Notes on Signals for Use in Hydrographic Surveying.
By Captain A. Rust, U. S. Navy.
As the rapidity with which a hydrographic survey can be conducted depends largely on the time saved in building the signals for extending the triangulation and controlling the hydrography, after two years test I have found the steel towers described herein vastly superior to the wooden signals in common use and employed at first by me, for the following reasons:
(a) They are more economical; for not only is the first cost less than that of wooden signals of the same size, but as they are made of galvanized steel, the large towers having composition bolts and nuts ten feet above the bottom, they will last indefinitely with proper care, and may be readily taken down and set up at other stations.
(b) There is a great saving of time and fewer men are required. A 60 foot steel signal tower may be put together, erected and decorated in one day, whereas a 60 foot wooden tower will take a week under the conditions usually met with, including the time taken to cut and fit the timbers.
Boatswain Thomas James, U. S. Navy, in charge of a party, erected, guyed and decorated a 60 foot steel signal tower on a sand shoal and the party was on its way back to the ship in 5 ½ hours. This is the best time ever made, I believe, though the work was done under most difficult conditions, the shoal being awash at the time.
(c) The steel signals are not disturbed by fishermen or the natives on shore as the wooden ones are liable to be.
(d) The small steel signals 30 or 40 feet high may be placed on shoals where the water is too deep to erect wooden signals.
(e) The steel signals may be made sufficiently rigid to support the transit directly without an interior instrument stand when occupied, though when the distances are very great a small tower may be erected inside the large one for an instrument stand.
(f) The steel signals are not so liable to be blown down, as the planks and canvas put on for making them visible at long distances being structurally weak, will blow away first, provided the signal be properly guyed, but in case these steel signals are blown down they are not injured to any great extent, unless there are rocks or stumps for them to strike in falling. It is difficult to guy up tall signals located on sand bars so they will stand a heavy wind and sea, and four 60 foot signals thus placed I found down at the beginning of the next season but practically uninjured. Son* of the angle irons were bent but the blacksmith straightened then; again in a short time.
Four sizes of signal towers are used by the Cape Craz-Casilda Survey Expedition: 30 and 40 foot tripods and 50 foot 4-post towers. The 60 foot tower is shown on KD-58 furnished by the Aermotor Company, 12th and Streets, Chicago, Ill., the firm which furnishes these tower. (See Fig. 1.) The 30 and 40 foot towers have 2-inch pipe at the top to admit a galvanized pipe for a flag-pole. Experience shows that these are more convenient than the heavy wooden poles. The upper ends of these 2-inch pipes should be threaded in order that a brass casting, which is portable, may be screwed to this pipe to which a wooden platform is attached to support the theodolite. I have been informed by the builders that a 3-inch pipe is the smallest that can be used with the 60 and 80 foot towers, therefore to the 3-inch pipe should be screwed a reducer into which a short piece of pipe the same size as that used for the 30 and 40 foot towers should be screwed and threaded on its upper end to receive the portable casting supporting the instrument.
To recapitulate:
(a) In ordering these towers specify for the 60 and 80 four post towers that they be fitted with 3-inch pipes at the top with reducers, and a short piece of pipe the same size as that used for 30 and 40 foot tripods, and be threaded at both ends with standard pipe thread and screwed into the reducer.
(b) Specify for all towers, 30, 40, 60 and 80 foot, the angle irons connecting the bottoms of the anchor posts are to be furnished as shown on Drawing KD-58.
(c) Require that all parts be so lettered or numbered that the towers may be readily assembled by the drawings furnished.
(d) The required distance from the end of the pipe to the platform which is most convenient for the observer with the transit should be given and the makers required to fit the platform at this distance.
(e) Furnish pipes for flag-poles.
To assemble the 60 and 80 foot towers.
