Chapter XIX: Appendix: Calculating Instruments (2)

[4] For full and excellent treatment of this subject, see the paper on “Causes of Unseaworthiness in Merchant Steamers,” by Mr Benjamin Martell, Chief Surveyor to Lloyd’s Register, with the ensuing discussion: Trans. Inst., N.A., vol. xxi., 1880.

Several of the causes above named it is doubtless the province of the scientific shipbuilder, and the duty of the shipowner, to obviate by furnishing the captain and officers—especially in the case of entirely new vessels—with particulars and data of the vessel’s technical character, such as are now left to be found out by slow and sometimes bitter experience. Of these it may be sufficient to instance:—Stability, steadiness, trim, carrying capability, and steaming powers. Mr William Denny, of Dumbarton, has recently publicly declared his firm’s intention of supplying such particulars to the vessels built by them. It is to be hoped this worthy example may be extensively followed.

[5] In this, as in other matters dealt with, the full appreciation of which involves careful technical study, readers are referred to the papers enumerated at the end of chapter, as well as to the “Manuals” already referred to in this work.

[6] The principle which underlies the experiment is this:—If any one body forming part of a system of heavy bodies be moved from one position in the system to another, the weight of the body moved multiplied into the distance through which it is moved, is precisely equal to the weight of the whole system of bodies multiplied into the distance through which the common centre of gravity of the whole has moved. If in a ship, therefore, a movable weight of known amount is moved across the deck through a given known distance, the centre of gravity of the ship itself, with all on board, has been moved in a line parallel to that through which the small weight has been transferred, and through a distance inversely proportioned to the weight of the whole ship to the weight moved. If, for instance, a weight of five tons should be moved through a distance of twenty feet, then multiplying this weight into this distance and dividing by the total weight of the ship, the distance through which the ship’s centre of gravity has travelled parallel to the deck is obtained. If, at the same time, an exact measure of the angle through which the ship has been inclined by moving the five tons through the distance named has been taken, and the position of the ship’s metacentre has been obtained, then the elements of a triangle are known—namely, the degrees in each of its angles, and the length of one of the sides—and from these the length of the remaining sides of the triangle is easily deduced. One of these sides will be the distance between the metacentre of the ship and its centre of gravity, and, consequently, the metacentre being known from calculation, the position of the centre of gravity becomes known also.

[7] The classification of strains here given is as contained in White’s “Manual of Naval Architecture.” To this authoritative source readers must turn who wish a full exposition of the several problems go shortly dealt with in these pages.

[8] This will be more fully referred to further on, but it may be stated here that the need for independent calculation is largely obviated, owing to the existence of “co-efficients,” deduced from investigations made by experts. Further, the existence and influence of the Registration Societies are such that the codes of scantling and the structural supervision instituted by them together constitute the only guarantee of structural strength generally desiderated.

[9] Suppose the dimensions of a proposed vessel to be 320 × 36 × 26½ feet, then, according to a method of approximation largely in use, the sum of these dimensions divided by 100 gives what is known as the “cubic number”—(320 × 36 × 26½ ÷ 100) = 3052 cubic number. Suppose that for a vessel already built, similar in type and dimensions, or of similar proportions, to the one proposed, the cubic number, when divided into the ship’s actual weight (_i.e._, the displacement _minus_ the weight of machinery and the dead-weight carried), gives say ·53, then this figure represents the “co-efficient” of ship’s weight, and applying it in the case supposed gives:—3052 × ·53 = 1620, the weight of hull for proposed vessel. This example illustrates the manner in which the weight of machinery is estimated, and indicates the nature and use of the general term “co-efficient:” frequently employed in this chapter.

[10] One such method, devised and followed by Mr C. Zimmermann in his daily practice as chief draughtsman to the Barrow Shipbuilding Company, and described by him before the Institution of Naval Architects in 1883, gives with very little preliminary calculation, and at once, a close approximation to the correct displacement. Another system, originated and used in practice by Mr Chas. H. Johnson, chief designer to Messrs Wm. Denny & Brothers, consists of an analysis of the lines of vessels of various degrees of fineness and fulness previously built, formulated for daily use in a series of curves of areas, giving, for sections at certain fixed distances from midships—in terms of percentage to the midship area—the particular area specially suited to afford the required displacement; and at the same time to maintain the general form of hull which in actual practice has proved satisfactory with respect to speed. In his later practice, Mr Johnson has found it preferable to use the block form of analysis of Mr A. C. Kirk (considered further on in matters relating to speed), using the three sides of that form as a basis upon which to group the water-lines.

[11] For illustrated descriptions of this and other improved calculating instruments referred to in this chapter, see Appendix.

