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Will the boat swing in time?
Thank you all for the excellent answers.
For your own curiosity, I filled in some details to support the figures I used.
The setting is Boston/Charlestown in April 1775.
The spring tides in Boston harbor around the full moon exceed +11 ft at high water and –1 ft at low water; currents at Boston Light are somewhat under 2 knots (at the Charles River the currents would have easily exceeded 2 knots- before all the landfills, dredging, etc.). There is an excellent free tide and current predictor online that quickly calculates present, past, or future tidal action (1970-2025) for hundreds of locations in the USA. It will give you the answers in table or graph form: http://tbone.biol.sc.edu/tide/sitesel.html
The warship described is an 18th century British Man of War, third-rate ship of the line, HMS Somerset. Displacement hull vessels have a draft of nearly half its breadth (draft <22.5 ft). It draws 19 ft fore and 18 ft aft without any of its 64 guns onboard, which add a few hundred more tons. It was my guess it draws 20 ft; it may actually be 21 ft; in any case, well over 18 ft.
If indeed the vessel could have swung around in time during tidal change, and I believe you have all convinced me that it would, then the Admiral was far more savvy than I had credited him on first impression.
A book I acquired on New Year’s Eve, “H.M.S. Somerset 1746-1778: "The Life and Times of an Eighteenth Century British Man-O-War and Her Impact on North America.” (1st ed., Marjorie Hubbell Gibson, Abbey Gate House, Cotuit, MA, 1992), is authoritative since the ship’s logs were consulted in its writing.
The author states,
“After careful measurements were made of the width and depth of the channel, the SOMERSET was warped into the narrow ferry way between Boston and Charlestown. This large ship with its fire power at this location prevented any attack from Charlestown.” (page 97)
Also, she summarizes the ship’s logs for April 13 to15, 1775,
“On orders from Adm. Graves the SOMERSET was moored a cable each way between Charlestown and Boston.”
The ferry channel depth is 3 fathoms and is verifiable from old nautical charts and encampment maps for that time.
I can now understand the need for careful measurements. It can be inferred that the 2 bow anchors were no further west than the eastern edge of the ferry channel. At high flood, her bow points east and her port side north towards Charlestown. During outgoing tide, her bow would face west over deeper water (4 fathoms), and well out of the shallow channel. Now, her starboard guns face Charlestown.
Charlestown was close to her guns, but the returning/retreating Regulars could not be easily protected as they crossed Charlestown Neck, which was a vulnerable position for the British Redcoats (April 19), unless the Somerset guns were in range. Charlestown Neck was well over a mile away from mid-channel and not in effective range if the Somerset was placed much further east. The admiral seemed very cleverly to have tactically positioned his ship.