Topmark............... Single red can
Light (when fitted). Red (any rhythm)
other than composite
group.
Topmark............... Single green cone, point up
Light (when fitted) . Green (any rhythm)
other than composite
group
Red markers are
PORT Hand
Green markers are STARBOARD Hand
Green markers are STARBOARD Hand
ISOLATED
DANGER MARK
DANGER MARK
SAFE WATER
MARK
MARK
Light White Group flash two
Light White, isophase, occulting , one long
flash every 10 secs or Morse A (. -)
LIGHTS EXHIBITED BY A POWER-DRIVEN
VESSEL OF LESS THAN 20 M IN LENGTH
LIGHTS EXHIBITED BY A SAILING VESSEL OF LESS
THAT 20 M IN LENGTH
2. Always carry charts for alternate ports - especially on long voyages when the weather at the planned destination will be unknown at the time / point of departure.
3. Always have an alternate navigational backup. Never rely on a single nav aid. Always have other data to cross check your position - heading and speed for a DR position, GPS for confirmation, depth sounder, radar etc.
4. Always lay your course to an anticipated upwind position from the destination.
5. Always ask "What if ???" What if the chart has an error? What if the current is different? What if the boat speed is different than planned or anticipated? What if the GPS fails? What if the engine fails? What if the electrical power fails? What if the intended course can't be layed?
6. Always stay clear of a lee shore.
In relatively benign conditions this can be estimated fairly accurately by sighting down the centre of the wake and comparing this path with the fore and aft axis of the boat. However in rougher conditions the wake is not so easily discerned and experience may be the best guide - with the current effect being most difficult to guess. With a GPS and some accurate sailing (holding a heading) the average leeway can be accurately found for that period. It can at best be only an average because the boat will encounter different wind and current during the fixing time interval.
When observations with sextants are taken on board ships at sea, the actual observed horizon can be used for measuring altitudes. On land this method of observation is seldom possible because of the irregularity of terrain. In this case a so-called artificial horizon is employed, consisting of a pool of mercury or some other horizontal reflecting surface. By observing both the star itself and the image of the star in the mercury, a sextant reading can be obtained that is equal to double the actual altitude of the star. In aircraft and on ships in a rough sea, when the visible horizon is not a clearly defined line, bubble sextants or bubble octants are used.
Stimulated by a Parliamentary award of £20,000 for a method of finding a ship's longitude anywhere on Earth to an accuracy of half a degree, Harrison spent nearly all his life perfecting a marine chronometer to solve the longitude determination problem. He reasoned that an extremely accurate clock set to the correct time when at the Greenwich meridian could be carried on a ship and its reading compared with local time, determined astronomically, to determine the ship's longitude at any place. Since the Earth revolves 360° in 24 hours, or 15° per hour, the time difference in hours multiplied by 15, is the ship's longitude in degrees.
Harrison's chronometer Number Four belatedly won the prize in 1773, long after a successful five-month sea trial in 1761. Carried across the Atlantic from England to Jamaica and back, Harrison's clock was found to be five seconds slow, corresponding to an error of only 1·25 minutes of longitude. The No 4 clock was also carried by Captain Cook on his 2nd Voyage of Discovery starting in 1772.
For more detail see the item below and more detailed item on FINDING LONGITUDE
HARRISON'S No 4 CLOCK
(13 cm Diameter)
(13 cm Diameter)
Sea Horizon (NM) = 2.08 X SqRoot(Ht of Eye in M)
Extreme Range add 2.08 X SqRoot (Ht of Object in M)
(When working in feet substitute 1.15 for 2.08)
Bearing (T) = Bearing (R) + Heading (T)
Bearing (M) = Bearing (R) + Heading (M)
Distance (NM) = (1.85 X Height (M)) / Vertical Angle (Minutes)
(When working in feet substitute 0.565 for 1.85)
Variation East - Compass Least..........Variation West - Compass Best
Deviation East - Compass Least.........Deviation West - Compass Best
For small angles up to about 20 degrees One Unit at a distance of 60 Units subtends 1 Degree
Therefore 2 Units at say 30 Units distance subtends an angle of 4 Degrees
Example : After travelling 15 miles a vessel is found to be 1 mile right of track. What heading changes are required to get back on track in another 10 miles and to then maintain track. - assuming constant current and leeway throughout.
Again from the diagram to regain track in another 10 miles means that 1 mile in 10 miles is 6 miles at 60 miles and therefore 6 degrees.
The following formula relies on the One-in-60 Rule. It is sufficiently accurate only for small angles when measured from abeam - or very close to abeam ie close to either 090R or 270R.
Distance Off (NM) = Time (Mins) X Speed (NM per Min) X 6
For Bearing changes other than 10 degrees the factor of 6 needs to be replaced by (60/ Bearing Change)
Note that every vessel is or could be 'Abeam Barrenjoey'. It depends which way Barrenjoey is deemed to be aligned. However only one vessel holds Barrenjoey abeam.
APPROXIMATE BEARINGS FROM BARRENJOEY
Red - 230 or SW Black - 315 or NW
Green - 020 or NNE Yellow - 045 or NE
Blue - 090 or E White - 135 or SE
Red - 230 or SW Black - 315 or NW
Green - 020 or NNE Yellow - 045 or NE
Blue - 090 or E White - 135 or SE
WHEN REPORTING A POSITION WITH REFERENCE TO A LAND FEATURE USE EITHER TRUE OR MAGNETIC BEARINGS FROM THE FEATURE COMBINED WITH DISTANCE
NB In the formula section above the Bearing to a feature is found by adding the Relative Bearing and the Vessel Heading. If the Vessel Heading is Magnetic then the resultant Bearing is also Magnetic. The bearing from the feature is then 180 degrees added or subtracted from the resultant Bearing to the feature.
Long before the European Age of Discovery, the Polynesian peoples had explored and populated the huge expanse of the Pacific. Indonesians had crossed the Indian Ocean to populate the island of Madagascar. The Vikings crossed the Atlantic to North America. During the Middle Ages Arab traders and huge Chinese expeditions sailed the Indian Ocean but European geography remained bound by ancient theories and scriptural interpretation.
It was the Arabs who developed navigation as a mathematical science measuring the height of the sun at midday to calculate their distance from the equator (latitude). Jewish map-makers in Spain and Portugal combined European and Arab learning in the late middle ages.
Mercator had devised a map on which a straight line represented a line of constant compass course, but the new Mercator's projection was not much used by sailors. Navigators could find their latitude but had no reliable way to find their longitude (distance east or west). Yet, the second Dutch fleet to sail to the Indies, on its return voyage sailed across the Indian Ocean and around the south of Africa without sighting land, then sailed up the South Atlantic to the tiny island of St Helena. They found their way to a small island after sailing 9,000 miles out of sight of land.
They knew that to the south of Africa was an area of relatively shallow sea (about 200 metres). They used a lead weight to sound the depth of the sea and put something sticky on the weight to bring up sand or mud from the bottom. In that area they expected to see a type of seaweed which they called "trompen" and many seabirds. These clues, plus their latitude, told them when they were south of the Cape of Good Hope so that they could change course to sail up the Atlantic. They headed for a point a little to the east of St Helena and when they reached the latitude of the island they turned due west to sail along the latitude until they found the island.
This method, or a variation of it, was used for many years in the early exploration of the earth by seamen navigators. It was not until a reliable timepiece that could withstand the rigours of oceanic travel was built by John Harrison that longitude could be established with reasonable accuracy. See the item about Finding Longitude