[12] This experimental method, it may be explained, has long been practised in connection with ships built for the Royal Navy, and for a considerable number of years it has been systematically followed in some leading merchant shipyards. Messrs A. & J. Inglis, Pointhouse, Glasgow, and Messrs Wm. Denny & Bros., Dumbarton, were amongst the earliest firms to systematically adopt the practice. With the former it has been customary to incline every vessel of distinctive type built by them since 1871, and with the latter the practice has been constantly followed from a date somewhat subsequent. For some years past other firms on the Clyde and elsewhere have adopted the method, the data so accumulated being found an admirable basis from which to estimate the height of the centre of gravity in proposed vessels. Tables giving the results of inclining experiments made on various types of merchant steamships and sailing vessels will be found in “White’s Manual of Naval Architecture,” pages 82-87.

[13] From the first volume (1860) of the Transactions of the Institution of Naval Architects, it is seen that Dr Inman, Samuel Read, and Dr Woolley had each already found different methods of simplifying Atwood’s calculations.

[14] Various other methods of simplifying the calculations based on Atwood’s theorem were subsequently proposed, and one or two different methods also brought forward—notably one in 1876 by the late Mr Charles W. Merrifield, afterwards improved by the late Professor Rankine, and one by Mr J. Macfarlane Gray, of the Board of Trade, described by that gentleman in 1875, but since considerably improved. Most of them were laid before the Institution of Naval Architects in papers which will be found enumerated in the list at end of chapter. While such propositions did not contribute directly to bring the problem of stability to its presently accepted form, they deserve to be remembered as tokens of the great labour and skill which have been expended in founding and developing this branch of scientific naval architecture.

[15] “On Cross-Curves of Stability; their Uses, and a Method of Constructing Them, Obviating the Necessity for the Usual Correction for the Differences of the Wedges of Immersion and Emersion.”

[16] A detailed description of this valuable instrument will be found in Appendix.

[17] Space forbids any detailed reference to these, but the names of the papers and their respective authors will be found enumerated in the list at end of chapter.

[18] An obvious means of dealing approximately with stability, to which limits of space will not permit more than simple reference, consists in so manipulating the data obtained by calculation for known ships that it may be made available, either in the form of curves or of tables, for determining the stability of proposed vessels. Methods of accomplishing this may of course vary to suit the ideas and convenience of designers. A well-arranged system was brought forward, jointly by Mr F. P. Purvis, head of Messrs W. Denny & Brothers’ scientific staff, and Mr B. Kindermann, one of his assistants, in a paper (see list at end of chapter) read before the Institution of Engineers and Shipbuilders in April last. While the results exhibited in the paper are immediately applicable to ships of one particular form, whatever the length, breadth, depth, or draught may be, this method still requires much development to make it at all universally applicable.

[19] It is the usual practice to assume vessels to be laden with homogeneous cargo of such a density as to fill the holds, and for this condition to estimate the position of centre of gravity to be used in calculation.

[20] See paper by Mr Kirk “On a Method of Analysing the Forms of Ships and Determining the Lengths and Angles of Entrance.”—Trans. Inst. N.A., vol. xii., 1880.

[21] With the view of effecting an economy in time, and to enable the trials at progressive speeds to be carried out while vessels are in a lengthened run out to sea, a method has been proposed by Mr J. H. Biles, naval architect to Messrs J. & G. Thomson, and adopted on board the vessels tried by that firm, and also experimented with on some of the vessels turned out by Messrs W. Denny & Bros., by which the necessity for running with and against the tide on the measured mile is entirely obviated. The principle of the method is to measure the time that a certain part of the length of the ship takes to pass an object thrown from the bows of the vessel well clear of the side. For full particulars, both of the apparatus employed and of the results of actual trials by this method compared with trials made on the measured mile, see paper on “Progressive Speed Trials,” by Mr Biles, in the Transactions: Institution of Naval Architects, vol. xxiii., 1882.

[22] A general outline of the operations conducted in Messrs Denny’s tank will be found in the description of their large works in Chap. VI. For a detailed account of the _modus operandi_ in the same establishment, see abstract of a paper delivered in Dumbarton by Mr E. R. Mumford, of Messrs Denny’s Experimental Staff, printed in the _Engineer_ for 15th February and the _Steamship_ for 15th February of the present year.

[23] From experimental data obtained by Mr Froude, this correction can be made with certainty. The reasons for it may be explained as follows:—If an extremely thin short plane is drawn through the water it meets a certain resistance due entirely to surface-friction; that is, supposing the plane to be thin enough to eliminate wave-making and eddy-making. If the length of the plane is doubled while the depth is kept the same, the resistance at the same speed is not, as might at first appear to be the case, doubled accordingly. Owing to the friction of (say) the first half of the plane, the water is made to partake of the motion of the plane, so that the second half of the length, rubbing not against stationary water, but against water partially moving in its own direction, does not experience so much resistance from it. Adding a third equal length, it would have less surface friction than the second, and so on to infinity.

[24] See papers by Mr Mansel, enumerated in list at end of chapter.

[25] For description of apparatus, see Trans. Inst. Mechanical Engineers, 1877.

[26] A body which shortly afterwards joined with a kindred society in forming the “Institution of Engineers and Shipbuilders in Scotland,” hereafter noticed.

