. What was the current?

Solving for Set & Drift

Step 1: Plot the 1st position.

Step 2: DR to the time of the 2nd position.

At 0919 your position is LAT 37°00.0'N, LONG 75°30.0'W. You are on course 270°T at 8.7 knots. At 1000 your position is LAT 36°59.5'N, LONG 75°37.0'W.

What was the current?

0919C – 270

S – 8.7

1000

Solving for Set & Drift

Step 3: Plot the 2nd position.

At 0919 your position is LAT 37°00.0'N, LONG 75°30.0'W. You are on course 270°T at 8.7 knots. At 1000 your position is LAT 36°59.5'N, LONG 75°37.0'W.

What was the current?

0919C – 270

S – 8.7

1000

1000

Solving for Set & Drift

Step 4: Complete the Triangle.

At 0919 your position is LAT 37°00.0'N, LONG 75°30.0'W. You are on course 270°T at 8.7 knots. At 1000 your position is LAT 36°59.5'N, LONG 75°37.0'W.

What was the current?

0919C – 270

S – 8.7

1000

1000CMG/SMGSet &

Drift

Solving for Set & Drift

Direction From DR to Fix = Set

At 0919 your position is LAT 37°00.0'N, LONG 75°30.0'W. You are on course 270°T at 8.7 knots. At 1000 your position is LAT 36°59.5'N, LONG 75°37.0'W.

What was the current?

1000

1000CMG/SMG

Set = 137° T

Solving for Set & Drift

Distance DR to Fix Converted to Speed = Drift

At 0919 your position is LAT 37°00.0'N, LONG 75°30.0'W. You are on course 270°T at 8.7 knots. At 1000 your position is LAT 36°59.5'N, LONG 75°37.0'W.

What was the current?

1000

1000CMG/SMGDistance = 0.6 nm

0.6 nm41 min X 60 min/hrDrift = = 0.9 kts

CMG/SMG

You are underway on course 315°T at 10 knots. The current is 090°T at 2.3 knots. What is the course & speed being made

good?

Problem Type #2Given the Course & Speed and a known

current, determine the CMG & SMG (Estimated Position).

CMG/SMG

Step 1: Plot the 1st position.

(If no position given, start in the center of compass rose or radar plotting sheet.)

Step 2: DR for one hour.

You are underway on course 315°T at 10 knots. The current is 090°T at 2.3 knots. What will be the course & speed being made good?

CurrentYou are underway on course 315°T at 10 knots. The current is 090°T at 2.3

knots. What is the course & speed being made good?

CMG/SMG

Step 3: Draw the Set & Drift Vector from the tip of the Course & Speed Vector

You are underway on course 315°T at 10 knots. The current is 090°T at 2.3 knots. What is the course & speed being made good?

CMG/SMG

Step 4: Complete the Triangle

You are underway on course 315°T at 10 knots. The current is 090°T at 2.3 knots. What is the course & speed being made good?

CMG/SMG

Step 4: Complete the Triangle

You are underway on course 315°T at 10 knots. The current is 090°T at 2.3 knots. What is the course & speed being made good?

CMG = 329° T

SMG = 8.2 kts

Course & Speed

You wish to make good a course of 015°T and a speed of 8 knots. The current is

330°T at 3 knots. What is the course to steer and engine speed to turn?

Problem Type #3AGiven a desired CMG & SMG and a

known current, determine the course to steer & required engine speed.

Current

Step 1: Plot the first position.

(If no position is given, use the center of a compass rose or plotting sheet.)

Step 2: Lay out the desired track (draw the CMG/SMG vector) for 1 hour.

You wish to make good a course of 015°T and a speed of 8 knots. The current is 330°T at 3 knots. What is the course to steer and engine speed to

turn?

CurrentYou wish to make good a course of 015°T and a speed of 8 knots. The

current is 330°T at 3 knots. What is the course to steer and engine speed to turn?

CurrentYou wish to make good a course of 015°T and a speed of 8 knots. The

current is 330°T at 3 knots. What is the course to steer and engine speed to turn?Draw the Set & Drift Vector from the 1st

position.

CurrentYou wish to make good a course of 015°T and a speed of 8 knots. The

current is 330°T at 3 knots. What is the course to steer and engine speed to turn?Complete the Triangle

CurrentYou wish to make good a course of 015°T and a speed of 8 knots. The

current is 330°T at 3 knots. What is the course to steer and engine speed to turn?Complete the Triangle

Course to Steer = 034° T

Required Engine Speed = 6.2 kts

CurrentYou wish to make good a course of 015°T and a speed of 8 knots. The current is

330°T at 3 knots. What is the course to steer and engine speed to turn?

Alternate Procedure – Draw the S&D Vector Backwards from the CMG Vector

CurrentYou wish to make good a course of 015°T and a speed of 8 knots. The current is

330°T at 3 knots. What is the course to steer and engine speed to turn?

Complete the TriangleCourse to

Steer = 034° T

Required Engine Speed = 6.2 kts

Current

You wish to make good a course of 315°T. Your engine speed is 8 knots. The current is setting due north at 3 knots. What is the course to steer and speed made good along

the trackline?

Problem Type #3BGiven a desired CMG, a known current & your engine speed, determine the course to

steer & speed made good.

CurrentYou wish to make good a course of 315°T. Your engine speed is 8 knots. The current is setting due north at 3 knots. What is the course to steer and speed

made good along the trackline?

Draw the CMG/SMG vector an indefinite length.

CurrentYou wish to make good a course of 315°T. Your engine speed is 8 knots. The

current is setting due north at 3 knots. What is the course to steer and speed made good along the trackline?

Draw the Set & Drift vector from the center.

CurrentYou wish to make good a course of 315°T. Your engine speed is 8 knots. The

current is setting due north at 3 knots. What is the course to steer and speed made good along the trackline?

Open dividers to engine speed.

CurrentYou wish to make good a course of 315°T. Your engine speed is 8 knots. The

current is setting due north at 3 knots. What is the course to steer and speed made good along the trackline?

Swing an arc from the S&D vector to the CMG line.

CurrentYou wish to make good a course of 315°T. Your engine speed is 8 knots. The

current is setting due north at 3 knots. What is the course to steer and speed made good along the trackline?

Complete the triangle.

CurrentYou wish to make good a course of 315°T. Your engine speed is 8 knots. The

current is setting due north at 3 knots. What is the course to steer and speed made good along the trackline?

Complete the triangle.

Course to Steer = 300° T

SMG = 9.1 kts

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