Antique of the Month - An Early 20th Century Maritime Relative Bearing Converter
The featured antique this month is another maritime one: an early 20th Century maritime relative bearing converter. This one was a gift from The Marconi International Marine Communication Company to a British sea captain, Captain H. Brown. Both the Marconi Company and Captain Brown’s names are engraved on this nautical device used in ship handling.
First, some quick background on the Marconi Company. It is most famous for providing both the morse code transmitting and receiving equipment, as well as the personnel to operate it, for the RMS Titanic. It was the Marconi telegraph system and operators that put out the distress call via morse code the evening of 14-15 April 1912 as the RMS Titanic was sinking after striking an iceberg in the Atlantic Ocean. One of these Marconi morse code operators died in the sinking, along with about 1,500 people – passengers and crew. This antique maritime relative bearing converter was likely manufactured later than 1912, probably just before or after World War I. The US Navy is documented as having begun to abandon morse code in favor of voice communications in 1916. Nevertheless, the connection with Marconi makes this nautical antique very interesting!
What is a maritime relative bearing converter and what does it do? Imagine you are at sea, inbound to port in bad weather that includes intermittent fog. You can’t see the port entrance so you are looking for the lighthouse that you know is there and on the chart. Soon, to your relief, you spot the lighthouse, but it’s off your starboard quarter (nearly behind you). You spot the light with your spyglass and report it to your officers in the bridge as, “lighthouse 130 degrees relative off the starboard quarter.” Relative bearing is always referenced from the bow or front of the ship to the location of the sighting.
But now you need to turn the ship around and go back. To do that, what course do you sail on, especially when you turn around the lighthouse disappears in the fog? The answer is you use your handy maritime relative bearing converter to determine the true bearing of the light. You then go into the chartroom, plot that bearing out from the light on the chart to see about where you are in the ocean on that bearing line and sail that course back to the lighthouse. Hopefully, when you get closer you’ll see the light again and can turn course to enter port.
The important thing is you must sail a true compass course back to the light. Compass headings in sailing use True headings, not relative ones. In order to determine the lighthouse’s true bearing from your ship, which you then turn the ship using the compass to match it, you would need to take into account the following variables: magnetic variation (caused by the earth’s magnetic core and determined by where you are relative to the magnetic core); magnetic deviation (caused by equipment aboard your own ship that can cause the compass to skew a few degrees; the ship’s compass heading as read off the compass on the bridge; and that relative bearing sight you just took when you saw the light. So, you use this nautical tool to calculate all of that by turning the dials. Once you determine the true bearing of the light from your ship, you order the helmsman to turn around until that true heading you gave him appears on the ship’s compass he’s looking at, right in front of the helm he’s steering.
Anyway, that’s an attempt by me to explain what this nautical instrument is and what is used for. Or perhaps I should say, used to be used for. That’s because today, all of this calculating is done by shipboard computer…the romance is gone! I hope you enjoy seeing this authentic maritime antique!