These are furnished in sections 20 feet long. The parts of each signal should be kept made up together in bundles, each section by itself, check out all bolts and lay the bundles in order. The upper section with the head is first assembled. This may be done on board ship and then landed on a catamaran made of two boats lashed to spars. Pick out a level place far enough from rocks or stumps to enable the tower to fall clear in case it is blown down. Lay the upper section down with the ladder up and complete the tower by adding to the bottom of the upper section and attach to steps for ladder. Secure the platform in place. For assembling these towers about a dozen double open-ended wrenches are needed to take 3/8 and ¼-inch nuts. Sometimes, where there is room on deck, time may be saved by assembling two sides of a tower first and completing it on shore. This is the method recommended by the builders, who will furnish printed instructions in detail for assembling and erecting these towers if requested. As a rule it will be well to adopt one method and stick to it, as in this way the men soon acquire the knack of putting the towers together very quickly.
To erect a 60 or 80 foot tower.
In order to give the tower sufficient rigidity for up ending, secure pieces 2 x 4-inches with wire to the corner posts from the bottom of the tower to the platform, AB, CD, also secure two struts 2 x 4-inches diagonally on each side from top to bottom of the lower section as shown at ED in Fig. 2.
It is thought that with the new towers fitted for pipe flag-staffs, as these will be so much lighter than the wooden ones, that the 2 x 4-inch, AB, CD, will not be needed, but only the struts ED. Without the struts ED there is danger of buckling the tower when hoisting it to a vertical position.
Gear required to erect a 60 or 80 foot tower.
(a) A pendant 20 fathoms long made of wire rope 2 inches in circumference, fitted with a thimble in the lower end and stout sister hooks in the upper end.
(b) A tackle 50 fathoms long of 2-inch Manila rope, with a steel 3-fold upper block, steel double lower block, standing part made fast to lower block and hauling part through a steel snatch block, the men walking away from the signal while erecting the same. Chief Boatswain James Glass, U. S. N., found after some experiments that the above gear was the best. This tackle should be unrove when not in use, and the blocks washed with fresh water, dried and oiled.
(c) Shears. These should be made by bolting two pieces of 4 x 4-inch together for each leg, the parts overlapping well, spread the heels about 15 feet apart and make the altitude of the triangle ACD,AB —35 feet. Bore a hole through each leg and put a stout bolt at A leaving a good crotch FAE, which should be well rounded out at the bottom and be well slushed, or chocks with rounded scores be secured to the legs. Nail temporary braces to the shear legs, and also place a lashing at A in the large angles.
(d) Planks and stakes at the foot of the tower. With the steel angle irons connecting the anchor posts, long planks should not be needed at the base of the tower, but to prevent the tower from skidding, it will be well to drive old boiler tubes or stout stakes near the lower anchor posts as the tower lies on its side, and to take loose lashings around the stakes and these posts, and if the lumber can be had it is well to bolt short pieces of heavy plank, 3 x 12-inches to the foot of each anchor post to prevent the legs sinking into the mud or sand when the ground is soft. List of articles required to erect a 60 or 80 foot steel tower.
- 2 heavy mauls or sledges.
- Stakes or old boiler tubes. A supply of tubes may generally be found at any Navy Yard and they will be found very useful.
- Frame work of the signal made up in bundles, each section to itself, and carefully checked up by the blue print, and tagged.
- Box of bolts.
- No. 7 telegraph wire for guys.
- Platform with lumber for enlarging same if necessary.
- Shear legs, four pieces 4 x 4-inches, 40 feet long over all when bolted together.
- Long 3-fold purchase, described in par. 5 (b) and leading block.
- Wire pendant.
- Sufficient 2 x 4-inch lumber for strengthening signal for hoisting. (u) Piping for flag-staff of the proper size to fit the pipe at the top.
- Horse for lifting head of signal tower for first pull.
- 2 heaving lines.
- 4 machetes.
- 2 axes.
- Hammers, nails, and all wrenches available.
- Planks, old canvas, paint and white-wash, if signal is to be decorated, also small stuff for stops.
- Flag and halliards.
N. B. Muster all tools before shoving off, and have a station bill for the party and hold each man responsible for providing and returning certain tools and gear. To erect the 60 or 80 foot towers.
