ON THE NICOLAIEFF FLOATING AND DEPOSITING DOCK.

By LATIMER CLARK, Esq., C.E.

[Read at the Seventeenth Session of the Institution of Naval Architects, 7th April, 1876; the Right Hon. LORD HAMPTON, G.C.B., D.C.L., President, in the Chair.]

THE Floating Dock now being constructed for the Russian Government at Millwall by Messrs. Clark, Standfield & Co., presents such novelties in design that it has been thought a description of it, and of their system of depositing vessels on fixed. platforms would prove of interest to the Institution of Naval Architects. This dock is designed for the purpose of raising the large circular iron-clads which have recently created such great interest in naval circles, and also for docking the ordinary iron-clads of the Russian Government.

The popoffka Novgorod already constructed has, it will be remembered, a diameter of 100 feet, and another circular iron-clad, the Admiral Popoff, has a diameter of 120 feet; and it has been stated that designs have been prepared for a third popoffka of 160 feet diameter, and there can be no doubt that even larger vessels than this will yet be designed. The largest dock entrances at present in existence do not exceed 100 feet, and it is believed that no entrance so wide as 120 feet has been hitherto proposed. It is evident that the enlargement of such entrances to a width sufficient to receive circular vessels of the size above mentioned would be a serious and perhaps impracticable undertaking the want therefore of some method of docking such vessels was one of the most serious difficulties to be overcome in connection with the introduction of circular war vessels. Admiral Popoff had been long studying this important question, and in one of his visits to this country he became acquainted with Messrs. Clark & Standfield's Gridiron Stage Depositing Docks, and at once perceived their merits and their adaptability to the wants of the Russian Government. Plans were accordingly prepared under the supervision of Admiral Popoff, who made numerous modifications to meet the special requirements of his Government, and the dock was commenced in January this year.

*Received 5th April, 1876.

Before describing the Nicolaieff Dock it may be well to give a general idea of the leading features of this kind of dock.

The Depositing Dock is quite unlike any other dock that has been proposed, inasmuch as it not only raises the vessels out of the water but when required deposits them high and dry on fixed stages of open pile work, where they can be cleaned or repaired at leisure. It can also readily be altered in its form so as to receive vessels of the ordinary type, or to dock circular iron-clads of whatever size.

The Depositing Dock in its general form is composed of a number of pontoons, either of square or circular section, which lie parallel to each other at fixed distances apart, and which range transversely to the length of the dock; each of these pontoons is permanently connected at one end to a longitudinal structure which forms the main side of the dock; the pontoons project outwards from the side of the dock in the same way as the fingers of the hand, so that the whole structure in plan resembles a comb. The pontoons, when the dock is lowered to receive a vessel, are submerged; but the side of the dock to which the pontoons are attached is never totally submerged, but is of sufficient depth to allow a freeboard of 6 or 7 feet when the pontoons are sunk beneath the bottom of the vessel. When the dock is raised the tops of the pontoons are well above water, and the side of the dock stands up some feet higher than the deck of the vessel which it supports.

Diagram 1 (Plate VIII.) shows the plan of the dock, A A A being the side of the dock and B B B the pontoons; and Diagram 2 shows an end elevation of the same floating upon the water with a vessel upon it.

Diagram 3 shows an elevation of the dock submerged with the outrigger attached ready to raise a vessel. It will be seen that this elevation resembles the letter L. It is obvious that such a form as this, viz.: a dock with only one side to it would be perfectly unstable when submerged, but the necessary stability is imparted to it by means of the outrigger arrangement shown in Diagrams 2 and 3. This outrigger consists of a broad flat pontoon divided into numerous compartments, and loaded with concrete ballast until it is half submerged. Its form gives it immense stability-it carries along its middle line a row of rigid upright columns, which project through the pontoon some distance above and below and are stiffened by struts. To the top and bottom of each column is hinged a pair of parallel bars or booms, CC Diagrams 2 and 3, which are also hinged at their opposite ends to the sides of the dock, as shown in the diagrams, so that the outrigger remains stationary while the dock is free to be raised and lowered vertically, being always retained in a horizontal position by the action of the parallel bars or booms; the movement is in fact exactly that of a parallel ruler.

Each of the fingers or pontoons is usually divided into about six separate compartments by means of five transverse vertical bulkheads. The side of the dock to which the pontoons are attached is practically a long box girder divided by numerous bulkheads into large water-tight chambers. Its height may vary from 20 to 50 feet or more, its width from 10 to 15 feet, and its length be about equal to that of the longest vessel intended to be docked. The pontoons are about twice the length of the beam of the vessel to be raised, so as to be available for paddle steamers; their height may be from 10 to 20 feet according to the weight of the vessel, and their width from 7 to 15 feet.

In the Nicolaieff Dock now in course of construction the side is 280 feet long, 44 feet 6 inches high, and 12 feet broad; the pontoons are 72 feet long, 18 feet deep, and 15 feet broad, and the clear space between them is 5 feet.

The machinery for working the dock is carried in the chambers of the side; it consists of a number of powerful pumps worked by steam engines in the usual

manner.

When it is necessary to submerge the dock the necessary valves are opened and the water admitted through pipes to the compartments of the pontoons; the dock is thus gradually lowered, its horizontal position being at all times maintained by its connection with the outrigger; the vessel is then floated over the pontoons, water is pumped out until the keel takes its bearing on the blocks, the bilge blocks are hauled into place by chains in the usual manner, and the vessel being firmly blocked and shored, the pumping is continued until the vessel is raised to its full height as shown in Diagram 2; the valves are then closed.

