CONCRETE SHIPS

Dominion, Volume 11, Issue 196, 8 May 1918, Page 8

CONCRETE SHIPS

IS LOCAL BUILDING LIKELY?

DESIGNS BEFORE N.Z, GOVERNMENT

The speed with which the construction of ships in reinforced concrete has gone ahead is more due to the activities of German submarines than to the considerable advantage of the ships being cheaper and quicker to build than those of steel. AVhile barges and smaller ves,sols constructed of concrete have been a success, however, marine architects and engineers have beon doubtful as to the suitability of concrete for the construction of larger ships. Thn several known weaknesses of concrete were a serious drawback, but a now method of construction whereby it is claimed larger boats can be built without difficulty has bsen devised by Mr. A. G. St. Clair Isbester, concrete expert, of Auckland. Jlr. Jsbester gave, a .Dominion reporter particulars of his improved designs and method of construction.

"Reinforced concrete has been accepted by all the leading experts us a suitable material for the building of ships of small dimensions," said Air. Isbester. "In the building of larger ships the designs and methods must be such as. recognise the proven weakness of concrete, mainly, (1) its low tensile standard; (2) the impossibility of preventing cracks from occurring when used in large jointless surfaces; (3) tho impossibility of testing tho structure so as to bo certain of every part coming up to the standard required, necessitating the allowanco of thickness in eicess of requirements to guard against a possible flaw. It is believed by engineers and marine architects of considerable standing that owing to the email amount of elasticity in concrete a ship would strain bersolf to pieces in a heavy sea. Many experts are of the opinion that the change from 'hogging' to 'sagging' experienced by a vessel when being driven against a heavy f'ea will be likely to dnmuge l the hull when mode in one piece, the sudden change from compression to extension alternately, not alloiving of the readjustment of the concrete to meet the varying etrain. "Now, admitting of the foregoing, it is my opinion," said Mr. Isbester, "that to i;et a successful concrete vessel of large dimensions, )he one-piece method of construction must be abandoned and pre-cast sections used; as no matter how the .enforcements are placed the different actions (under stress) of concrete and steel are such that in the case of varying changes in the direction and intensity of thrusts the effect upon them individually will bo Fo different as toi eventually deteriorate tho bond between them, defeating the purpose for which they were joined. Further, more, vessels frequently havj to stand the impact of heavy masses of water, the jar of which in a vessel with practically no life would have a tendency to cause splits or cracks in the concrete, especially where two sections join at an angle. "By my method of construction planks or plates of concrote are used, being joined by a dowel speciallv designed for the purpose, which dowel is forced into position hy pressure in preference, as jarring stich as caused by the use of a hammer is liable to damage the concrete planks. A prepared felt or other suitable caulking is used between the planks to ensure water-tightness of joint, and it also gives a degree of elasticity to the hull when finished. By using planks of concrete it is possible to have them properly cured and tested before use, consequently it is unnecessary to make them thicker than actually required. "In constructing tho hull of a vessel by my method, tha diagonal Drc-cast conCrete lining is arranged so" as to take | .the thrusts in the manner which concrete < is most adapted for. The framework oi!' the vessel i.s built of concrete so rein, forced and placed as to develop the best results under tensile strain, together with the plaiddng which takes the compression.