Great care must be taken to have everything rigid and well braced in order not to buckle or drop the tower when it starts from the ground. When the ground is soft or sandy take the strap for the lower block of the tackle around a spar in front of which stakes or old boiler tubes are driven at an angle, drive a second row opposite, and lash the stakes or tubes together as shown at AB, Fig. 7. Sometimes it will be found convenient to use a steamer's anchor and back it up in the same way. Lift the head of the tower from the ground and rest it on a wooden horse. Take the end of the pendant around the tower just below the platform and hook the sister hooks around its own part and mouse them well. Hook lower block of tackle to a strap taken around several boiler tubes or stout stakes driven well down, well backed, and placed as far from the head of the tower as the length of the tackle when overhauled will allow. Place pendant in the crotch and upend shears and place the feet near the bottom of the tower,' but far enough away for the head of the shears to cant a little towards the head of the tower. Get a good strain on the tackle and see that everything is well secured. Attach two guys leading off at angles of about 45° from the tower to stakes in the rear. Drive stakes in front of the bottom angle irons to prevent skidding, and bend a line to the shear legs to prevent them falling when the pendant comes out of the crotch. This line should be tended as the tower goes up, and the shears eased down by it. When the tower is up it should be well guyed by a sufficient number of guys made of No. 7 telegraph wire secured to old boiler tubes or stout stakes and set up with Spanish windlasses. At least eight guys will be required for these towers. The telegraph wire is better than wire rope or strand, as not only is it not so liable to be disturbed by the fishermen and natives on shore, but the guys made of several parts may be more easily set up taut. The guys should be secured near the top of the signal, just below the platform is a good place, so as not to interfere with hoisting the planks for decorating the signal to make it visible. Anchor stakes should be driven at each corner and secured to the anchor posts by wire lashings. To make the tower rigid for occupation, wire diagonals should be put in between the legs and set up with Spanish windlasses. Procure 24 planks each 16 feet long, 6 inches wide and ½ inch thick, and 6 pieces 2 x 4-inches, about 16 feet long. Nail 12 of the 6-inch planks at right angles to 3 of the 2 x 4-inch, spacing the 6-inch planks about 2 or 3 inches apart (not more than three inches). If the signal is to show against the sky, paint these planks black, if against the land, use the white paint or white-wash. Carry a tackle to the top of the signal, bend on the frame work so that the 6-inch planks will be vertical, bend on tripping line and hoist away and secure the frame-work close up under the guys on one face of the tower, and the other at right angles to the first one. We thus have an area 16 feet square visible 10 miles with the naked eye.
Flags in a region of variable winds are very unsatisfactory, but it is necessary to use these until the occupation of the signal has been completed, when beacon heads made of planks 6 x ½-inch, and from 8 to 12 feet long placed on the flag-staffs were found very good. Another very good decoration for the top of a signal in place of a flag, and one that can easily be removed, if necessary to occupy the signal, was a canvas rig which was made like an old fashioned navy target with three wings making an equilateral triangle of 21 feet on a side from any direction. These wings are spread out by riggers nailed to the platform. A large supply of old canvas should be carried which should be painted or white washed when used instead of lumber for decorating signals.
Lowering and transporting 60 and 80 foot signals.
Before lowering these it is necessary to rig the wooden frame work and diagonals used in erecting the signal, otherwise the teg5 are liable to buckle, the tower dropped and more or less injured. To transport the 60 and 80 foot towers it is necessary to take them apart except the top section which need never be disturbed, but brought off on a catamaran made of two boats and be triced up along-side or hoisted on board ship.
Erecting 30 and 40 foot signals.
The 30 and 40 foot steel tripods may be easily put together in the gangways on board ship and be triced up alongside, three or four each side. These need never be taken apart when once on the working ground. They are sent ashore on two boats with spars lashed between and may be erected and decorated in a couple of hours, and are very convenient for use on shoals where the water is smooth and from 6 to 8 feet deep, provided the bottom is of such a nature that long stakes may be driven down to which the guys may be made fast.
By using the steel signals in place of wooden ones the amount of work done by a given party in a given time may be trebled.
Floating signals.
