Black Diamond Sound Systems

Operation of Ocean Navigator

Ocean Navigator is a very handy program for navigating at sea. Use it to compute way-points, great circle routes, distances, positions, and other useful parameters, as the folks at Black Diamond Sound Systems did aboard the 40 foot yacht Marishanna in th e recent Pacific Cup Race from San Francisco to Hawaii.

The Navigate Program was written to simplify the process of calculating a number of quantities that are of special interest to the typical mariner when he or she is at sea. These include such questions as "Where am I?", "Which way should I steer to get to where I want to go?", "How far away is my destination?", "How can I generate Way Points with a minimum of bother?", "How far away is that rock?", and "Will that approaching vessel collide with me?" The attempt has been made to enable one to determine answers to these questions by entering the fewest number of parameters possible. Each function provided is explained as succinctly as possible. Most of the functions were derived by the author while he was sailing in the Pacific Ocean (from Japan to California) or the Caribbean. It has been found in practice that this program provides a very useful adjunct to one's navigational prowess, especially if the GPS goes dead, the GPS manual is misplaced or indecipherable, or when you forgot to bring all the tables you should have brought.

Distance Calculations:
1) Length of a Degree of Latitude and Longitude:

Because the Earth is spheroidal, as you approach the North or South Pole, the distance between lines of Longitude converges. A typical question is "What is the length of a degree?" for any arbitrary position, so that you can easily estimate distances on a typical Mercator chart. The program calculates this for you: simply enter the Degree of Latitude, click "Compute", and the program calculates the corresponding lengths in Feet, Nautical miles, and Statute miles (results are shown in red). Click "Clear" to reset the values.

2) Distance to an Object by Two Sequential Bearings:

Suppose you are cruising up a coast and you observe waves splashing off a rock at some unknown distance off to starboard. Lacking radar, you naturally wonder how far away the rock is, and are you going to clear it? You can easily determine this by taking two sequential bearings of the rock, and noting the distance traveled between bearings (distance traveled = (your velocity / time between bearings)). Simply enter Bearing 1 and Bearing 2, and Distance, and click "Compute" and the program will calculate your distance to the object at the time you measured Bearing 2, as well as the distance that the object will be from you when it is abeam. Click "Clear" to reset the values.

3) Distance by Vertical Angle to an Object beyond the Horizon (eg, an Island):

Suppose you have been at sea for 10 days en route from San Francisco to Hawaii and you are convinced that you should soon be sighting Mona Kai. First you see a cluster of clouds, which gradually appears to be the tip of a mountain. How far away is it? Th e program will calculate this for you. It needs to know the Vertical Angle subtended by the visible portion of the Object above the sea, your height above the sea, and the height of the Object. Enter these values and click "Compute" to calculate the result, or press "Clear" to reset the entries. As an example, suppose you are 6 feet above the sea, you sight Mona Kai (height = 10,000 feet above sea level), and it subtends 3 degrees of arc: the island is 29.391 nautical miles away.

4) Will Another Vessel Pass Ahead, Astern, or Collide?

A typical situation occurs at night when the lights of a cargo ship rise up off the horizon and make themselves clearly distinguishable from the stars. You note, with growing alarm, that the lights of the cargo ship are getting bigger and bigger, and wonder whether it will hit you if neither you or it changes course. Your dinky radar deflector seems to make no effect on the relentless track of the approaching vessel. Persistent efforts to hail it on your VHS remain unacknowledged. You naturally want to know if you are going to collide with the vessel, and if so, when, in order to plan whether to take evasive action. The program will answer these questions rapidly: simply enter a Bearing to the vessel at Time 1 and Time 2, and the Angle subtended by the vessel at Time 1 and Time 2, and click "Compute". Click "Clear" to reset the entries. Note: a handy rule of thumb, so to speak, is that the width of your thumb held at arm's length subtends approximately 1.19 degrees of arc. (Note: these calculations are based on the assumption that the Observer's eye is 6 feet above sea level, and that the Object is 50 feet above sea level.)

Great Circle Route
A common problem in navigation is to compute the shortest route between two points. Typically one is tracking one's course on a Mercator projection chart, which shows latitude (degrees North or South of the Equator) and longitude (degrees East or West of Greenwich, England) lines at right angles. In reality, since the Earth is a sphere, the longitude lines curve so as to converge at the poles, and the shortest distance between two points is not a straight line, but a curved line, called a "great circle" . This function computes the great circle route to follow from an Initial Latitude and Longitude position to a Final Latitude and Longitude position anywhere on the surface of the Earth. (Note: it assumes that you are sailing the shortest distance between the two positions, not the "long" way around the globe.) The results are shown in red: the total distance along the great circle route is shown in both nautical miles and statute miles, and the program displays the Initial heading to steer in the upper panel.

Position Calculations:
This function enables you to calculate your latitude and longitude from a single observation of the Sun's position at your local noon. Operation:

Simply enter the observed angle between the Horizon and the Sun at Local noon (Deg, Min, Sec [set to 0 by default]), Local Noon, GMT, and the date, and click "Compute". To reset the entries, click "Clear".

Time Calculations: Convert Local Time to GMT
Many navigational observations and calculations are made with reference to GMT (Greenwich Mean Time, also called Zulu time). This function provides a useful means of making the conversion from local time to GMT. This is especially useful if you are making sextant observations and marking the observation times with a wrist watch: you can simply write down the observed times, then determine the "watch correction" from an accurate chronometer. Simply enter the watch correction once and the time zone (which depends on your longitude), and then enter each observed local time and press Compute in order to calculate GMT.