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May/June 2014 Issue 218: Wreck of the brig Jezebel

Apr 24, 2014
An example of a 19th-century brig in the merchant trade.

An example of a 19th-century brig in the merchant trade.

The Story of Sail

Editor’s note: This fictionalized account is an amalgam of several true incidents, most notably the wreck of the brig Commerce off west Africa in the 19th century.

The two-masted brig Jezebel, built on the Clyde in Scotland in 1855 was 80 feet on deck with a draft of 11 feet. The brig was commanded by R. Kenneth Hamilton who had developed a reputation for reliability and honesty in the trade from Cape Town to European ports. Jezebel was designed to carry cargo up and down the coast of west Africa from Cape Town to Monrovia and then to Gibraltar. Capt. Hamilton had previously been involved in the trade of dried fish stock in Bergen and was in command of Jezebel only a few short months.

Jezebel was in the ivory and whale oil trade from Cape Town up the west coast of Africa. It was a well known but tricky passage. The prevailing winds in that part of the world are from the southeast, giving northbound sailing ships a starboard tack to keep them off the land. The hot winds mixed with the cold Benguela current and produced deep fog that made celestial navigation difficult. To make matters worse, numerous sandstorms blowing off the coast created visibility problems. Most, if not all mariners at that time, used the most basic form of celestial navigation, which was the observation of the sun at local apparent noon (LAN) in order to find latitude. They would then combine the noon sight with the DR and calculate an estimated position and proceed accordingly. The captains of the day rarely calculated to find longitude. It was both too complicated and the accuracy of their chronometer — if they carried one — was suspect.

The ship loaded with ivory and departed, bound for Monrovia in Liberia at a position of 6° 18’ N by 10° 48’ W. The route was a new one to the captain and he underestimated the drift of the current which was actually moving him faster through the water than he had anticipated.

We are in the month of April and we are using the 2014 Nautical Almanac. The captain believes the ship is at a DR of 2° 5’ N by 2° 40’ W. The height of eye is 15 feet. Because the captain was not working with an accurate timepiece, he goes on deck a quarter hour before taking his sight and gets a lower limb shot of the sun at around 1215 GMT with an Hs of 84° 22’. Using this number, calculate the latitude. Use the declination for 1200 GMT. Plot out the latitude and see why Capt. Hamilton was about to get into serious trouble.
 
We need to remember that the noon sight was a sight that was never predicated on exact time — that was its beauty. The navigator would guesstimate the time of LAN and go up on deck about 15 minutes before the expected event. You didn’t need an accurate timepiece and the sight was relatively simple to calculate. It also has its downside. You could be up on deck for 20 minutes and if the sun was hidden by clouds, you were basically using dead reckoning and hoping for the best.

A: What is Ho?
B: What is latitude?

See answers on the next page.

 

Answers:
A: Ho is 84° 34’
B: 4° 24.4’ N
 

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May 18, 2014 10:04 pm
 Posted by  Jack

I agree with the Ho. Can someone explain how we get to that latitude? It. Doesn't seem to fit the equations I know.
Jack

May 19, 2014 02:35 pm
 Posted by  Ocean Navigator

Thanks for your comment, Jack.

In this case the latitude equals the Declination minus the Co-Altitude.

Co-Altitude is 90° - Ho = 05° 26.0'

Declination is N 09° 50.4'

So, 09° 50.4' - 05° 26.0' = N 04° 24.4'

Hope that helps.

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