Square Pocket Protractor

A grid map tool that is modified with 1/16" drilled holes makes for the perfect pocket chart plotting protractor. Drill the holes by hand using a 1/16" bit spun with the index finger and thumb. Make a pilot hole at the spot to be drilled using the point of a divider leg.

Longhand Sight Reduction

An obscure sight reduction formula derived by a Frenchman named Doniol is the only reduction method that can be quickly done longhand without logarithms. Needed are four place cosine, sine, and haversine tables which can be found in the 1958, 1962, 1966, 1975 vol 2, and 1981 vol 2 Bowditch editions.


http://fer3.com/arc/m2.aspx/Haversine-how-derive-it-Bernecky-jun-2015-g31663


Doniol Formula

Sin h = n - ( n+m )a

Same name:
n = Cos ( Lat-Dec )
m = Cos ( Lat+Dec )
a = Haversine LHA

Contrary name:
n = Cos ( Lat+Dec )
m = Cos ( Lat-Dec )
a = Haversine LHA

rule:
Lat + Dec > 90 use (-)Cos of Complement

When haversine tables are not available then substitute
a = ( 1-Cos LHA )/2

Ditty Box

Every boat should have a go to ditty box for storing miscellaneous extra items collected over the years. Rubbermaid tubs work well and are in various sizes to fit available onboard space.