For off shore soundings where the water is shoal enough to moor or anchor them securely, excellent floating signals may be made as follows:
Procure three empty barrels, test them to see that they are perfectly tight. Make two equilateral triangles of planks 2x6 inch x 12 feet long, place the three barrels between these at the vertices of the triangles, and secure the triangles together by six bolts made of ½-inch iron rod, a, b, c, d, e, f. If these bolts do not seem to make the structure sufficiently rigid for rough water, use six more at g, h, i, k, 1, m. Filling pieces, n, o, p, for the heads of the barrels should be fitted to prevent the barrels working adrift. If the barrels are placed close together the signal will be more suitable for use where there is much wind or sea, and will at the same time be more easily handled, but it will be more difficult to make it stand upright in the water when the wind blows, and the weight at the lower end of the flagstaff must be increased. Two stout planks should be bolted across the frame, top and bottom, to support the mast, which should extend 15 or 20 feet below the surface of the water and have about 150 or 200 pounds of old grate bars lashed to the lower end. If made of 12 foot planks the mast may extend 25 or 30 feet above the water and carry a beacon head made of 12 or 16 planks y2 x 6-inch x 6 or 8 feet, painted black, nailed at right angles to each other 6 or S on a side, with the space between filled in solid with black cloth. This increases the visibility very much. Three triangular wings may be secured below this like an old fashioned navy target. A strap of wire rope or chain should be secured around the flag-pole above the upper part of the platform to which the anchor chain is secured with a pelican hook moused with wire. The mast should be made of two pieces of 3 x 4-inch or 4 x 4-inch bolted together. It will be found that by placing the barrels tangent to each other, or nearly so, that the float may be more easily handled, but then instead of a beacon head it will be found necessary to modify this by pacing all the planks in one plane only and nail them to the flag-pole as shown in Fig. 5, otherwise in a fresh breeze the signal will list over so far as to lose much of the height of the mast. This signal will stand upright and show up well except when the observer is directly to leeward or to windward. Weave black cloth in between horizontal planks. Nail a vertical plank ½ x 6inches to the horizontal planks, and at the lower end of this plank secure a brace to the flag pole, see f and g. To render this signal visible for a distance when directly to windward or to leeward, a few pieces of plank ½ x 6-inches x 8 feet may be nailed to the bottom of the mast at right angles to upper ones; these should be painted white. A regular system of triangulation may be extended off shore on soundings with these signals, occupying each in turn with a sextant, and cutting in as many as possible signals on shore or those on the shoals.
Hydrographic signals.
As it is not necessary to locate these with great accuracy, as the case with the principal signals used in triangulation, much time and lumber may be saved by building them as follows:
Signals of this kind are recommended on shoals in water from 6 to 8 feet deep when for any reason it is not convenient to erect a steel signal.
Provide a stout stake about 6 inches in diameter and long enough to stick about 6 feet above the water, bore two holes about one foot apart and sharpen the lower end of the stake. If there is too much water to handle the signal without a boat, rig a catamaran of two boats, anchor it, and from this drive the stake down with a heavy wooden maul until the top is about a foot from the water. Take a piece of 4 x 4-inches about 15 or 20 feet long for a derrick, bore two holes in it to match those in the stake and secure a tail block with a line rove through it to the other end, or better still put a sheave in the upper end of this spar.
Provide two long ½-inch bolts long enough to go through the upright for the signal 3 x 3-inches, or 4 x 4-inches, the stake and the derrick of 4 x 4-inches, put a bolt through the three as shown in Fig. 6, upend the derrick, and put the other bolt through from the opposite side of the stake. Bend the line to the mast at a distance from the first bolt to the center of the sheave in the derrick. Nail on the cross planks ½ x 6-inches x 8 feet and weave the cloth in to fill up vacant spaces which should be tacked on with plenty of battens to hold it, take the wire around for the guys, and hoist the beacon upright and after securing the guys unship the derrick and secure the heel of the mast with the second bolt.
