Will the boat swing in time? - SailNet Community
 
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post #1 of 9 Old 01-08-2003 Thread Starter
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Will the boat swing in time?

Hello sailors,

I need help in trying to understand if a large heavy ship will turn during tidal change in time.

A large wooden sailing vessel (warship) is moored facing east in a channel with a large heavy anchor off her starboard side and a small anchor off her port side, both off the bow. When the tide ebbs out west-to-east, how long will it take for the boat turn 180 degrees towards the outgoing current? Will it make much difference if the current is 1.5 knots or 3 knots?

Some information about the ship that may be important: the topmasts and yards have been removed for repair; the length is160’ (131’4” keel for tonnage), breadth is 45’, depth in the hold is 19’4”, the draft is around 20’; and the burden is about 1415 tons. Assume no wind. The water depth at high water is 29’, but will be 18’ at low water. It will go aground unless the ship gets turned around to deeper water in time.

Though this sounds risky to anchor in potentially shallow water, there may have been a military tactical reason for the captain to do this.

Thanks for your help,
John
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post #2 of 9 Old 01-09-2003
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Will the boat swing in time?

A ship, especially one without rigging, will turn with the tides to the current. Obviously the speed of the current will affect how quickly that occurs. In the 1/2 to 3/4 knot cuurent on the Chesapeake, the modern ships seem to take 20 to 30 minutes to turn with the tide.

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post #3 of 9 Old 01-09-2003
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Will the boat swing in time?

John (jcmannone),

I''ll take a crack at your question. Assuming no wind (as you stated) means that the only horizontal forces acting on the vessel is from the moving water (current), and the horizontal component of the anchor rodes. Yes, the force will vary greatly with the speed of the current; I believe the force would be proportional to the square of the current speed.

I don''t have the ability to mathematically model the exact situation you describe, so I''ll go out on a limb and make a guesstimate. I would say that the vessel will start to move in the direction of the ebb current almost immediately, since by our discussion above, there is no other force to oppose that motion. As the ebb current builds, the vessel should move faster until the rodes are taking strain in the opposite direction (assuming the anchors don''t drag). My guess (and that''s all it is) is that the vessel will swing around in short order; probably less than a half hour.

Assuming the tidal height doesn''t drop too far in the brief time period from when the ebb current starts, I think the vessel will not go aground given the depths you cite.

Hope that helps.

Duane
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post #4 of 9 Old 01-09-2003
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Will the boat swing in time?

Two questions from a rookie.... A tide range of 9 feet is incredible, does that happen much. Also a Boat that drafts 20 feet is very limited in which bay they can manuever isn''t it? How many boats draft that deep, or better yet what does a destroyer size military vessel draft?
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post #5 of 9 Old 01-09-2003
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Will the boat swing in time?

There are a couple of places in the world where tides of over 27 feet are common. One of them is in Homer Alaska at the Cook inlet.

Oil Tankers and Aircraft carriers have a 30 foot draft when fully loaded.

As for tidal movement and boats. A boat will move at exactly the same speed and distance as the water it is sitting in unless there is an opposing force such as wind regardless of it''s size. The boat does not resist the movement anymore than a leaf. Actually a steel ship will alline with the magnetic poles if there is no wind at all and sitting in still water. Newtons law.
Jim
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post #6 of 9 Old 01-10-2003
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Will the boat swing in time?

Actually, under Newton''s Laws boats do swing at differing rates. It is a matter that a heavier vessel has greater inertia relative to the force that the water is placing on the hull. Additionally, when a vessel swings with the tide, it first moves downcurrent without swinging until it fetches up on the anchor rode. Once the anchor rode is taut, it is the force of the anchor rode that causes the vessel to swing. A big ship has a lot more rode out than a small boat and must travel further before the anchor pulls taut, and so takes longer to reach the end of the rode. This can require a very significant amount of time given the slow current speeds at the change of a tide and slow acceleration of a big ship.

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post #7 of 9 Old 01-10-2003
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Will the boat swing in time?

I thought that''s what I just said.
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post #8 of 9 Old 01-10-2003 Thread Starter
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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.

John
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post #9 of 9 Old 01-12-2003
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Will the boat swing in time?

Ah go figure. It was a history test!!
Jim
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