On chart 3: EP with a tidal diamond
How to think it through
- 1You know where you are, course and speed for an hour; you want where you will be, with the chart's real tide.
- 2Read the tide at the nearest diamond: the row for the hour from HW Victoria, the rate between neaps and springs.
- 3Lay off the water track first (DR), then the tide from its end: the tip is the EP.
Worked example
Every step says why you do it, which formula you use, the working and the answer.
An EP on chart 3, with diamond H
At 13:30 you are at 46°03.6′N 006°15.2′W, near diamond H. You sail for one hour on 040°T at 5.5 kn through the water, no leeway. Today HW Victoria is 11:20, range 2.7 m. Where are you at 14:30?
- Start
- 13:30 · 46°03.6′N 006°15.2′W
- True course and speed
- 040°T · 5.5 kn
- HW Victoria · range
- 11:20 · 2.7 m
- Diamond H, row HW+3
- 293° · 2.0 / 1.1 kn
For the hour 13:30–14:30 (mid-point 14:00), which row of diamond H do you use? (hours from HW Victoria, −6 to +6)
Why: The chart's tidal stream table is referred to HW Victoria: each row covers the hour centred on it.
Working: 14:00 − 11:20 = +160 min → row HW+3: 293°, 2.0 kn springs, 1.1 kn neaps
Answer: 3 h
What is today's rate, between neaps and springs?
Why: The diamond gives two rates: today's is interpolated with the range at Victoria (springs 4.9 m, neaps 2.4 m).
Working: (2.7 − 2.4) ÷ 2.5 = 0.12; 1.1 + 0.12 × 0.9 = 1.2 kn towards 293°
Answer: 1.2 kn
Where would you be after one hour with no tide (DR)?
Why: The DR is just the water track for the distance run: it is the first side of the triangle.
Working: DR: 46°07.8′N 006°10.1′W
Answer: 46°07.8′N 006°10.1′W
Now add the tide: where is the estimated position (EP)?
Why: In one hour the tide moves the boat its set for its rate: add it to the end of the DR.
Working: EP: 46°08.3′N 006°11.7′W
Answer: 46°08.3′N 006°11.7′W
What is the course over the ground (COG) from the start to the EP?
Why: The line from the start to the EP is the real track over the ground: two arrows.
Working: COG 027°T, SOG 5.3 kn
Answer: 027°