Lectures in Navigation Part 3

1. I desire to sail a true course of NE. My compa.s.s error is 2 points Westerly Variation and 1 point Easterly Deviation. What compa.s.s course shall I sail?

2. I desire to sail a true course of SW x W. My Variation is 11 W, Deviation 2 pts. W and Leeway 1 pt. starboard. What compa.s.s course shall I sail?

3. I desire to sail a true course of 235. My compa.s.s error is 4 pts. E Variation, 27 W Deviation, Leeway 1 pt. port. What compa.s.s course shall I sail?

4. I desire to sail a true course of S 65 W. My compa.s.s error is 10 E Variation, 3 E Deviation, Leeway 1/4 point starboard. What compa.s.s course shall I sail?

FRIDAY LECTURE

THE PROTRACTOR AND s.e.xTANT

The protractor is an instrument used to shape long courses. There are many kinds. The simplest and the one most in use is merely a piece of transparent celluloid with a compa.s.s card printed on it and a string attached to the center of the compa.s.s card. To find your course by protractor, put the protractor down on the chart so that the North and South line on the compa.s.s card of the protractor will be immediately over a meridian of longitude on the chart, or be exactly parallel to one, and will intersect the point from which you intend to depart. Then stretch your string along the course you desire to steam. Where this string cuts the compa.s.s card, will be the direction of your course.

Remember, however, that this will be the _true_ course to sail. In order to convert this true course into your compa.s.s course, allow for Variation and Deviation according to the rules already given you.

In case you know the exact amount of Variation and Deviation at the time you lay down the course--and your course is not far--you can get your compa.s.s course in one operation by setting the North point of your protractor as far East or West of the meridian as the amount of your compa.s.s error is. By then proceeding as before, the course indicated on the compa.s.s card will be the compa.s.s course to sail. This method should not be used where your course in one direction is long or where your course is short but in two or more directions. The reason for this is that in both cases, either your Variation or Deviation may change and throw you off.

Practically all navigation in strange waters in sight of land and in all waters out of sight of land depends upon the determination of angles.

The angle at which a lighthouse is seen from your ship will give you much information that may be absolutely necessary for your safety. The angular alt.i.tude of the sun, star or planet does the same. The very heart of Navigation is based upon dealing with angles of all kinds. The instrument, therefore, that measures these angles is the most important of any used in Navigation and you must become thoroughly familiar with it. It is the s.e.xtant or some member of the s.e.xtant family--such as the quadrant, octant, etc. The s.e.xtant is the one most in use and so will be described first.

Put in your Note-Book:

The s.e.xtant has the following parts: (Instructor points to each.)

1. Mirror 2. Telescope 3. Horizon Gla.s.s 4. Shade Gla.s.ses 5. Back Shade Gla.s.ses 6. Handle 7. Sliding Limb 8. Reading Gla.s.s 9. Tangent Screw 10. Arc

In getting angles of land-marks or buoys, the s.e.xtant is held by the handle No. 6 in a horizontal position. The vernier arrow in the sliding limb is set on zero. Now, suppose you wish to get the angular distance between two lighthouses as seen from the bridge of your ship. (Draw diagram.)

[Ill.u.s.tration]

Look at one lighthouse through the line of sight and true horizon part of the horizon gla.s.s. Now, move the sliding limb along the arc gradually until you see the other lighthouse in the reflected horizon of the horizon gla.s.s. When one lighthouse in the true horizon is directly on top of the other lighthouse in the reflected horizon, clamp the sliding limb. If any additional adjustment must be made, make it with the tangent screw No. 9.

Now look through the reading gla.s.s No. 8. You should see that the arc is divided into degrees and sixths of degrees in the following manner:

11 10 | | -------------+-----------------| | | ----|--+--+--+--+--+--|--+--+--+ | | | | | | | | | | ----+--+--+--+--+--+--+--+--+--+

Now, as every degree is divided into sixty minutes, one-sixth of a degree is 10 minutes. In other words, each of the divisions of a degree on this arc represents 10 minutes.

Now on the vernier in the sliding limb, directly under the arc, is the same kind of a division. But these divisions on the vernier represent minutes and sixths of a minute, or 10 seconds.

