Module 10 - Chartwork
Worked EP and Course-to-Steer Example
Active recall
Section 5 of 7
Explain the main idea from worked EP and Course-to-Steer Example in your own words.
Worked EP example: start from a fix at 1000. You steer 090 degrees true for one hour at 5 knots, so the DR is 5.0 M due east of the fix. The tidal stream during the hour sets 180 degrees at 1.0 knot, so plot a 1.0 M vector due south from the DR. The resulting triangle point is the EP at 1100.
Worked CTS example: your intended ground track is 090 degrees true for one hour. The tide sets 180 degrees at 1.0 knot and the boat speed is 5 knots. Plot the 1.0 M tide vector south from the start, then from the end of the tide vector swing a 5 M arc to cross the intended 090 degrees track. The line from the end of the tide vector to that crossing is the water track to steer in true degrees before compass correction.
The arithmetic check helps: in one hour the tide will push you 1.0 M south. To make good an eastbound ground track, your water track must include a northerly component to cancel that set. That is why the CTS is not simply 090 degrees true.
- Step
- DR
- Example input
- 090 degrees true for 1 hour at 5 kn
- Result
- 5.0 M east of the fix
- Step
- Tide for EP
- Example input
- Set 180 degrees, drift 1.0 M
- Result
- EP is 1.0 M south of the DR
- Step
- CTS tide vector
- Example input
- 1.0 M south from the start
- Result
- Shows the offset to cancel
- Step
- CTS water track
- Example input
- 5.0 M arc from tide-vector end
- Result
- True course to steer before CDMVT
| Step | Example input | Result |
|---|---|---|
| DR | 090 degrees true for 1 hour at 5 kn | 5.0 M east of the fix |
| Tide for EP | Set 180 degrees, drift 1.0 M | EP is 1.0 M south of the DR |
| CTS tide vector | 1.0 M south from the start | Shows the offset to cancel |
| CTS water track | 5.0 M arc from tide-vector end | True course to steer before CDMVT |
Key points
- DR uses course and distance through the water only.
- EP adds tidal stream and leeway to the DR.
- For CTS, plot the tide vector first, then the boat-speed arc.
- Convert true course to compass course only after the vector solution is found.
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