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MARINE REVIEW. : : [June 13, CONSTRUCTION OF TORPEDO BOATS AND DESTROYERS. BY GEORGE HERBERT WILSON. PROPELLER STRUTS. The rudder applinaces having been well outlined and the steering details gone into, I will now turn to those important parts of the pro- pelling machinery, the propeller struts and shaft hangers. The great dependence placed upon these parts when the boat is under way demands that they be so constructed and so connected as to withstand the severe strains put upon them. In addition to these requirements they must be brought down to the minimum of weight, and the connections to and _ through the shell must be perfectly water-tight under the conditions inci- dent to high-speed boats. The designing of these parts, therefore, to meet the various requirements, and to stand the severe strains and tests put upon them, must be carefully gone into and considerable time and space can be devoted to them. The strains incident to the conditions required are principally those of thrust and vibrations. : : : When the excessive high speed of these 'boats is taken into considera- tion, with the great number of revolutions necessary to the same, some ss Clo -4 °| /o-Les TEXTE RZ Las. 2k 52¢x2-Lane ° So Llosa ole ea /5- Las. oft? z= 2 £ > x ' «x 252 5" ' Hi a) : Ono oh DEO 250! ce pees Ose. On On Oe nO 0 oye ae O20 Ofeq, vy ' le ? ° ° No af: © of of MeO O Jefe! ee of az: , 2 ° ofo ° of z os Oo 2 : a 2 ollo Slee. delfic v Oi». oO: z > = | oO Barve. 'Nolllo ge oO lINY 3 8-2 ollo olllo pe = allo = ° ° ° O10 ° ° ° o ° ol ° 24.392 lO 0 ° egos let. /Gan az passe (Shiec 5 | Ceo ces Pee ae [b= ee ee JM) idea of the constant vibration taking place on these parts may be gained. The thrust of the propeller is, of course, taken up at the thrust block, but there is a certain amount of thrust in the transverse direction which is constant and varying in locality, being the principal cause of this vibration. The weight of the shafting and the propellers is, of course, carried by the strut, and this weight increases in moment as the vibrations multiply until it reaches a maximum, at which point the strut must resist the strains with a considerable factor of safety. In outlining the different constructions employed and the various types adapted, I will take them separately, dealing with the propeller struts first and the shaft hangers later. The propeller strut, the more important of the two, takes its name from its nearness to the propeller, but is some- times known as a shaft strut, as that is its principal function. The first and principal matter for consideration is the material to 'be employed in the construction of the strut. The general practice has been to use ham- mered scrap iron, but of late there has been a tendency to use cast steel, and this practice has again been followed by using a strut built up of plates and bars, carrying the hub casting. As regards these different materials the first has the advantage of being very strong and rigid, but rather heavy and very expensive. The second has sufficient strength, but it is gained at the expense of weight; it has also the advantage of tbeing less expensive. The third is built up to the required strength, which also brings up the expense to about that of the first, but it has the great ad- vantage of being lighter than either of the two. Dividing the question of material into three headings, namely strength, weight and cost, I will deal with each separately to a certain extent. In deciding the relative properties of the materials generally em- ployed, it is a well known fact that great dependence is to be placed in hammered scrap iron, but the difficulty in forging such a shape as a strut and making the proper welds creates a certain amount of doubt as to the presence of flaws and other imperfections. With a perfect forging, how- ever, a more satisfactory piece of work could hardly be desired. The question of weight in these fittings is of very great importance, coming as they do at so great a distance abaft the center of flotation, and having a tendency to trim the boat by the.stern. Ina forging the shape and size of the average strut a great deal of weight is necessarily entailed and some of this weight could be dispensed with, as will be shown, in another method. To gain the best effects of a forged strut as regards material, the expense of a good forging is relatively high and the price has a great tendency to fluctuate, while delivery of such goods is a ques- tionable item. The past demand for forgings of all descriptions, com- bined with the high price of iron, are examples of what may be expected at any time. : : ce In dealing with the next material, cast steel, a different proposition is at hand. Owing to the shape of the casting there are places where blow- holes, especially in this material, generally appear and it may take a great many castings and the relative number of molds, before a perfect one is secured. If these imperfections are discovered in the foundry, and not after machining, a great deal of time to the purchaser and money to the producer is saved. The location of these blow-holes is generally where a large mass of material is connected to a smaller mass, such as the hub to the strut arms and the palms to the strut arms. I have frequently found this to be the case. In some instances the fillet between the arm and the hub was like a honeycomb and in others the imperfections extended well down towards the center of the hub. These conditions can be remedied, but as I said before at the cost of time and money. In dealing with cast steel, the area of section should be increased over that of a forging, as the factor of safety is less. This naturally increases the weight over that of the forged stut, much to the disadvantage of the boat. In the matter of expense this strut has the advantage of the forging, as regards the price per pound, but the question of time of delivery has also to be considered and where "time is money" this is of vast importance. Proceeding now to the third and last construction employed, in which cast and wrot steel are used, I will describe its relative merits as regards strength, weight and cost. In the matter of strength it merely depends on the thickness of the*plating used and the distribution of the metal em- ployed. The question of flaws and blow-holes are hardly to be considered in this construction, care being exercised in the selection of the plates, angles and rivets used in building it up. The plates used in forming the strut arms following the contour of a pear-shaped section, leave a hollow arm, and the decrease in weight is of considerable import. The distribu- 'tion of the metal, however, gives equal strength to the strut arms pre- viously mentioned. The shaft bearing, or hub, is made of cast steel and from the nature of its shape it is not difficult to obtain a good casting. In the matter of expense attached to this style of construction, the "time clause" is of no great import, as the majority of the parts can be taken out of stock, the "hub" being the only part about which there is any ques- tion. The "hub," however, not being a difficult casting, can be relied upon for quick delivery. With the elimination of the "time clause" must neces- sarily follow a decrease in expense. The principal item of expense attached to this type of strut is in the labor and where a proper equipment of tools is at hand this can be reduced to a minimum. Taking labor and material into consideration it still remains for this type of strut to lead the others in cheapness, especially where the demand for forging and cast- ings is great. The question of making a water-tight connection where the strut pierces the structure demands considerable attention and is still the bone