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1902] MARINE REVIEW. - DRY DOCKS-THEIR ORIGIN AND DEVELOPMENT. Parker H. Kemble in the Boston Evening Transcript. The use of iron and steel in ship building, requiring as it does con- stant attention to avoid corrosion, has made the dry dock a more neces- sary adjunct to a modern ship than ever before. When ships were small, in the infancy of the art of building, they were hauled up on a shelving beach on all occasions when needing repairs or when not in use, as is still the custom with the French and English fishermen with their smacks and fuggers. As, however, they increased in size they became too unwieldy for such rough and ready methods, and were beached at high tide to be worked on during the short space of time they were dry at low water. Where the coast permitted a narrow cove running well inland and shel- tered was chosen, and there, supported by ropes from the masts to either shore, with a hard, sandy bottom to rest on, the germ of the dry dock began to develop. Some lazy genius objected to losing his night's sleep ut the tide happened to be low too early in the morning and conceived the idea of damming the mouth of the cove when the tide was out--and there was a dry dock. When dry docks were scarce and vessels had no machinery as engines and boilers that would break loose, careening was a favorite method of getting at a ship's bottom. This consisted in hauling the ship down on her side by means of ropes fastened to the mastheads and to heavy anchors on shore until the keel or center timber on the bottom was out of water. When the work on that side was finished the vessel was righted, turned round, and the operation repeated for the other side. This method pos- sessed the advantage of requiring no stationary machinery, as all the requisite apparatus, anchors, ropes, etc., were carried by the vessel her- self for everyday uses. In looking over the logs of both old merchant and navy vessels it is not unusual to find entries of careening on the coast of Africa to repair copper or plug a leak. Before the almost universal use of iron and steel for ship construction the more usual method of getting at the bottom of a vessel was by means of the sunk or graving dock. This is a hole in the ground lined with some water-excluding material such as wood, stone or concrete, any or all, furnished with means of access to a navigable waterway which can be closed when it is desired to pump the water out, and provided with pumps to remove the water when the gates are closed. For constructive reasons the dock is made wider at the top than at the bottom, and for convenience of access and shoring vessels the sides are made like a continuous flight of steps called altars. The gates open outward, and when closed form a broad V with the point towards the outside, so that the pressure of the water on the outside ' when the dock is empty only closes them the tighter. A modern method more prevalent at present employs a caisson instead of gates. This is a single hollow gate made thick enough to float itself when it is empty. When it is wished to close the dock it is towed across the entrance, and by admitting water sunk into grooves made for it, and when the water in the dock is pumped out the outside water presses the caisson onto its seat with force enough to make a water-tight joint. On the bottom of the dock are three rows of blocks for the vessel to rest on. The center ones are fixed and are called keel blocks; those on either side are made to slide in or out by means of ropes so as to fit against the bottom of the ship near the sides. These are called bilge blocks. In order to have room to work under the bottom these blocks are usually made 4 ft. high, experience having shown that a man cannot do efficient work with less space, and higher blocks increasing the danger of accident to the vessel. Concrete is at present the favorite material for graving docks where economy is not a prime factor. It has many advan- tages over granite or masonry, both in expense and ease of construction, and while more expensive than wood, its superior durability warrants the greater initial outlay where the capital can be obtained. From its nature as an outside or containing vessel a graving dock must exceed in all its dimensions the largest vessel it is intended to handle. For naval vessels alone a length of 550 ft. for cruisers, a breadth of 78 ft. for battleships and a draught of 28 ft. is-required by vessels now under way or projected. For merchant ships the dimensions of breadth and depth would be sufficient, but that of length would have to be extended beyond 700 ft. to accommodate existing ships, and 750 ft. would be as short as safe to make it, in view of the present tendency of dimensions. As, however, a modern navy includes in time of war the use of merchant- men as auxiliary cruisers, they would need to be considered. This gives us then a length of 750 ft., a beam of 78 ft. and a draught of 28 ft. to be arranged for. For width 10 ft. on either side must be allowed for light, air and access; and owing to the shape of the midship section of the pres- ent type and the prevalence of bilge keels this width must be that of the bottom of the dock. When we come to the depth the conditions are more complex. A merchant vessel drawing 32 ft. loaded will draw in the neighborhood of 20 ft. light, so that even if damaged and partly filled with water there is a margin enough in a 28-ft. depth. With warships, however, the case is different. Our battleships average about 28 ft. when fully loaded for sea, and a result of battle in the shape of an increase of 5 ft. is quite possible and should be guarded against. Then, again, there is the tide question. Imagine a badly crippled vessel waiting for high tide to get into dock, with the possiblity of sinking while waiting. With a rise and fall of 9 ft. in Boston harbor it would not be practicable to allow for the whole tide, but a 3-ft. allowance should be made, which would give a depth at high water of 36 ft. over blocks, or a depth below mean high water to bottom of dock of 40 ft. We have then the following for a fist-class grav- ing dock: Length, 750 ft.; breadth on bottom, 98 ft.; total depth below high water, 40 ft.; at entrance, 36 ft. The entrance should be nearly the full width of the dock proper, as with bilge keels a very small list would prevent entrance. The construction of a dock of this depth would be of the greatest difficulty. For structural reasons, out of place here, it would be a task in any but the best of soil impracticable, if not impossible. The floating dock undoubtedly is derived from the pontoon, and the origin of the latter can be carried back to the primitive age when our forefathers found one log insufficient to carry their cargo, and put another on either side. In early history the use of pontoons in getting deep- draught war