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How Lighthouses Aid Navigation

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Postby island » Sun Apr 14, 2013 7:01 pm


How Lighthouses Aid Navigation

Navigation is the action of directing a ship on its journey while at all times maintaining situational awareness a significant part of which is knowing the ships position at all times. Navigation utilizing landmarks on the shore is known as Piloting. Ships engaged in trade along the coasts were Coasters, the navigation known as Coasting of Coast Piloting. For this there are two forms of navigation.

One form of piloting is following along the coastline, using a navigation chart as a guide while observing aids to navigation and other natural and man-made features on the land and thus to know ones position during the voyage. The navigation chart is a detailed topographic map of the coast showing features both on land and below water. This is basically traveling from lighthouse to lighthouse and following the contour of the coast line and the downside of being close to “foul” waters with submerged hazards. This was the method used when navigating the confines of harbors, narrow waterways and rivers.

The second form of navigation utilizes the navigation chart, a clock, the speed indicator and compass. From this the position at any time is deduced from time and speed, thus the position “reckoned” by deduction, ded or dead reckoning, otherwise abbreviated “dr”

This 'dr' form of piloting is utilized by ships proceeding along the coast at a distance from the shore roughly parallel to the shore of the ocean and on The Lakes and following the general curvature of the coast within sight of the shore. This with long straight runs in open waters was shorter distance between port of departure and port of destination and would require far fewer changes in compass heading. This compares on land to driving on a thruway.

“DR” navigation is accomplished by first planning the trip on the navigation chart, marking on it a line representing the first leg of the journey (this line the mariner’s road on the road-less navigation chart). At the known point of the start of the journey the compass direction is determined and time of departure recorded. When in route the position of the ship is tracked from the speed and time of travel from the starting point and until the end point of this first leg is reached as determined from the estimated time of arrival at the speed traveled. Then assume the compass heading for the next leg.

The “dr” alone would work well if not for other factors influencing the ship in transit, these being wind and waves and at some locations ocean currents, all of which influence the ship's real position with respect to the coast. Wind from the left side pushes the ship to the right thus the ship would be to the right of the course line (“dr” line on the navigation chart) This side motion is known as “leeway” and combined with headway determines the real location of the ship. Leeway is difficult to accurately measure. A second factor is it is difficult to hold directly on the desired compass heading, more so when subjected to waves acting on the boat. If one degree to the right then the ship would be about one half mile off course to the right in thirty miles of travel. Here is where the lighthouses come in as a most invaluable aids to navigation.

Using a hand-held compass the compass bearings from ship to lighthouses on shore are determined and from these bearings applied to the navigation chart the navigator can then pinpoint the exact location of the ship on this chart. This position thus obtained is known as a Fix. Comparing the Fix to the course line on the chart reveals how far from from the “dr” course the ship is and a compass heading correction may be made if too far from this “dr” course line. Such a Fix identifies the distance traveled and the distance the ship is from the shore thus enables correction for any leeway or compass error that may exist. By determining a Fix at numerous locations and correcting the “dr” during the journey the ship compass headings be appropriately corrected. The Fix may be considered as new known starting point. Thus the bearings of lighthouses and other objects observed on land that also appear on the navigation chart enable the mariner to closely follow his desired course of travel and to accurately know his position to maintain situational awareness. For not knowing ones position one is basically lost but may not realize it until later, and if so lost preferably not learning this from direct intimate contact with land.

To most accurately navigate by 'dr” recording times of observsations and changes in speed and heading and accurate measurement of speed are required. Lighthouses offer the opportunity to verify speed of a ship. This from the distance between two lighthouses divided by the time between passing of each, this the speed of the ship relative to the land.

Further, by looking ahead on the chart the mariner can then anticipate from his speed and distance to those landmarks and aids to navigation yet to be visible and thus know in advance the identity of and the estimated time when a lighthouse will be seen. Most important to do this at night when the only reliable lights seen are the aids to navigation lights, and to not mistake one light from another or a civilian light and thus be lost.

To aid off shore navigation prominent Primary and Secondary Seacoast Lights were established at intervals along the entire lengths of the coasts although where they exist inshore harbor lights may also serve to aid when navigating by dead reckoning. These seacoast lights were most valuable in years past on the sections of coast where few if any other landmarks were available such as the length of Cape Cod, Long Island, sections of the Virginia Coast, the Outer Banks, the east coast of southern Florida and likewise in sections along the west coast. And running by 'dr” on The Lakes was often most appropriate in the transport of iron ore or grain.

With today’s aid of electronic equipment and computerized gps coastal navigation is quite simple. However, it may be of great value for a mariner to know how to navigate by dead reckoning in event of a failure of the electronics. To qualify as a coxswain (the small boat operator) in the Coast Guard it is necessary to demonstrate ability to navigate by dead reckoning.

The image below illustrates how a fix or a single line of position from compass bearings are applied to a navigation chart and compare to the 'dr' course of travel.
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Postby MontaukPoint » Sun Apr 14, 2013 7:02 pm


As usual, another good post David!
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Postby island » Sun Apr 14, 2013 8:42 pm


Thank you. I did put a fair amount of time and effort in planning and then creating this post. In the past I have participated in both classroom and on the water training in DR navigation. It is much easier to describe this stuff using a blackboard or when out in a boat. Show and tell without the show part is a challenge. And hoping this might add to the appreciation of lighthouses.
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