Swinging Ship at Anchor
(See page 1223, September, 1942, Proceedings)
Lieutenant Commander W. A. Wiley, U. S. Coast Guard (Retired).—During the spring of a certain year (I think it was about 1906 but I am a little uncertain as to just which year), I reported for duty as Watch Officer and Navigator, on board the old cutter Perry, then anchored in San Francisco Harbor, Captain W. A. Failing, U.S.C.G., Commanding.
The Perry was propelled by a single right-hand screw; and had her standard (Ritchie liquid) compass fitted or installed within a “starboard-angle” compensating binnacle. Because this vessel had just undergone extensive overhauling, and was then fitting out for an extended Alaskan cruise (and for other reasons), her compasses were much in need of compensation; also tables of residual deviations were needed.
But as I was unable to coax the Captain to get the vessel under way and let me compensate compass and then “swing for residuals” in the usual manner (you see he always had other plans or wanted to do something else), I did what seemed to me to be the next best thing. As the vessel swung, from day to day, to her starboard bow anchor (with 15 fathoms of chain), to wind and tide, I took as nearly a complete set of “compass bearings” upon Algetras Lighthouse (upon equidistant ship’s headings) as was obtainable, and filled in the missing ones from a Napier’s diagram curve. With this data and in this manner, I compensated ship and likewise “swung for residuals.”
At the time, I seemed to have gotten good results; but when we went outside and started “up the coast,” we found that our compasses were all wrong. So we returned to San Francisco Bay and swung ship in the usual manner, and then got most excellent results which held good all the way to the Bering Sea, change of latitude in our instance making no appreciable difference.
The trouble had all been caused by the starboard bower and its 15 fathoms of chain. It was then found that a very different compensation was required to correct the standard compass with the anchor and chain on board, as compared with what would correct the compass, with the anchor and amount of chain out of the ship (that is, with the anchor on the bottom of the bay in say 7 fathoms of water, or less).
It may be of interest to mention that the Perry had an old-fashioned steam launch which hoisted and secured in a cradle, just abaft the pilot-house upon the starboard side. During my work with the Perry's compasses, I found that this steam launch exactly balanced the starboard bower anchor and 15 fathoms of chain (heeling error excepted), in its effect on compass deviation. Either this launch had to be up and secured in its cradle, and the anchor up and secured (with its chain in the chain locker, or both had to be out of the ship).
When I made my first (and faulty) compensation the steam launch was hoisted but the anchor was resting on the bottom of the bay. In other respects, things aboard the Perry were about as they would have been at sea. I did not experiment with the port bower anchor; it was not quite as heavy as the starboard one.
Some years later, while I was in temporary command of the cutter Onondaga and taking her from Norfolk to Washington, D. C., the steel foremast was struck with a bolt of lightning, when abreast of Mt. Vernon, and the foremast so highly magnetized as to render both standard and steering compasses for the time being utterly useless—they just pointed toward the mast no matter which way the vessel turned.
We anchored off Georgetown in a harbor too small and cramped to get under way and swing ship in the usual manner, but where there was very little current or tide. It was not deemed safe to attempt to return to Norfolk with useless compasses, especially if fog set in. I bore in mind my previous attempt to correct (i.e., compensate, etc.) the Perry's compasses, with a heavy bower anchor down, so I went about the matter differently.
The Onondaga had 2 kedges of her own and I borrowed 2 more kedges from another vessel, making a total of 4 kedges; none of which were very heavy, and so would not have much magnetic effect (upon compasses), and no chains were attached to any of them; instead Manila hawsers were used.
Three of the kedges were then planted in the form of an equilateral triangle with Manila hawsers led from each of them to on board ship; the Onondaga being in the middle of the triangle.
Both bowers and all anchor chain were then taken on board and secured as for sea. The vessel merely was anchored to the fourth kedge with a very short scope of Manila hawser. The ship was then swung by pivoting upon the bow (or central) kedge and warping upon the other three kedges.
The Dome of the National Capitol was the distant object used (not the sun). The angle of parallax of the different bearings taken on the “distant object” was very small.
Thus after one day of 12 hours of hard work the maximum (deviation) error of the standard compass was cut down from 67½ to 2¼ degrees, and we left the Georgetown anchorage the next morning prepared to run in fog, if necessary. The type of Standard Compass and Compensating Binnacle upon the Onondaga was slightly different from that on the Perry, in that the magnets in the binnacle were arranged “fore and aft” and “athwartship” instead of according to the “starboard angle” method.
As time went on, the electrified foremast gave up some of its magnetic charge, which made recompensating every few months advisable.
These incidents occurred before radio and radio-compasses had come into general use; and gyrocompasses had not up to that time been installed on board cutters.
My instructor in compass compensation had been Captain S. W. B. Diehl, U. S. Navy, who was then regarded as a first-class expert.
Author’s note.—The possibility of error in compass calibration brought up by Lieutenant Commander Wiley is a very real one and requires careful consideration by the navigator. In the case of the vessel discussed in the article “Swinging Ship at Anchor,” the residual errors were found to be small after the ship got under way.
A Diagram for Great-Circle Sailing
(See page 62, January, 1943, Proceedings)
Commander A. A. Ageton, U. S. Navy. —I found Mr. Getchell’s article most interesting and illuminating. This is the first time I have seen such a solution of the great-circle problem in print. With properly prepared diagrams, this scheme should provide a rapid solution with a minimum of effort. A practical use not mentioned by Mr. Getchell is the easy determination each day of the great-circle course your ship should steer from its present position.
The Hydrographic Office could very conveniently and inexpensively construct the diagrams required. I would suggest they be made on the scale used for the full chart of the Pacific and the Atlantic and other small-scale, large-area charts. This one diagram would suffice for most great-circle voyages contemplated. If the diagram be printed on a flexible plastic, the curves could be punctured at intervals (about every 5 degrees) for ease in plotting the great-circle curve on the Mercator Chart.