"An Inquiry into some Points of Seamanship, by a man, Tower, Towson - some such name - Master in his Majesty's Navy"

Conrad is probably conflating two books: J. T. Towson's navigation tables, and Nicholas Tinmouth's An Inquiry relative to various points of seamanship (1845).

Towson published two volumes of navigation tables (in 1848 and 1849), but he did not write a handbook on points of seamanship. The opening chapters of Tinmouth's book inquired 'earnestly into the breaking strain of ships' chains and tackle'; the book also contains the diagrams and tables of figures mentioned by Marlow. But Tinmouth was not a 'Master in his Majesty's Navy'  – he was a Master-Attendant at Her Majesty's Dockyard at Woolwich.

In The Political Novels of Joseph Conrad, Eloise Knapp Hay also suggests A.H. Alston's Seamanship and Its Associated Duties (1860), which she describes as 'one of Conrad's favourite books', as a possible model.

 

Further reading:

J. A. Arnold, 'The Young Russian's Book in Conrad's Heart of Darkness'