Talk:Pelorus (instrument)

Explanation for the unitiated please

This article badly needs an intro describing, in layman's terms, what it is, what it is used for and by whom. As it stands now it is meaningless to the layman. Rcbutcher (talk) 07:04, 13 August 2013 (UTC)

Six years later, and still nothing done… :-( How does the instrument work? The photo shows more the stand on which it rests than the instrument itself. What is the physical explanation behind the instrument? What makes it “keep” the direction? CielProfond (talk) 20:15, 20 October 2019 (UTC)

Second the motion

Not happy with the write-up - scanty and confusing, not much above a stub. I have some material I placed under clock position but it was pointed out it does not really belong there. It would belong here primarily so I'm moving my write-up to here, but it needs to be worked into an integrated article. I will do it gradually but for now I'm putting it in the talk here until I can get to it.

Pelorus

Main article: Pelorus (instrument)

Student being shown how to sight through the vanes of a pelorus.

The pelorus is a superposition of dials and sighting vanes or telescope on a stand bolted to the deck in the wheel house or bridge of a vessel next to the helm. The purpose of bolting is to provide an immoveable base for a fixed fore-and-aft engraved line, the lubber line, marking the course of the longitudinal axis of the vessel. In an open container at the top of the stand is mounted a 360-point clock face with the degrees marked and identified with a 3-digit number. The face is swivel-mounted to keep it horizontal as the vessel pitches and rolls. The face can be rotated clockwise or counter-clockwise. Over it is mounted a rotating arm with pointer to mark a particular point. On the arm are mounted two sighting vanes or a telescope with cross-hairs.

To use this pelorus to take a relative bearing, the observer positions 000 with the lubber-line notch, clamps it, sights the external object, and reads the number to which the pointer points. This number alone is not of much value unless the observer can tie it to a true bearing. There is one such: the compass course of the vessel, which, if the helmsman is careful to maintain it, is the compass reading of point 000. Usually the helmsman and observer coordinate their reading, tossing the word "mark" back and forth. As the compass reading is in compass points, but the clock position is in circular degrees, it is necessary to convert the circular points to compass points. To assist in this effort, a compass also is printed on the clockface, with north at 000. One may obtain the compass bearing of the observed object by adding the relative bearing to the vessel's course, or heading.

Still more information can be gleaned from the relative bearing. If the navigator takes two relative bearings, one at each end point of a known distance, he may construct a triangle of two sides along the lines of sight and the third the known distance. The base angles are calculated from the raw pelorus readings. From the known angle-side-angle information the navigator uses trigonometry (or tables) to calculate the length of the last line of sight, which is the separation distance of the object.Botteville (talk)