To read the angle, the zero point on the vernier is used as a starting point. If it exactly coincides with one of the lines on the scale of the arc, that line gives the measurement of the angle. In the following ill.u.s.tration the angle is 10-1/2 degrees or 10 30':

10 9 ----------------+-----------------------+ | | ---------|---+---+---|---+---+---|---+---+---| | | | | | | | | | | -+---+---+---+---+---+---+---+---+---+---+---+ | | ^ ----+---+---+ 0

If however, you find the zero on the vernier has pa.s.sed a line of the arc, your angle is more than 10 30' as in this:

11 10 ------------+-----------------------+ | | ------------|---+---+---|---+---+---| | | | | | | | ------------+---+---+---+---+---+---+ | | | ^ ------------+---+---+---+ 0

You must then look along the vernier to the left until you find the point where the lines do coincide. Then add the number of minutes and sixths of a minute shown on the vernier between zero and the point where the lines coincide to the number of degrees and minutes shown on the arc at the line which the vernier zero has pa.s.sed, and the sum will be the angle measured by the instrument.

Now in measuring the alt.i.tude of the sun or other celestial body, exactly the same process is gone through except that the s.e.xtant is held vertically instead of horizontally. You look through the telescope toward that part of the sea directly beneath the celestial body to be observed. You then move the sliding limb until the image of the celestial body appears in the horizon gla.s.s, and is made to "kiss" the horizon, i.e., its lowest point just touching the horizon. The sliding limb is then screwed down and the angle read. More about this will be mentioned when we come to Celestial Navigation.

Every s.e.xtant is liable to be in error. To detect this error there are four adjustments to be made. These adjustments do not need to be learned by heart, but I will mention them:

1. The mirror must be perpendicular to the plane of the arc. To prove whether it is or not, set the vernier on about 60, and look slantingly through the mirror. If the true and reflected images of the arc coincide, no adjustment is necessary. If not, the gla.s.s must be straightened by turning the screws at the back.

2. The horizon gla.s.s must be perpendicular to the plane of the arc. Set the vernier on zero and look slantingly through the horizon gla.s.s. If the true and reflected horizons show one unbroken line, no adjustment is necessary. If not, turn the screw at the back until they do.

3. Horizon gla.s.s and mirror must be parallel. Set the vernier on zero.

Hold the instrument vertically and look through the line of sight and horizon gla.s.s. If the true and reflected horizons coincide, no adjustment is necessary. If they do not, adjust the horizon gla.s.s.

4. The line of sight (telescope) must be parallel to the plane of the arc. This adjustment is verified by observing two stars in a certain way and then performing other operations that are described in Bowditch, Art. 247.

Do not try to adjust your s.e.xtant yourself. Have it adjusted by an expert on sh.o.r.e. Then, if there is any error, allow for it. An error after adjustment is called the Index Error.

Put in your Note-Book:

How to find and apply the IE (Index Error):

Set the sliding limb at zero on the arc, hold the instrument perpendicularly and look at the horizon. Move the sliding limb forward or backward slowly until the true horizon and reflected horizon form one unbroken line. Clamp the limb and read the angle. This is the IE. If the vernier zero is to the left of the zero on the arc, the IE is minus and it is to be subtracted from any angle you read, to get the correct angle. If the vernier zero is to the right of the zero on the arc, the IE is plus and is to be added to any angle you read to get the correct angle. Index error is expressed thus: IE + 2' 30" or IE - 2' 30".

Quadrants, octants and quintants work on exactly the same principles as the s.e.xtant, except that the divisions on the arc and the vernier differ in number from the sixth divisions on the arc and vernier of the s.e.xtant.

If any time is left, spend it in marking courses with the protractor and handling the s.e.xtant.

a.s.sign for Night Work the following Arts. in Bowditch: 134-135-136-138-142-144-145-151-152-157-158-159-160-161-162-163.

SAt.u.r.dAY LECTURE

FIXES, ANGLES BY BEARINGS AND s.e.xTANT

There are five good ways of fixing your position (obtaining a "fix," as it is called) providing you are within sight of landmarks which you can identify or in comparatively shoal water.

Put in your Note-Book:

1. Cross bearings of two known objects.

2. Bearing and distance of a known object, the height of which is known.

3. Two bearings of a known object separated by an interval of time, with a run during that interval.

4. s.e.xtant angles between three known objects.