In this position it will be seen that the dock with the ship on it has very great stability quite independently of that afforded by the outrigger, the outrigger having in fact performed its functions-viz.: that of controlling the dock when submerged-is no longer of any service; and it might, should occasion demand, be entirely removed as shown in Diagram 4. It may be remarked that the dock in this condition is much narrower than any other form of dock, and it might with great facility be taken through any narrow entrance or channel; there is however no necessity to remove the outrigger for any other reason. While thus docked the vessel can be examined, painted and repaired as in any ordinary dock, or can be removed from place to place.

The great feature of this system is that this vessel can now be readily lowered on to a fixed staging along the shore, and there deposited high and dry as shown in Diagram 10, leaving the dock free to raise or lower another vessel, or any number of other vessels.

This staging consists of a series of piles driven into the ground in rows parallel to each other, these rows standing at right angles to the shore, as shown in Diagrams 1 and 10. The rows of piles are capped by horizontal timbers which are exactly the same distance apart from centre to centre as the spaces between the pontoons are, so that if the width of the pontoons is 10 feet, the clear space between the piles is 12 feet, leaving 2 feet for clearance. The height of the vessel above the water is greater than the height of the staging, so that when the dock with the vessel on it is brought alongside the staging the pontoons can enter freely between the rows of piles, and the vessel is carried directly over the staging without touching it; the dock is now slightly lowered by admitting water into the pontoons until the vessel rests upon the keel blocks on the fixed staging; bilge blocks are now placed under the bilges, and the vessel being securely shored, the dock is lowered just clear of the vessel and drawn out from the staging, and is then of course ready to receive other vessels.

The lifting the vessel off the stages and lowering it into the water is simply the reverse of this process.

In considering the question of the stability of this dock during the process of submersion and raising, it is of vital importance to regard the dock as consisting of two very distinct portions, viz.: the side and the pontoons. It must be borne in mind that in this dock, as in all others, the side or sides take no part whatever in the raising or supporting of the vessel; on the contrary, their office is simply and purely to give stability during submersion. Instead of being of any assistance in raising the vessel, 'their own weight has to be raised and carried by the pontoons or dock-bottom, in addition to that of the vessel itself.

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It might appear at first glance that in this dock the great bulk of the single side would have a tendency when submerged to rise and cause the dock to list on one side, -and this would really be the case if the water were to be pumped out of the side and not out of the pontoons; but it must be clearly understood that the lifting power applied solely and wholly in the pontoons, and directly beneath the vessel. The pontoons are provided with a certain number of hermetically-sealed chambers, into which the water cannot enter, and these are sufficient at all times to give the pontoons a surplus of buoyancy, so that they can only be submerged by being forced down by the weight of the side; and when the dock has to be submerged water has to be pumped into the side to give sufficient weight to carry down the pontoons. Similarly, in raising the dock, the water is pumped only out of the pontoons, allowing that in the side to flow out by gravity. The lifting power is thus applied by the pontoons alone, and that directly under the vessel, and the dock has no tendency to turn either one way or the other. It

is however necessary to apply some power to keep the dock horizontal in all states of submersion, and this is provided for by the outrigger and parallel booms before alluded to. This outrigger consists, as before stated, of a large flat rectangular pontoon entirely decked, and of the same length as the dock itself and of about two-thirds its width. It is connected with the dock by booms in the manner before described, acting like a parallel ruler, so that its horizontality is exactly the same as that of the dock, and any tendency of the dock to list to one side or the other must be accompanied by a similar movement of the outrigger to the same extent, necessitating the raising out of the water of one side of the outrigger and the submersion of the other. A very simple calculation shows that the force requisite to thus move the outrigger pontoon, through even a small angle, is enormous, amounting to several hundred tons; taking the Nicolaieff Dock in its worst position as regards stability, the force necessary to make a dock and vessel list through an angle of 3o, is equal to 2,776 foot tons, or to 2,776 tons acting on the vessel with a leverage of 1 foot. This is shown in Diagram 5. With an angle of 10° this righting-power would increase to 16,080 tons.

Although we have pointed out the great righting-power which the dock with its outrigger possesses, it is necessary to remark that with ordinary management not more than a ton or two of this power ought ever to be brought into action.

The valve engineer is provided with a spirit level, and as soon as the dock shows any tendency to list to one side or the other, or to rise at one end more than the other, he adjusts his valves or shuts off his pumps so as to bring the vessel immediately horizontal.

The water compartments of the dock are more than a hundred in number, and are connected with the pumps by means of separate pipes, which are all brought to the valve-house and are then divided into four groups, corresponding with the four quarters of the dock; every pipe has also its own valve. Each of these four groups is governed by a principal valve, and a glance at the spirit level at once enables the valve engineer to ascertain and to control the action of each group and compartment.

Although in docking ordinary vessels and depositing them on the stages it is not absolutely necessary to make use of cradles, it is obvious that they may be employed with advantage in many cases. These cradles may either be provided with side-shoring frames as in Diagram 4, or may consist only of a platform of longitudinal iron girders stiffened transversely, and provided with the usual keel and bilge blocks. In either case the cradle with the vessel upon it is lifted bodily and deposited on the staging, and again removed with it when the vessel is lowered into the water.