Coastal Navigation

[island]

A lighthouse may serve to aid navigation in several ways. One way is illustrated in the following.

It is 1890 and a small freighter is cautiously following a NNE course along the coast at night. The shore is not visible in the darkness. The master of the freighter observes in the distance off the port bow the distinctive flashing light of a lighthouse. This is the only light he can see. When the light gradually gets closer the master will use this light to determine how far he is from the shore as he continues on this voyage along the coast.

At 10:15 P.M. when the light is now 4 points (45 degrees) off his port bow the master tells the helmsman to hold steady on NNE and to maintain headway at 10 knots. A few minutes later at 10:27 P.M. the master observes that the lighthouse is now abeam, directly beside his position. The master now determines that he is then 2 miles offshore from this lighthouse.

How did he determine this distance?

[ron]

1/5th (12 mins at 10 knots) of the distance travelled would be 2 miles given the angles

[boats]

Not so, what is the tide and drife. How high is the lighthouse over the water. You may be at 10 knots but what is your true speed. In 1890 the sexton was your only tool, plus the compass and compass rose. The other main item in all of this is a watch. Wind is another factor, sea state. Easy to do if you know all the above. "Boats"

[island]

Ron. The freigher in this example did travel two miles in 12 minutes. So how did the master determine he was two miles from the lighthouse when it was on his beam?

Boats. Wind and current would be factored in if they had significant impact on headway and leeway. Using a sextant or similar device to determine vertical angles the distance from the lighthouse could be determined if one knew the height of the light focal plane above sea level, and the height of the tide at the time of the observation if one was close to shore.

[ron]

in an equalateral triangle he would be two miles from the light when abeam

[boats]

Dave, what is this ship carrying anyway. In most cases you can see the shoreline at two miles. But in most cases you want to be no less then five miles from the beach. The east coast in not kind to ships at two miles. You can hit bottom in many places. Just so you know, most ships stay out 20 too 50 miles in the east. And no less then 50 miles on the west coast. You got to ride a big ship a few times to understan way, deep water is "MORE BETTER". "Boats"

[island]

Ron you are correct although what we have here is an isosceles right triangle having two 45 degree angles. More importantly to the mariner is that two sides are of equal length, i.e. the distance traveled equals the distance from the lighthouse.

This example is known as a 4-point bearing (45 degrees) and it is one of several bow and beam bearings mariners often used to determine position from one object on the shore. Though it is not as accurate as using bearings from two or more fixed objects that are identified on a navigation chart, bow and beam bearings were useful and often used in the days before electronic equipment navigation equipment was available.

[island]

Boats.

This trivia scenario was set in the late 1800s and at night when the shore line could not be seen. In those days mariners followed the coast closer to the shore and this is why the primary lights were established along the coasts.

No question deep water is better and safer, but these big ships you refer to do have to approach the coast to come in to port and then it gets a bit touchy to say the least.

[ron]

you know i meant to type isoceles but thought better. its been a long time since high school

[island]

I did not remember for sure either so I had to check it out.

Unlike celestial navigation on the open ocean, the methods of horizontal bearing triangulation, "Pythagorean navigation" if you will, were extremely useful for mariners in coastal waters. It is now has become a lost art with the advent of gps.