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Re: Automatic deviation calculation by electronic compasses
From: George Huxtable
Date: 2009 Dec 10, 15:29 -0000
From: George Huxtable
Date: 2009 Dec 10, 15:29 -0000
Lu Abel asked a fair question, which I will do my best to answer- "Why do you assume that an area of low deviation is required as a location on a steel vessel for a mounting place for a fluxgate compass? It seems to me that ANY spot, regardless of how esoteric its deviation table or Napier diagram, would be a suitable mounting place so long as the deviation (table) does not change with vessel heading or activation of machinery. " But first, let's dispose of this one- "(before you object, I'll admit that I'm assuming that a proper constant-angular-velocity turn can be executed to compensate the compass) ". I ask Lu to examine my posting [10799] of 23 November. There, I pointed out (after belatedly realising it) that there was no need at all for constant angular velocity. As long as the vessel passed through every point of the compass, and didn't turn too fast, all the necessary information was there to deduce deviation, at ANY inconstant speed-of-turn. Now, back to Lu's question. First, lets deal with the deviation of the compass as it affects most small-boat-owners: those with fibreglass or wooden hulls, in which the deviation is the result of odd bits of iron and steel in its vicinity; particularly an engine. In that case, deviations are small, both the sine(heading) or "hard-iron" component, and the sine(2 x heading) or "soft iron" component. It can be handled by the algorithm we have discussed, simply making a 360-degree turn. That would also apply to the rare stainless-steel hull, and perhaps even to a construction of "very, very, special steel", if it was a Nickel alloy of low magnetic permeability. But an ordinary ferrous hull is a different matter. Its magnetic effects can be enormous, and there's nowhere within the hull they can be escaped. The history is informative here. In the 19th century, it was a powerful and valid argument against iron and steel construction; that it wasn't possible to correct a compass properly. Mariners knew how to make a deviation table, but that wasn't sufficient. Deviation varied, not just with heading, but with (magnetic) latitude, with heel-angle, and with cargo, and with time, in ways which were not understood. There were countless accidents as a result of compass errors. The work of Airy and (particularly) Kelvin helped to sort it out. It resulted in the familiar polished binnacle, placed as high as possible above the hull, in solitary splendour on the bridge so there could be no undetected interfering objects. That carried permanent magnets to correct for the ships "hard-iron" component, soft-iron hollow balls placed thwartwise to correct for the "soft-iron" component of induced lengthwise magnetism, and the Flinders vertical iron bar to correct for vertical induced magnetism. Even after all that, there was the need to create a deviation curve, and a prosperous trade of compass-adjuster to bring it about. Kelvin (then William Thomson) wrote a very readable account of this in "Terrestrial magnetism and the mariner's compass", collected into vol 3 of his Popular lectures and addresses" of 1891. This is the volume on "Nivigational Affairs", which I thoroughly recommend. Kelvin owned a fully-crewed 130-ton schooner, and did a lot of sailing aboard her. ===================== Let's try a thought-experiment to demonstrate the sort of problem that can arise, as simply as possible. We will be at the (magnetic) equator, so there's no dip, and we can choose a spot with no magnetic variation, so the field points North. Represent our ship by a length of board, with a compass mounted a foot above it. As we turn our ship, the card will ride over the lubber-line correspondingly, and indicate true heading with no errors. But next, attach a little bar-magnet to the board, directly below the compass, pointing lengthwise, such that when the ship points North its field enhances that of the Earth. That magnet represents the "hard-iron" component of a ship's hull; we are ignoring the "soft-iron" component here, for simplicity. It's clear that it causes no compass-error on a Northerly heading, nor (as long as it's effect is weaker than the Earth's) on a Southerly heading, and the maximum error will be on Westerly or Easterly courses. The error will vary, more or less sinusoidally, as long as the field from the magnet is small compared with that of the Earth. However, in a steel ship, that can no longer be assumed. If the magnet happened to produce a stronger field than that of the Earth, it would override that of the Earth when the heading was South, so the compass card would turn with the ship and still indicate a North heading. It would then be impossible to create a deviation table. All you could say, if the compass read North, is that the ship was pointing due North or due South! That may seem an extreme case, but the effect of a steel hull can be so great that it is by no means far-fetched. The binnacle components compensate so as to reduce the hull effects to manageable proportions; single-valued at least, that can then be handled by a deviation card. George. contact George Huxtable, at george@hux.me.uk or at +44 1865 820222 (from UK, 01865 820222) or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK. -- NavList message boards: www.fer3.com/arc Or post by email to: NavList@fer3.com To , email NavList+@fer3.com