PART VI SECTION A.-DETAILS OF LIFTING HOOKS, KEEL PLATES, CHAIN SLINGS, ETC. Lifting Hooks. The fitting of suitable lifting hooks in a lifeboat is of the greatest importance. Provision is made to meet the ordinary conditions of lowering a boat over the side of a vessel into the water, with a disciplined crew, and, in addition, the "unusual" or "extraordinary extraordinary" circumstance has to be well considered. The passengers may be in a condition of nervous excitement or panic during the operation of lowering, with a heavy sea running, and the work of launching the boat may be undertaken with the help of untrained persons. In such circumstances, stresses may be exerted on the lifting hooks which under ordinary conditions would not be anticipated. Therefore, in the first place, the lifting hooks and their attachments must be designed so as to be strong enough to carry the weight of the boat, full equipment, the total number of persons appropriated to the boat, and having in reserve a factor of safety. This factor of safety is the only feature which is open to argument or criticism, as its magnitude is intended to cover all requirements and the "unusual' circumstance. Little attention at one time was paid to this important question when fitting out lifeboats intended for cargo vessels, and until within a matter of three or four years ago it was quite the common practice to insert welded lifting hooks having shanks which were driven through the keelson and keel, and simply clenched over a small plate about in. in thickness. The weight of some three or four tons was therefore hanging in mid-air, which depended entirely on the efficiency of a very doubtful clench. To say that the same principle was commonly carried out in the case of lifeboats for passenger vessels would be an exaggerated statement, but, nevertheless, the quality of the lifting hook connections was often open to serious doubt. Recent regulations issued by the Board of Trade, it is hoped U will prevent the difficulties, to which reference has been made, occurring in the future. "Safety first" is a warning which has riveted itself on the minds of many people in Great Britian, through their intimate contact with the posters displayed on the municipal cars, and it is one that is well advertised throughout the United States of America, and should be made applicable to all the various considerations associated with the life-saving appliances on board a merchant vessel. The present-day type of lifting hook may look heavy in appearance as compared with hooks used for other purposes in the shipyard, and particularly with those fitted in lifeboats previous to the issue of definite regulations, but in dealing with ships' lifeboats, there should be no doubt on the question of strength, and every opportunity taken to avoid error or the slightest appearance of weakness, especially when discussing the suitability of lifting hooks. Before further investigation, the instructions issued by the Board of Trade in July, 1916, should be considered. Recent experience has added much to these requirements, but they still form the basis of the regulations, and are as follows: "All sling hooks fitted in open boats should be of the fixed type, unless, on the special application of the owners, some other approved type of hook is adopted. All hooks must be of ample strength having regard to the load carried, and should be of wrought iron or steel. Welded hooks should be of wrought iron. "The proportions of all parts taking the weight of the boat should be such as to provide a factor of safety not less than four, and the arrangement should be strong enough to carry the boat, equipment, and full load of persons with that factor. Unless the bolt and fittings are tested as described below, the material is to be assumed to have an ultimate strength in tension not exceeding 18 tons per square inch. "If a higher tensile strength is claimed for the material, or if the surveyor is not satisfied that the strength is sufficient, the lifting hook, together with the sling bolt and fittings for attachment to the boat, is to be subjected to a proof test of at least the total weight of boat, equipment and persons. (Modified by recent instructions, see Scantlings of Lifting Hooks.) A sample should also be tested to destruction to determine the approximate factor of safety. In welded parts, the calculated tensile stress is not to exceed three tons per square inch with the dead working load. 66 Sling hooks depending for their security upon a clenched head alone, are not to be passed in new boats in future. In all lifeboats exceeding 24 ft. in length, the sling hooks should be secured to eye-plates or to clamps of wrought iron or steel fitted on the top of the keelson, and fastened to it by iron or steel screw bolts nutted on the inside of the boat, and the bolts upset. The heads of the bolts should be forged with ample bearing surfaces, and should bear on a substantial plate fitted on, but not let into, the underside of the keel. Any other method equally satisfactory may, however, be adopted. When the lifting hooks are required to be so near the ends of the boat that they cannot be directly connected to the keelson and keel, special strengthening arrangements should be fitted to prevent the boat being strained when it is lifted. Means should be provided for preventing the hook bolts from turning. Before we can successfully determine the correct sizes of lifting hooks, to enable them to withstand the effect of a certain definite dead load, it is essential that sufficient and accurate data should be at our disposal upon which to base the scantlings. We need to go beyond the investigations of theoretical calculations and secure this data from the results of actual tests. At the request of the writer, Messrs. Scott's Shipbuilding and Engineering Co., Ltd., Greenock, very kindly carried out a series of extended tests on various sizes of lifting hooks, connected to different types of keel plates. The firm went to a great deal of trouble to secure sufficient information upon which to base a full scheme of scantlings for sizes of lifting hooks that would be strong enough for the purpose required. The boats were classified into lengths ranging from 15 to 30 ft. With each length were associated dimensions considered to be of maximum proportions. Upon these dimensions the maximum dead load coming upon the lifting hooks was obtained, which included the weight of the boat, equipment and full complement of persons. As an example, take an open lifeboat of Class I. of the following dimensions, which is not to standard size, but considered as extreme or of limiting proportions, viz. length 26 ft., breadth 85 ft., and depth 3-5 ft. The total load carried by the lifting hooks would be 5'4 tons; each hook would therefore have to sustain 27 tons under working conditions. In other words, certain maximum loads were allotted to each particular length of boat, and the sizes of the hooks were appropriated in accordance with these loads. From the results of these tests the sizes of lifting hooks and keel plates, given in Table XIX., were based. The factor of safety was three on the elastic limit. With riveted joints and other combinations it is somewhat difficult to obtain the elastic limit during the process of testing, and it is usual for the factor of safety to be based on the ultimate breaking strength, which is known as the nominal factor of safety. Breaking Stress Nominal Factor of Safety = Working Stress When we come to solid drawn lifting hooks made from cable iron, where the difficulty of obtaining the elastic limit is not so great, it is very evident that immediately this point is passed, alteration to form and permanent set take place. If the load which produced permanent set, or the stress which came upon the material at the elastic limit, was not increased, but allowed to continue for a sufficient length of time, the hook would eventually straighten out so that the actual strength of the bolt is the elastic limit or yield point, and the ratio between this and a working stress is termed the true factor of safety. True Factor of Safety Stress at elastic limit From the point of view of obtaining data for the purpose of formulating a scale of sizes for lifting hooks, testing material to destruction after once the elastic limit has been reached does not provide us with much useful information, beyond giving us the additional load which is required to break the hook, or produce fracture within a reasonable length of time. The difference between the stress which discovers the elastic limit and that which produces fracture, is the reserve strength of the hook beyond the elastic limit. No useful purpose will be served by the insertion of any details of the tests referred to, these were very carefully carried out with small increasing loads to ascertain the elastic limit of each individual hook, and the deflections were measured and tabulated. Considerable amount of work is put on the largest type of hooks in forming them into the required shape, the effect of which was made evident at the test. The material at "A," Fig. 163, immediately in way of the critical section, was under a very acute stress due to the bending moment, and consequently became fatigued. When a certain load was reached it produced a fracture to a depth of about half an inch, the load was increased, and after the hook was fully broken the fatigued portion at "A" was found |