The black beacon head made solid with cloth makes a fine signal on water, and shows up well against the sky; when a single pole signal like this is used on shore with trees or bushes as a background, a white beacon or three triangular wings hauled out by means of the guys makes an excellent signal and one that may be quickly built. On shore a hole may be dug to hold the lower end of the mast instead of the stake. These signals have been described to the smallest details, as experience shows that much time may be lost by the signal building parties in trying unnecessary experiments, and thus greatly retard the work.
When prominent trees or hummocks are used for signals they should be marked by beacons or flags to distinguish them before they are cut in. A piece of 2 x 4-inches, or a pole cut in the woods with boards nailed across at right angles to form a beacon with a flag above or below is a very good distinguishing mark. Empty flour barrels white washed or painted black make good marks. As each hydrographic signal is built it should be occupied with a sextant even though a transit be used later, and all signals in sight from it cut in, a regular angle book being always carried for recording these angles.
In erecting signals for the control of the hydrography it is well to remember that it is better to have too many signals than too few. The soundings are delayed very much when the steamers are compelled to stop work to build signals.
Marking centers.
The center of a tall signal cannot be accurately located with a plumb bob if there is any breeze, so as a rule it is better to use a transit. Set up, level, and bisect the flagstaff, and line up two pegs so that the center is between them. Shift the transit so that a line to first pegs will be at right angles, or nearly so, to the first line, and line up two more pegs in the same way, and with cords connecting the two pairs of pegs, find the center and drive an old boiler tube, first flattening the lower end, until the end is nearly flush with the ground and fit a plug in the end of tube and mark the center with a tack. Dig a hole around the boiler tube and having straightened up the tube so that the back comes at the intersection of the cords, place a section of terra-cotta pipe with the flange down as far below the surface as desired, then fill around the tube with cement. Drive the plug down in tube, fill end with cement and place a copper nail in center. This forms a mark that no one can remove without considerable work. Suitable reference marks should be established also, for while the iron tube will last for many years it will in time go, and it may be necessary to dig for the sub-surface mark. If no terra-cotta pipe be available, place a box 6 inches square and two feet long around the tube and fill in with cement. If the top of the terra-cotta pipe be left above the ground the earth should be heaped over it and a pile of stones placed over the mound if they are found to be near.
Sub-surface marks, unless they have pipes driven close to them, are hard to find unless the reference marks are piles of stones, as stakes rot in a few years. When no pipes are available to use with the cement mark, it should be made so long and heavy that it cannot be readily disturbed. While the cement is soft, place a copper nail in it to mark the center.
A signal record book should be kept in which the officer or surveyor in charge of the triangulation should be required to make an entry over his signature describing all center and reference marks for each signal built, with sketches showing their locations and giving angles and true bearings to prominent objects.
Buoy for floating signals.
In Fig. 8, at A and B are shown views of a buoy for floating signals which will be found more convenient to handle than the raft shown in Fig. 4. If made of the dimensions given in Fig. 8, 3 feet in diameter and 2 feet high, it will support about 600 pounds dead weight. A similar buoy 3 feet in diameter, 2 ½ feet high will support about 800 pounds dead weight. These buoys should be made of galvanized iron or steel about 3/16-inch thick or 7 ½ pound plate.
a, b, c, Pad-eyes to which slings for handling buoy are made fast.
d, Pad-eye for mooring chain or pendant.
e, Ring for slings.
f, Pipe at least 4 inches in diameter for mast. This mast should be in two sections, the upper section with the beacon head is bolted to the lower which passes through f, and has about 200 pounds or more of ballast on its lower end and extends 15 or 20 feet below the water.
M, Man-hole.
The mast for this buoy had best be made of two sections of galvanized iron pipe, the lower section of the proper size to pass through f, with a collar or flange around its upper end to prevent its slipping through f. The upper section slides into the lower when the buoy is not in use. When the buoy is in use the upper section is kept in position by a through bolt above the flange. A cast iron ballast ball weighing about 200 pounds, with a short section of 4-inch pipe cast into it should be attached to the lower end of the lower section by a through bolt. The ballast balls will be found much more convenient than the improvised ballast made of old grate bars. The diameter of cast iron sphere weighing 200 pounds is about 11.3 inches.
