Dutchman's Log

When the knot log fouls and the GPS goes on the blink then there is always the Dutchman's Log for estimating the speed through the water. Just count the number of seconds a bit of floating material takes to travel from bow to stern to get knots.

Use the formula:
Boat Length in nautical miles / drift seconds from bow to stern = Knots / 3600 seconds in an hour

Example for 33 ft. boat given 6076 ft. in a nautical mile and 3600 seconds in an hour:

Formula: .00543 NM boat length / drift seconds = knots / 3600 sec.

Knots = .00543 x 3600 / drift sec.

Knots = 19.6 / drift sec.


Table of Speed Though the Water for 33ft. Boat

9s = 2.2 kts.
8s = 2.5 kts.
7s = 2.8 kts.
6s = 3.3 kts.
5s = 3.9 kts.
4s = 4.9 kts.
3s = 6.5 kts.

http://en.wikipedia.org/wiki/Chip_log

Sextant Scope Filter

Index error (I.E.) can be determined more precisely during the day by using a custom fitted solar mylar film filter (the type used to observe a solar eclipse) over the objective lens on the sextant scope. The result will be a pair of sharp uniform orange Sun images. Overlap the Sun images for direct index error readings or take half of the difference between the two limb to limb readings.

note:
-When off the arc the reading is 60' minus the micrometer value.

-Shading the sextant scope directly allows the horizon mirror shades to be pulled aside which opens up better access for the adjustment tool.

Link: http://www.thousandoaksoptical.com
Thousand Oaks Optical (928)692-8903

Ex-Meridian by Formula

The Ex-Meridian can be done by formula for improved precision over Bowditch vol.2 tables #29 and #30. (current edition table #24)

Formula :

(a) = 1.9635 x COS Latitude x COS Declination / SIN (Lat. ± Dec.*)
*RULES (+) if Lat. & Dec. are contrary in name. (-) if Lat. & Dec. are the same name.


Ex-Meridian Correction in minutes of arc to be added to Sun (Ho) near LAN (upper transit) :

Ex.Correction' = (a)( Time from LAN in minutes and tenths)^2 / 60

or

Ex.Correction' = (.267)(a)( Meridian Angle in degrees and tenths)^2

Example:

Lat.34°N Dec.20°S (Sun observed 20 minutes 30 seconds before LAN)

(a) = 1.9635(Cos 34)(Cos 20)/Sin (54) = 1.89
Ex.Corr.' = (1.89)(20.5)(20.5)/60 = 13.24' added to (Ho)

or

Ex.Corr.' = (.267)(1.89)(5.125)(5.125) = 13.24' added to (Ho) *

The corrected (Ho) is then reduced as a normal LAN to get the Latitude at the time of observation.


*Try a formula variant that uses meridian angle vs. time
.267 (a)(Meridian Angle)^2 = Ex.Correction' file:///Users/greg/Desktop/factors.html

Symmedian Point

Shift a celestial navigation LOP triangular plot center slightly toward the short side (sides) for an improved probable position (symmedian point)

http://en.wikipedia.org/wiki/Symmedian

Horizontal Sextant Angle Plot Made Easy

Make short work of plotting horizontal sextant angles by determining the arc radius using a chart plotting board, radar plotting sheet, or the simple formula (0.5 mark A to mark B)/ sin(horizontal sextant angle) = radius. Once the radius is known then a chart plot is performed by arcing the radius off navigation marks A & B then plot the arc of position from the radius point intersection to determine an estimated position. The chart plotting board image above shows a radius solution for a horizontal sextant angle of 45° and a distance between marks A and B of 16.6 nautical miles.

Unique Short Table Sight Reduction

Combine Pub. 208 Dreisonstok with a modified 2102-D star finder for quick determining of the Sun's azimuth and intercept. Use the star finder to first derive azimuth which is then preplotted through the assumed position. Follow up with a calculated altitude using Pub. 208 Dreisonstok to get the intercept.

Set the 2102-D Star Finder without a Nautical Almanac

Set the 2102-D star finder without using a Nautical Almanac by scaling a radar plotting sheet as depicted by the above image.

1. Set Aries zero to month and day (clockwise)
2. Set blue latitude template to zone time of twilight (counter clockwise)
3. Select stars for twilight viewing.

The image example is for 12 October 2010 zone time civil twilight 1800 at 35°N 120°W

Try penciling in the Sun onto the 2102-D base disc using the slotted red template.

1. Set Aries to month and day.
2. Set red template to time of meridional passage.
3. Mark Sun's declination through the slot onto the base disc.

The twilight sky can now be set by rotating the blue latitude template so the Sun is 5° to 10° below zero altitude. For the southern hemisphere reverse the zone time direction to clockwise.