|Sketch between Decks, May 75, Gabriel Bray, 1775, National Maritime Museum.|
The first thing we have to recognize in examining literacy among common sailors of the eighteenth century is that literacy is a spectrum. We should be careful not to conflate being literate at the time as translating to both reading and writing. As Tamara Plakins Thornton wrote in her excellent Handwriting in America: A Cultural History:
Because reading and writing were understood to serve entirely different ends, instruction in one was divorced from instruction in the other. Reading was taught first, as a universal spiritual necessity; writing was taught second, and then only to some.Thornton was writing about Americans, with a focus on New England, but the point still holds that the ability to read or write was not mutually inclusive or exclusive.
To teach reading as religious education was seen as a basic need. Olaudah Equiano, enslaved aboard a naval vessel, made friends with a captain's servant named Daniel Queen who 'taught me to shave and dress hair a little, and also to read in the Bible.' There does not appear to have been any objection to enslaved Africans like Equiano learning to read, though he does state that some objected to his learning navigation as such a skill could be directly applied to escape. This implies that reading was not seen as having the same value toward freedom as other skills. More to the point, other sailors on Equiano's ship were competent enough in reading that they could teach others.
As N.A.M. Rodger stated in his The Wooden World: An Anatomy of the Georgian Navy:
How many men would or could read is difficult to say. Many were undoubtedly illiterate, probably more than the average of their class, but there are chance references to men off watch reading in their hammocks, and very likely it was common.Historians agree that some degree of literacy was present in a majority of sailors. There is no foolproof system for examining this, but the most commonly used framework for getting a ballpark estimate of literacy is through examining documents that sailors had to sign. If a sailor made his mark, rather than a signature, it is taken as a possible sign of illiteracy. N.A.M. Rodger does this in The Wooden World. Marcus Rediker, in his Between the Devil and the Deep Blue Sea: Merchant Seamen, Pirates and the Anglo-American Maritime World, 1700-1750, argues for a high literacy rate among sailors:
As many as three-quarters of the sailors employed in the merchant shipping industry between 1700 and 1750 were literate if judged by the standard of the ability to sign one's own name...All captains, mates, and surgeons were literate, but, at best, only two of three common seamen could even sign their name.There are many documents with signatures that one can use to measure potential literacy. For example, I've been examining the wills left behind by sailors killed on the Braddock Expedition, or those who died shortly thereafter. Midshipman Thomas Gill, and able seamen Nathaniel Gee and Alexander Frazier all signed their name by their own hands, but carpenter Jonathan Hill and boatswain's mate John Cain only left a mark. Rodger and Rediker both work with far larger sample sizes than this, but merely for the sake of example this would indicate that 60% of the detachment was literate.
Rodger acknowledged this rough system in measuring literacy is of limited use:
How many of those who could sign their names were functionally literate for everyday purposes one cannot say, but a proportion must certainly have been.Rediker similarly cautions:
There is reason to suspect that the actual proportion of the literate may have been considerably smaller, because not all who could sign their names could read and write.Ira Dye, in his paper 'Early American Merchant Seamen' in The Proceedings of the American Philosophical Society, looked at later generations of sailors, but used the same method for examining literacy. Dye worked under the assumption that being able to sign one's name was 'midway between the capacity to read and the capacity to write.'
The assumption that signing one's name was an indication of a broader literacy was one that existed at the time. Jacob Nagle was called by the captain on a naval vessel to answer for a cup he had written on:
I trimbled, though I new I got the cup onestly. I went aft, where the capt was sitting by a small table, and the purser giving him the cup, "Is this your cup?" "Yes, Sr." "Is this your righting?" "Yes, Sire." He calld his steward and desired him to bring up pen, ink, and paper. When it was brought, he desired me to right. I asked hime what I should right. He made the answer, "What was on the cup." I rought, "Jacob Nagle, Born in the Town of Reading, Berks County, Pennsylvania State, N. Amrice." With my fright and trimbeling I did not right it half so well, but he saw it was the same hand. "You must act as ships steward under the purser," the capt replyed. I made answer that I did not now my own allowance. He replied, "What is the pusser for, but to give you instructions and books."Given that Nagle wrote more than just his name, we might even assume that the captain made a broader judgment than he would for sailors who could only sign their names. This assumption was not misplaced, despite Nagle's creative phonetic spelling.
Mary Lacy, who disguised herself as a man and went to sea as a carpenter's apprentice, felt guilty for leaving her family. She related this telling anecdote of trying to seal a letter to her parents:
I observed the people now on board were employing themselves in writings letters to their friends; which put a thought into my head to write to my mother, to inform her where I was; which I knew would be a great satisfaction to my parents; I could write but very indifferently; and to entrust any person with my thoughts on this occasion I imagined would be very improper. At last; I resolved to write myself; but, after having wrote my letter, I had nothing to seal it with; and thinking a bit of pitch would do, I went to the pitch-tub for some, which, when I thought I had got, it proved to be tar; so that with using it I soiled the letter very much. I was now greatly perplexed to contrive a method to seal it up. At length, one of the men, who observed I had been writing, gave me a wafer, which did completely.Not only were multiple common sailors ('the people') engaged in writing, they possessed material culture specific to the act of writing letters: sealing wafers.
Focusing on those sailors who could read and write should not distract us from the fact that illiteracy was undeniably present. Both John Nicol and Hannah Snell, sailors whose recollections were compiled and sold as memoirs, were incapable of writing and relied on others to pen their stories.
In short, I don't have a good answer as to how present the ability to read or write was among common sailors of the eighteenth century. Literacy rates are quite probably much higher than is commonly assumed, while illiteracy was still a common condition.
 Rodger, N.A.M., The Wooden World: An Anatomy of the Georgian Navy, New York: W.W. Norton, 1996, page 45.
 Rediker, Marcus, Between the Devil and the Deep Blue Sea: Merchant Seamen, Pirates and the Anglo-American Maritime World, 1700-1750, New York: Cambridge University Press, 1989, page 158.
 PROB 11/826/161, 11/821/148, 11/821/151, 11/820/386, 11/823/229.
 Rodger, Wooden World, page 45.
 Rediker, Between the Devil, page 158.
 Dye, Ira, 'Early American Merchant Seafarers,' The Proceedings of the American Philosophical Society, Volume 120, No. 5, October 15, 1976, page 340.
 Nagle, Jacob, The Nagle Journal: A Diary of the Life of Jacob Nagle, Sailor, from the Year 1775 to 1841, edited by John C. Dann, New York: Weidenfield & Nicolson, 1988, page 60.
 Thornton, Tamara Plakins, Handwriting in America: A Cultural History, New Haven: Yale University Press, 1996, page 5.
 Equiano, Olaudah, The Interesting Narrative and Other Writings, edited by Vincent Carretta, New York: Penguin, 2003, page 92.
 Equiano, Interesting Narrative, pages 122-123.
 Slade (Lacy), Mary, The History of the Female Shipwright, London: M. Lewis, 1773. in The Lady Tars: The Autobiographies of Hannah Snell, Mary Lacy and Mary Anne Talbot, Tucson, Arizona: Fireship Press, 2008, pages 72-73.
 Nicol, John, The Life and Adventures of John Nicol, Mariner, edited by Tim Flannery, New York: Atlantic Monthly Press, 1997; Snell, Hannah, The Female Soldier; Or, The Surprising Life and Adventures of Hannah Snell, London: R. Walker, 1750, in The Lady Tars: The Autobiographies of Hannah Snell, Mary Lacy and Mary Anne Talbot, Tucson, Arizona: Fireship Press, 2008.