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SKYSAIL Chartwork card - including Course to Steer, EP, Tides, weather, etc
Course to Steer Process (Yachtmaster) – you know your present position and required Ground Track. You want to find the Course to Steer to allow for the tide, and stay on the Ground Track
pdf version - Course to steer process
Plot the initial fix and CHECK it. Add the START time. For SPDST subtract 2 hours from the start time to find UT time.
Estimate boat speed through the water.
Draw ground track (2 arrows) from fix beyond your destination D.
Estimate approx time of passage = distance to destination / speed
Determine tide set and drift for each hour of the passage – normally one hour, but could be ½ hour or more than 1 hour.
Look up the Tide diamond or
Tide Atlas for direction and speed of tide – springs or neaps?
You may need to interpolate the tide speed if the Range is between Spring Range
(4.9m) and Neaps Range (2.4m). See ‘Tidal Streams’.
Plot the tide from the FIX to point T (3 arrows)
Set your dividers for boat distance travelled,
based on boat speed S, from end of tidal stream plot to cross the
ground track at intercept X.
(NEVER join the tide to the destination!)
Use plotter to find bearing T to X of Course to Steer (° T) - 1 arrow
Apply Leeway if present – into the wind.
Apply Variation to find Magnetic Course º (M), then find Deviation (back page of Almanac) to get the Compass course
Speed Over Ground SOG = Distance from Fix to X
Time to D = Distance FIX to D x 60 minutes
(for 1 hour passage)
SOG
Course to Steer worked example
See also Course to steer process
RYA Chart 2 and Practice Almanac. Use variation 7°W.
On
the morning of Monday 13th May a yacht is sailing between North and South Coombe
Islands. At 1110 BST the boat crosses the 20m contour at a position 2 cables due
south of the South Cardinal buoy off Mill Head.
What
is the Latitude and Longitude?
What is the course to steer and ETA to the green buoy (FI.G.5s) at the entrance
to the River Dart?
Deviation on the required heading = 5°W
and Leeway is 10°
in a fresh westerly breeze.
Boat speed 5 knots. Use Diamond <D>.
Draw the Course Over Ground COG from A through B and extend the line, give it 2 arrows (2 feet on the ground)
Distance
to Buoy B = 4.6 M Boat
speed = 5 knots 2 cables = 0.2 M
Use
time to buoy = 1 hour.
The time period of the passage is 1110hrs to approximately 1210hrs
Time relative to HW and Tidal Stream calculation:
| HW Plymouth |
=1440 UT |
=1540 BST |
|
HW 1540 |
4.8m |
|
|
LW 0928 |
1.4m |
|
|
Range |
3.4m |
|
|
HW - 4 |
1110 |
|
|
1210 |
||
|
HW - 3 |
1210 |
|
|
1310 |
||
|
HW - 2 |
1310 |
|
|
1410 |
||
|
HW - 1 |
1410 |
|
|
1510 |
||
|
|
1510 |
|
|
HW |
1540 |
|
|
|
1610 |
Tide
diamond <D>
HW - 4 214
°(T)
2.3 sp 1.1 np
lnterpolation 1.7kn from Computation of Rates chart when range is 3.4m
Plot the tide from A to T - give it 3 arrows.
Arc
off the boat speed (5 knots) to cross the COG. The intersection of the arc and
the COG is the point where you will be after one hour
of sailing, allowing for the tide. So if you steer in that direction you will
stay on the required ground track. This is the Course to Steer.
| Course to Steer | 340° (T) |
|
| Variation | 007° W | |
| Magnetic | 347° (M) | |
| Deviation | +005° W | |
| Compass | 352° (C) | |
| Leeway (wind from West, steer into wind to offset leeway) |
-10° | |
| Course to Steer | 342° |
Time
for passage =
Distance
to go A-B 4.6M
x 60 = 66 minutes
ETA
=1216 BST
SOG 4.2 kn
Posted 29th May 2011
