An interesting discussion here, some of which has addressed the OP's query.
I would like to make a couple of observations, neither of which relate closely to that query:
First, while discussion of steering
while cruising is interesting and somewhat contentious, I do not believe that many cruisers actually pilot their boats this way, at least while at sea. Rather they establish a distant waypoint, determine the approximate course to steer, and then direct their vessel in that direction. The course may be followed by hand steering
, by autopilot
or by a wind
steering device. Then later they compare their actual position (as determined by GPS
usually) with the course line -- usually displayed as "cross track error". If the boat
has drifted off the desired line, a course change is entered into whatever system is steering. Repeat as required until reaching the destination
. Nowhere in this process is absolute accuracy of the ship's compass
And yes, I do realize that the process fails if the GPS
fails. At this point, one reverts to celestial position finding. Now you discover your position less frequently, but still follow a similar plan, adjusting the course steered to aim at the desired destination
. Again, absolute accuracy is not required.
Now, as to constructing a deviation table -- one technique not mentioned yet is using a simple device (name unknown to me) consisting of a vertical pin which casts its shadow upon a plane marked off with 360 degrees (like a plotting sheet). By placing this in the sun, the shadow shows the relative bearing of the sun. The sun's azimuth can be determined accurately from the almanac, or by calculation, and thus the ships heading can be derived without reference to charted landmarks or even knowing the ships position. Quite some years ago Latitude 38 published (in the Max Ebb column) a short program written in Basic for determining the azimuth of the sun. Made the process pretty easy.
Jim and Ann s/v Insatiable II lying Towlers Bay, NSW, Oz