1 only one plate in the event of damage-a remote contingency which should not be allowed to interfere with good construction. Where a doubling is required by the classification societies' rules it will be found advantageous, where practicable, to increase the plate keel to a sectional area equivalent to that of the keel and doubling, and if double buttstrap be required, the inside one may be fitted in two pieces. Scantlings and riveting will be as specified or to rule requirements. At the forward and after ends the keel plate must efficiently incorporate with the stem and stern frame respectively, a short "breeches" plate being usually worked for this purpose. In small con 66 FIG. 136. struction a spoon "plate is welded to the bottom of stem bar in lieu of the short plate referred to, and a similar plate of "gutter" form welded to stern frame. STEMS. The remarks on bar keels apply equally as regards details to stems. The classification societies' rules allow a reduction in sectional area at stem heads, but as the practice is now to make the stem from universal rolled bar, it will prove no economy to taper it. The usual method of connecting lower part of stem to keel plate are shown by Fig. 137. In straight stems the profile line should be cambered about " to 3" from where it joins the forefoot curve to stem head, to guard against the illusion of the contour line appearing hollow. STERN FRAME. These frames are mostly forged or cast in steel in one piece for small and moderate sized steamers, and in two or more parts for the larger vessels. As in the case of stems, bar keels, etc., the scantlings are determined from the corresponding numeral of the societies' rules to which the ship is being constructed. The two posts comprising the stern frame, viz., rudder and body posts with the joining arch, are of similar scantlings, but the keel piece connecting the posts at bottom while of the same sectional area as the posts, is flattened out to allow of the keel line being curved upwards to the clump for keel pintle bearing of rudder for protection to the latter in the event of grounding. Gudgeons are forged on the rudder post of frame from 4 to 5 feet apart to take the pintles; one, or two in large vessels, being so shaped as to engage the rudder stop at hard-over. This post is connected to the main structure on a deep transom plate clipped to its fore side, and in vessels of over about 300 feet in length the forward or body post must also be carried up and secured in a similar manner. The body post is swelled around the stern tube, having a sectional area through the eye equal to the frame and meeting the post above and below in a fair curve; the spur or keel part of frame must not be too long to facilitate handling, the general rule being about 21 frame spaces before the body post, where it incorporates with, or scarphs into, the keel as already described. In steamers over 350 feet length where these frames are of considerable weight, the riveting connecting body post to hood ends of shell plating should be treble below boss and of increased diameter and an addition made to the plating thickness. As in the keels, these holes must be carefully drilled and where scarphs are introduced as in the case of frames of two or more pieces the riveted connection should be developed to equal the bar. It is common to make the contour of body post curvilinear, thus effecting an appreciable saving in weight over the straight line, besides giving a more graceful form. In small steamers the after or rudder post may be dispensed with, a spur being carried aft from body post to support heel pintle. For single screw steamers classed to Lloyds the weight of stern frame may be very closely approximated by taking the first numeral to upper deck and multiplying it by 240 for vessels over 300 feet in length, or by 155 for those under this dimension, as first number × 240 weight in pounds. = RUDDERS. Some of the more common forms of rudders are shown in Figs. 138 to 143. The stresses to which they are subjected and the method of determining the diameter of stock has already been fully described. The single plate rudder, Fig. 138, is the type most commonly adopted in merchant steamers, and is usually built in three parts, viz. the frame, norman head and plate. The frame may be either cast or forged, BOLTED COUPLING The norman head or stock should be forged in iron or steel with a coupling palm at lower end to connect with a similar palm on head of frame. Allowance should be made on this forging for machining a key to lock the norman head to the frame, and in addition turned coupling bolts are fitted with nuts on under side, threads turned off to a thimble point and split pins fitted. These bolts are from one to three inches in diameter in practice. Their size, however, is not important, as the shearing stresses are all taken on the key. The stock need only be turned in wake of the rudder quadrant where it is sometimes increased in diameter to compensate for cutting the key way. FUTT FIG. 138. The single plate forming the rudder blade is fitted between, and riveted to, the supporting arms, besides engaging a groove cut down the back of rudder stock. Its thickness ranges from about ?" in small steamers to 11 inches in liners. Braces are formed at the ends of supporting arms which are turned out to take fitted pintles. One (two in large rudders) of these braces must be shaped to act as a stopper when the rudder is put hard-over. The pintles should preferably be fitted separately and of the cone type shown in the detail. It is bad practice to forge pintles on the frame, as besides the difficulty of turning them in a lathe they have the disadvantage of not being readily renewable. The best manner of bushing the pintles is a matter of opinion, the simplest and probably the one most favored being to make the bushes of hard steel with a flange to take the tap screws securing them around the eye of the braces. The weight of the rudder in small vessels is taken on a hard steel disc placed in the heel step bearing with a hole through the heel step for backing it out. In large steamers, however, where the weight of rudder is many tons, the weight should always be taken by a carrier seated inside the counter. Various types of these are shown by engravings 144 to 146. Provision must be made on the back of rudder well clear of water line to fit a jew's harp shackle for securing the emergency chains, which are from thence carried up the counter, being stopped with ratline stuff to tapped eyes spaced about thirty inches apart. Next in favor to the single plate is the cast steel rudder, Fig. 139, although where only one is being made its cost is against it. For the largest sizes its difficulty of successful manufacture is also to its disadvantage, although this is got over by casting it in two or more pieces, see Fig. 140, keying these together and riveting them through coupling flanges. When rudders are designed to be cast in one piece, the ribs which are cast on the blade to act as stays should be of easy section, so as not to interfere more than necessary with the contraction of the casting in cooling. The oldest method of making the rudder for steel ships is the built type, Fig. 142, which consists of a forged frame having stock, stays, and back piece in one, with two side plates riveted to same after having the inside filled in with fir coated with tar. Its great objection is the cost of forging, especially for large rudders. It has gone completely out of favor unless for yachts, where its appearance commands its use, and in light craft of the torpedo boat kind where sufficient stiffness would not be obtainable in a single plate without going into a thickness which would make the weight prohibitive. It is also often used with the frame cast in gun metal and the side plates of 16 gauge brass sheet, for wood speed launches, vedettes, pinnaces, etc., although for these craft a cheaper and lighter rudder may be obtained by casting it complete in gun metal or bronze. |