A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Brian Walton
Date: 2021 Mar 6, 03:26 -0800
The Gypsy Moth, Moth Major, and Tiger Moth have identical straight parallel upper and lower edge longerons to their rear cockpits. The first 2 have lower wing rear edges extending outwards at 90°, from a position vertically below the rear pilot's head. The rear pilot cannot see the sea forwards, due to the lower wing.
If the wind at sea level exceeds about F4, whitecaps will form. Looking straight down over the upwind cockpit edge, the pilot can see a whitecap emerge from the point where fuselage and wingroot touch, and travel backwards. When the whitecap has travelled by eye 2'6"(30") backwards, the pilot assesses how far out from the parallel fuselage edge it has moved. If the whitecap has moved out say, 5", the drift angle is 5" over 30", = 10 over 60 = 10°.
The drift assessment can be checked by turning back the assessed angle, when drift should be zero.
The process is repeated on headings at 30° either side of the original heading. Plotting the 3 drift angles, as descibed, gives wind velocity. The wind strength can be checked by sea state, F4/5/6 being visibly different.
Chichester carried a hard backed roller map he had made himself.