I trust that engine builders will make a step further in the fitting of these liners on these shafts, as it is against the interest of the shipowner to keep ships in dry dock from such causes as defective liners, and I think it will be only a matter of time when the screw shaft will be completely protected from sea water, at least inside the stern tube; and when this is done, I would have no hesitation in using steel for screw shafts. Though an easier forging than a crank shaft, these shafts are often liable to flaws of a very serious character, owing to the contraction of the mass of metal forming the coupling; the outside cooling first tears the center open, and when there is not much metal to turn off the face of the coupling, it is sometimes undiscovered. Having observed several of these cavities, some only when the last cut was being taken off, I have considered it advisable to have holes bored in the end and center of each coupling, as far through as the thickness of the flange; when the shafts are of large size, this is sure to find these flaws out. Another flaw, which has in many cases proved serious when allowed to extend, is situated immediately abaft the gun metal liner, in front of the propeller.
This may be induced by corrosion, caused by the presence of sea water, gun metal, and iron, assisted by the rotation of the shaft. It may also be caused under heavy strain, owing to the over-finishing of the shaft at this part under the steam hammer.
The forgemen, in these days of competition and low prices, are instructed to so finish that there won't be much weight to turn off when completing the shaft in the lathe. This is effected by the use of half-round blocks under the hammer, at a lower temperature than the rest of the forging is done, along with the use of a little water flung on from time to time; and it is remarkable how near a forging is in truth when centered in the lathe, and how little there is to come off. The effect of this manipulation is to form a hard ring of close grain about one inch thick from the circumference of the shaft inward. The metal in this ring is much harder than that in the rest of the shaft, and takes all the strain the inner section gives; consequently, when strain is brought on, either in heavy weather or should the propeller strike any object at sea or in the Suez canal, a fracture is caused at the circumference. This, assisted by slight corrosion, has in my experience led in the course of four months to a screw shaft being seriously crippled.
I show you a section of a screw shaft found to be flawed, and which I had broken under the falling weight of a steam hammer, when the decided difference of the granules near the circumference from that in the central part conveyed to me that it was weakened by treatment I have referred to. I think more material should be left on the forging, and the high finish with a little cold water should be discontinued. Doing away with the outer bearing in rudder post is an improvement, provided the bearing in the outer end of screw shaft in the stern tube is sufficiently large. It allows the rudder post to have its own work to do without bringing any strain on the screw shaft, and in the event of the vessel's grounding and striking under the rudder post, it does not throw any strain on the screw shaft. It also tends to reduce weight at this part, where all the weight is overhung from the stern of the vessel.
A paper read before the Institute of Marine Engineers, Stratford, 1889.
EXPERIMENTAL AID IN THE DESIGN OF HIGH SPEED STEAMSHIPS.
By D. P.
The achievement of one triumph after another in the matter of high speed steamships, and especially the confidence with which pledges of certain results are given and accepted long before actual trials are made, form one of the most convincing proofs of the important part which scientific methods play in modern shipbuilding. This is evident in the case of ships embodying novel or hitherto untried features, and more especially so in cases where shipbuilders, having no personal practical experience or data, achieve such results. This was notably illustrated in the case of the Fairfield Co. undertaking some five years ago to build and engine a huge craft of most phenomenal form and proportions, and to propel the vessel at a given speed under conditions which appeared highly impracticable to many engaged in the same profession. The contract was proceeded with, however, and the Czar of Russia's wonderful yacht Livadia was the result, which (however much she may have justified the professional strictures as to form and proportions) entirely answered the designer's anticipations as to speed. Equally remarkable and far more interesting instances are the Inman liners City of Paris and City of New York, in whose design there was sufficient novelty to warrant the degree of misgiving which undoubtedly existed regarding the Messrs. Thomson's ability to attain the speed required. In the case at least of the City of Paris, Messrs. Thomson's intrepidity has been triumphantly justified. An instance still more opposite to our present subject is found in the now renowned Channel steamers Princess Henrietta and Princess Josephine, built by Messrs. Denny, of Dumbarton, for the Belgian government. The speed stipulated for in this case was 20½ knots, and although in one or two previous Channel steamers, built by the Fairfield Co., a like speed had been achieved, still the guaranteeing of this speed by Messrs. Denny was remarkable, in so far as the firm had never produced, or had to do with, any craft faster than 15 or 16 knots. The attainment not only of the speed guaranteed, but of the better part of a knot in excess of that speed, was triumphant testimony to the skill and care brought to bear upon the undertaking. In this case, at least, the result was not one due to a previous course of "trial and error" with actual ships, but was distinctly due to superior practical skill, backed and enhanced by knowledge and use of specialized branches in the science of marine architecture. Messrs. Denny are the only firm of private shipbuilders possessing an experimental tank for recording the speed and resistance of ships by means of miniature reproductions of the actual vessels, and to this fact may safely be ascribed their confidence in guaranteeing, and their success in obtaining, a speed so remarkable in itself and so much in excess of anything they had previously had to do with. Confirmatory evidence of their success with the Belgian steamers is afforded by the fact that they have recently been instructed to build for service between Stranraer and Larne a paddle steamer guaranteed to steam 19 knots, and have had inquiries as to other high speed vessels.