Even navigational instruments can make enjoyable collectibles. Today, most skippers simply dial in a waypoint and set their course, but, in the days of paper charts, a parallel rule was essential for striking a course on the chart. The most common parallel is the "walking" parallel, with two rulers linked by movable bars. More interesting as a collectible is the rolling ruler, which had a pair of knurled rollers to move it accurately across a chart. Usually made of brass and with the maker's mark inscribed, these polish up to a high shine for display. An all-brass rolling parallel from World War II with the British Navy emblem, housed in a wooden box, is currently on the market for $350 (Antiques of the Sea; www.antiquesofthesea.com).
Like the parallels, a set of dividers was in every captain's navigation case. These were often from a base metal because they were made for engineers and draftsmen ashore, and then taken to sea. But, unlike the plain dividers found in marine stores today, early dividers were often adorned with inscribed filigree or ornate locking screws. Expect to pay $150 and up, depending on the age, which can date back as far as the 1600s (but those are a lot more).