Proportional Logarithms. Used when 'Clearing a Lunar Distance'. Was the logarithm of 10,800 (the number of seconds in 3 hours) diminished by the number of seconds by which a quantity dealt with was less than 3 hours. Proportional log. of 2 hours would be log. of 10,800 minus log. of 3600. Now quite obsolete for this purpose.
Propulsion. The driving forward of a vessel.
Protection and Indemnity Association. Association of shipowners who combine to indemnify any of their members against third party claims and against risks not normally covered by marine insurance.