"The planks are cast in small pieces to avoid the cracks roused by expansion and contraction. This in further ensured by a degree of give and* take being- permitted of in the joint between plank and plank, through the felt or other caulking being softer than thot concrete. This also eliminates the jar caused by the impact of water, etc., which tlie hull might be subjected to. The moulds used to form the planks arc of simple construction, and are easily adapted to various shapes as desired. At the present tiino my niothod would have a great advantage, inasmuch as tho planks could bo cast, at various places where concrete manufacturers' plants aro situated, and sent to the building yards to be assembled. This system is Aioro especially suited to the building of standard-sized ships at present in vogue. "In building ehips by ray method, concrete is used in sizes comparable with every day work, tho method of jointing being such as permits of the actual (strength being used according to the success they aro intended for. "The method of tying planks to ribs, beams, etc., all allow of a degree of elasticity, insuring a livo ship adapting herself readily to the various strains which slip is subjected to. "As concrete is known lo expand in water and contract in tho air, it will be seen that ii,ship which is built by ono piece method has a tendency to shear at the water-line, necessitating the use of a great deal more reinforcement than would bo required where the unit method 01 construction is used; consequently I maintain that it is better to accept tUeso facts re expansion and contraction of concrete in shipbuilding as engineers have already done in tho building of bridges and houses. Furthermore, it haa been proved that tho concrete made in nnit sections under factory conditions cau compete favourably with, the onepieco method of construction, in regard to cost, and excels undor tests for .strength. The hardening of concreto is bo affected l>y the atmospheric conditions that tho advantages of factory methoda must be apparent to anyone with experience m the making of concrete. "In this system of construction the dowel used in securing the joint between tho concrete planks is of simple design, and is grouted in with a cument ivash which preserves the steel, Rnd also fille in any space which there might) be be. tween the .iowel and the concrete, consequently !ho plank develops the samo compressive strength in the cross section through the dowel as it does in the cross .section between the dowel in sections of concrete planking. Tho top edgo is set back the thickness of the plank, and to a depth of 1 inch, forminga rebate which takes tho lower edge of the adjoining board. This detail ensures the joint developing tho same resistance to compressive strain as is obtained in tho board which Ime been so joined. The tensile strain :s taken off tho dowels themsolves, which will bo spaced at euch centres as denned necessary from actual tests of the particular dowels to be used. Tho tendency of shearing in the- longitudinal joints of a ship is prevented by casting concreto planking without the. extended backing (which forme the rebate in tho planking) at such intervals as the ribs aro spaced at to the width of tho rib, so that the rib is run against the plank parallel to tho face of the bull. Before casing rite a piece of felt is placed in position so as to be between tho rib I iind the pinnks of the hull, and this jmrniits oi' a movement between them. I This niovemont, I claim, 16 certain from I tho different: conditions under which the I concrete Js placed, urio part being in ; water and tho other purt being subject to a! dry and possibly hot temperature. I In tbo onc-pieco mothod of construction the tendency in <rack at tho union be. tween the rib and tho outeido wall of I tho hull will bo very groat, necessitating I the use of reinforcement in excess of that required for atrungth of ship, as it wilt have lo rwist tho stress 6et up by tho I concreto itself.

"In oonchiswn," said Mr. Isbester, "I have treated concre'-o entirely on a basis iliat permits of fee advantageous development of its known qualities favourable to shipbuilding, eliminating the doubts entertnhgd in connection with. tlio knmro in the one-, piece method of construction. This makes it possible for a fhip'e architect to design and specify a concrete vessel with the samo dogroo of certainty as is obtained in vessels con&tructed of steel, iron, and wood. Furthermore, concrete has tha advantage of beinjj rustless, and would not require painting, as when the planks used in the hall are cast and cured they may be polished by the application of a suitable friction, thereby securing a smooth, glass-like surface, which

would leavo the water freflly without any hftng or drag. 'Ilieso pinnies being cast individually nre done in one operation, ensuring perfect bond throughout th« composition. Concrete as a material is \vell adapted for bn!k-li»ad.s and decks, as it does Jiot buckle, and if designed r» ihat 111/ strain which it ha.- to aland «;ill 1)6 con/prp'.H'iisivD, >'!iould prove u .solution of the wfltoi'-tight liuik-lii'n.-l constructiou which'is so much in denmud at tho present tinie." Mr. Isbeater has fiiibmiitcrl his design? to the New Zealand Government for it to do in the matter of constructing ships in Xew Zealand what it considers most advantageous in- meeting tho Umpires present urgent need of tonnage. No conditions whatever, Mr. Jsboslcr Enys, havo been made by him. On behalf of the Government several leading marine surveyors, architects, niu! other authorities on ship construction, have carefully examined Jlr. Isbestcr's improved plans, and thoy gave, him to understand that favourilb'lo reports would hr made. however, the decision of the Governmentis unfavourable, Mr. Isbester's method, or if it is decided that the building of concrete vessels 13 too bis a proposition to start-in New Zealand at this time, |ir Natal Adviser has promised to submit t lie idea to tho Admiralty. It is expected that the Government's decision will bo announced in a few days. It was stated in an interview that a reporter had wi!h an authority at Auckland that thn quantity of steel needed for building ships in concrete would,hamper (he work in New Zealand at. present owing to steel being an unprocurable commodity irr. Isbcstcr states that by Ins method only a small quantity of fteel (for the dowels) is needed, and Jhat supplies could be procured in New Zealand, as there is a firm in this country manufacturing email pieces of reinforcement steel out of scraps.