The Commodore by Patrick O'Brian
To whom does one's true allegiance lie? Jack Aubrey's long service has at last been rewarded with promotion to the rank of commodore, and a squadron of ships to command. His new commission is twofold - first, inhibit the slave trade off the coast of West Africa, and then, on his return, intercept a French fleet loaded with weapons intended for the disaffected Irish. But will the conflict of loyalties be insurmountable for his friend, and Irishman, Stephen Maturin?