vessels over shallow bars at river entrances is mentioned. In a very old book on marine construction in the possession of the writer, there is a print showing a vessel supported between two pontoons by means of ropes passing underneath the keel and tightened by- windlasses on the pontoons. These are provided with valves for the admission of aK water, and with pumps for its removal. . The descriptions in quaint old French call it a "chameau," and go on to tell how it was used by the Hol- landers to take heavily laden ships over the shallows to where they could continue their way to Amsterdam. The two pontoons were towed out to the vessel, partially sunk by admitting water; the ship hauled between them; the ropes under the keel tightened till the pontoons, which were roughly shaped to the form of a vessel, came alongside, then the pumps were started and the whole com- bination rose enough in the water to' pass over the shallows, where a reverse operation restored the ship to her independence. The first few pages of this book are unfortunately missing, having been devoured by some rodent with a taste for literature, but from the text, the date of pub- lication appears to have been around 1700. If in this old machine you substitute a third pontoon for the ropes joining the two sides together you have a floating dry dock. ae _ A floating dock of today is designed and built under the same rules and theories as a vessel as regards stability or ability to stand up, and strength. The more generally used type for large docks consists of two sides about four-fifths the length of the dock, and three bottom pontoons, one long one in the middle and a short one at either end. These are all rigidly joined together, and rise and fall as one. The function of the sides is chiefly steadying and controlling, although they exercise some lifting power, but the main lifting force of the dock lies in the bottom pontoons. : ok Engines, boilers and pumps are stowed in the sides, and the ingress" and egress of water to all the various compartments is controlled from one room, where the whole control of the dock is centered. The machin- ery outfit usually includes as well as pumps, and electric light plant, cap- stans and windlasses, an air compressor and various pumps for washing of the dock or pumping out ships' tanks. The method of use is the same as described for the pontoons except, of course, that the ship rests on keel and bilge blocks instead of on ropes. The advantages of the float- ing system are many. First of all the site. The excavation required is not greater than that for a graving dock even if harbor space is not avail- able and dry land has to be encroached upon. It needs no expensive lining or foundation as required by a graving dock. Greater speed of construction while the dock is building at a.shipyard. As a graving dock may be likened to an oblong box without a cover, so a floating dock may be likened to ihe same box with the ends removed. The result is that the length of the ship to be lifted is no longer governed rigidly by the length of the dock. The model and construction used in the modern vessels allow considerable overhang of ship unstipported at either end, and for a 700-ft. ship a floating dock 560 ft. long would be ample. ie The beam of a floating dock is, of course, fixed, but as structural reasons allow the sides to be made straight instead of sloping, as in the graving dock, this can be made wide enough without difficulty. Draught of water is simply a matter of dredging and' height of sides and presents no great difficuities, as the extra expense of building up the sides a foot "or more and dredging the bed is the small side of the extra masonry and foundation for the other dock.' While an excess of an inch in draught would bar a vessel from a graving dock, if the water were smooth a float- ing dock could be, under stress, sunk 10 or 12 in. more without danger. _And the latter is, of course, independent of tide. The floating dock has one absolute limitation, and that is lifting power, which the other does not possess. if the ship weighs more than the capacity of the dock it cannot be lifted out. It may, however, be lifted part way, and a dock that could not lift a vessel for a patch on the bottom could lift her enough to slip in a new shaft. Should a ship have a list the dock may be listed to suit, and when the ship is firmly on the blocks, dock and all are straightened up. There is a source of economy in the floating system due to the fact that only an amount of water equal to the weight of the dock plus the weight of the ship needs to be pumped out. In the gray- ing dock the amount of water is greatest when the dock is empty, and least with the Jargest ship. As a consequence, only about one-third the pumping power is required in a floating dock of like capacity. The con- nections between the bottom pontoons and the sides while rigid are made detachable, and by unfastening any one of the bottom sections it may be docked 'by the other two, while the bottom of the sides may be reached by letting in water to the opposite side, thus careening the dock. The dock is thus self-docking, and any part may be reached at any time. The following dimensions are of two of the largest floating docks in existence: U. S. N. floating doek at Al giers, La.-- ve bei fength oe ea British: government 4° Breadth .over: all....° 126 {t. 2 in. ve : gth over all...: d545ft Breadth between Breadvh: fh @eaveen WaALIS 2550... 3 ee 100 ft. Depth over sills... 8it | ptth over sil 1.) aR ie ee 0% RE ee Lifting power ..... 15,000 tons. Lifting power ...:. 18,000 tons. There are other systems of reaching the bottoms of vessels, notably the marine railway, which is the development of the hauling up the beach method. It consists of a slanting track running from a little below the maximum draught to be lifted, up onto the shore above high water mark. On this is a car or cradle which is lowered into the water, the ship floated over it and both pulled out together. It is applicable only to relatively small vessels. : A very ingenious form of dock is that built by the Union Iron Works, San Francisco, the builders of the Oregon. An excess of mud and other local considerations rendered a graving dock undesirable and with char- acteristic independence they built a dock which is in fact a big hydraulic elevator. A platform 436 ft. long, 65 ft. wide, and with 22 ft. of water over the keel blocks at high tide, capable of taking a ship weighing 6,000 tons, is bodily lifted up above water by a row of hydraulic rams placed on either side of it. It has been a great success, having been in con- tinuous service some fifteen years. The Baltimore, Chesapeake & Atlantic railway is preparing plans for a new excursion steamer to run on the route between Baltimore and Clai- borne. The new steamer will be about 200 ft. long and will have capacity for 1,500 passengers.