NavList:

Howard you asked ‘Why do it’

Ships can be out of sight of land for weeks at a time; they have more iron in them than aeroplanes; they can travel over large differences in latitude; and until recently their charts were often older, so their magnetic variation was less certain, if known at all.  The values of the horizontal and vertical components of the Earth’s magnetic field vary with latitude (and longitude), so the deviating effect of the vessels ‘soft iron’ changes.  My father always contended that ‘hard iron’ could be beaten out in rough weather.  He claimed that was why ships were turned around after launch so that magnetism driven in by riveters hammering could be beaten out again by those hammering fitting out after launch, but he might have been pulling my leg.  It can certainly be affected by passing through magnetic storms.  All good reason to be sure of what your magnetic compass is telling you when what really matters is your true course. DaveP

William Bligh is believed to have had amplitude tables with him in Bounty’s Launch, but he didn’t have much of a watch (the Gunners?) and he didn’t have charts so would have had to have had a good memory to remember variation as well as his geography.  I’ve a feeling Lecky had something to say about sunrise heading checks.  If I can find it, I’ll post it. DaveP