A sketch of a ballast ball is shown in Fig. 10. While the lower section of the mast should always be made of a pipe, the upper section may often be conveniently made of wood.
Boat chain made of iron 3/8-inch in diameter should be heavy enough for any of the signals described herein, and the chains should be galvanized. When the water is deep, wire rope, on account of being lighter, should be used. Before having the rafts or buoys made, figure out from the approximate depth of water the weight of chain or wire pendant that will be needed and increase the size of the buoys if necessary.
Sinkers for floating signals.
At C and D, Fig. 9, are shown two views of cast iron hemispherical sinkers similar to those used for moorings in the U. S. Lighthouse Service.
Dimensions | ||||
Type | Radius of sphere | Distance co. | Distance pq. | Weight |
A | 11 ¾ inches | 3 inches | 20 ½ inches | 770 lbs. |
B | 13 ½ inches | 3 inches | 23 ½ inches | 1200 lbs. |
C | 15 ½ inches | 3 ½ inches | 26 ½ inches | 1850 lbs. |
A supply of these should be carried for mooring the signals, type A for the buoy shown in Fig. 8, and also for the float shown in Fig. 4, when in smooth water. In an exposed pro type B should be used with the float in Fig. 4, though much will depend upon the character of the bottom.
Pumping down foundations for signals.
When the bottom is sand or soft mud, the foundations for signals may be planted by means of a pump in the same way that concrete piles are sunk. If the foundation is of pipe, attach a hose from a steam pump to the upper end. When the pump is started the pipe will sink into the bottom rapidly.
In the same way, by attaching a pipe to a pile or stake by means of wire spikes driven alongside it and bent over, to hold the pipe securely but loosely, the pile may be sunk six or eight feet into the bottom with ease. Withdraw the pipe before stopping the pump, otherwise if working in sand, it cannot be done on account of the sand settling around and filling up the pipe. (See Fig. 11) Keep the ship or tender in position by kedges off either quarter if necessary. I am informed by Commander J. H. Hetherington, U. S. Navy, that he has planted piles 12 inches in diameter in this way.
General remarks on the proper outfit for an efficient surveying expedition.
In the past, as a rule, surveying ships have been too small to be economical and efficient. A ship for this service should be large enough to carry an ample supply of coal and provisions, lumber, old canvas, old piping and boiler tubes, and other material which may be picked up at a navy yard without cost, and which would be expensive to obtain on the working ground, either in money or at an expenditure of time and labor, as well as enough regular stores to make the ship self-sustaining, and thus enable her to remain steadily on the working ground. She should be large enough to carry at least four 40-foot steamers on deck, or on the rail, with suitable means for lowering and hoisting them quickly.
The ship should have large and efficient evaporators, tanks for a reserve supply of fresh water, electric lights, ice machine, cold storage plant and a good drafting room. In addition to this large ship there should be a smaller vessel to act as a tender to be used for building signals, planting buoys, carrying the triangulation party back and forth, and at times go to neighboring ports for fresh provisions or other supplies if necessary, and also do the off-shore soundings. The movements of the larger ship will be restricted, due to the necessity of her remaining in the vicinity of the sounding boats in order to be accessible in case of bad weather coming on. Besides this, and the danger to the larger vessel due to cruising about in unknown waters, she cannot ordinarily approach sufficiently near the shore or to shoals to be used efficiently for signal building.
While working around shoals in unsurveyed waters the tender must at times get aground, therefore, in case she does not happen to have two Scotch boilers, she should be fitted with water tanks forward and aft whose capacity is at least 4 per cent of the vessel's displacement. These should be kept full when on the working ground, and so connected to the pumps that they may be quickly pumped out in case of grounding. Cloud shadows on the water often present the appearance of shoals, and are so deceptive that they will worry one considerably at first, and when the sun is ahead or right overhead, it is almost impossible to distinguish a shoal by eye; the lead is the only thing to depend upon, but with the greatest care, when working among reefs, or when the water shoals suddenly, the tender is at times liable to get aground, hence the great utility of the water ballast. If in a territory where you suspect rocks keep a boat ahead of the tender unless working in a protected place.
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