[27] Following the methods laid down in the Treatise on Shipbuilding, edited by Prof. Rankine, Mr John Inglis, Pointhouse, instituted calculations in 1873 of the longitudinal strains of two steamers built by his firm, the form of the waves being assumed trochoidal. The result of these calculations—which, under Mr Inglis’ directions, were got out by Mr G. L. Watson, subsequently distinguished as a yacht designer, and then in the employ of Messrs Inglis—appeared in the form of curves of hogging moments in _Engineering_ for 1st May, 1874. Mr Inglis found that entering upon the work of calculation had a very decided effect in giving him clearer ideas of how distribution of weight and buoyancy affected the structure of a vessel.

[28] The substance of this paper is contained in a series of three articles on the Strength and Strains of Ships given in “Naval Science” (vol. i. and ii., 1872-3), a high-class journal ably edited by Sir E. J. Reed, but unfortunately abandoned after the fourth year of publication.

[29] It should be stated that under certain circumstances of lading and support the value assigned by Mr John for the maximum bending moment may be exceeded in merchant vessels, and that in some special classes of ships—particularly light-draught vessels in certain circumstances of lading and support—the sagging moment may prove of most consequence. Instances are indeed on record of light-draught vessels giving way completely under the excessive sagging strain brought upon them at sea.

[30] The Institution of Engineers and Shipbuilders in Scotland was formed in 1865 through the amalgamation of two separate bodies—“The Institution of Engineers in Scotland” and “The Scottish Shipbuilders’ Association.” The former of these was founded in 1857 and the latter in 1860, the same year in which “The Institution of Naval Architects” was established. The membership of the Institution at the present time numbers nearly seven hundred, and comprises honorary members, members, associates, and graduates: the latter being a special section of the Institution, designed to embrace students or apprentices in the profession, and fulfilling a very useful end. The various offices have long been filled by gentlemen more or less actively engaged in the practice of shipbuilding or of engineering on the Clyde, and the proceedings have assumed, on this account alone, a richer practical interest. Scientific subjects have also received their share of attention, and of the members taking the lead in this connection the names of Mr J. G. Lawrie and Mr Robert Mansel are worthy of special mention. Along with Mr Robert Duncan and Mr Lawrance Hill, these gentlemen have, from the foundation of the Institution, taken a specially warm interest in its prosperity, and have contributed not a little thereto by the numerous valuable papers they have brought before its meetings. The secretary of the Institution is Mr W. J. Millar, C.E., himself the author of numerous papers, and the editor of the Transactions.

[31] For interesting and reliable information on this head, as well as on other matters dealt with in this and the preceding chapter, see Sir E. J. Reed’s excellent treatise on “Shipbuilding in Iron and Steel.”

[32] This method of graphically representing tonnage output was applied for the first time by the author to the Clyde district from the figures supplied by the _Glasgow Herald_ for each of the years since 1860, and appeared, with much of the descriptive matter now given, in the issue of that journal for March 4th of the present year.

[33] The following fragmentary returns have, through the kindness of a friend engaged in shipbuilding on the Tyne, been forwarded while those sheets were in the press. They have been gathered from occasional records in the local press, supplemented by personal knowledge, but may only be taken as approximate:—

Year. | No. of Vessels. | Tons.
| |
1864 | 97 | 49,820
1865 | 123 | 77,500
1866 | 110 | 51,800
1867 | 81 | 34,080
1868 | 86 | 45,390
1869 | — | ——
1870 | 95 | 86,420
1871 | — | ——

[34] These instruments, and the others here noticed, are supplied in this country by Mr W. F. Stanley, the noted scientific instrument maker of Great Turnstile, Holborn, London. They are described in his treatise on “Mathematical Drawing Instruments,” from which work, it should be stated, some of the present notes concerning them are derived. A source of accurate information on the theory of planimeter, to which Mr Stanley himself expresses indebtedness, is the paper by Mr —now Sir—F. J. Bramwell, read before the British Association in 1872, and contained in the Association Reports for that year.

[35] The following is a list of the multipliers for converting the planimeter readings to square feet for any required scale:—

1/16-in. scale = 256
⅛-in. do. = 64
3/16-in. do. = 28·44
¼-in. do. = 16
5/16-in. scale = 10·24
⅜-in. do. = 7·11
½-in. do. = 4·00
¾-in. do. = 1·77
1-in. scale = 1·00
1½-in. do. = ·44
3-in. do. = ·111

BOOKS RELATING

TO

APPLIED SCIENCE

PUBLISHED BY

E. & F. N. SPON,

LONDON: 125, STRAND.

NEW YORK: 35, MURRAY STREET.

_A Pocket-Book for Chemists, Chemical Manufacturers, Metallurgists, Dyers, Distillers, Brewers, Sugar Refiners, Photographers, Students, etc., etc._ By THOMAS BAYLEY, Assoc. R.C. Sc. Ireland, Analytical and Consulting Chemist and Assayer. Third edition, with additions, 437 pp., royal 32mo, roan, gilt edges, 5_s._

SYNOPSIS OF CONTENTS:

Atomic Weights and Factors—Useful Data—Chemical Calculations—Rules
for Indirect Analysis—Weights and Measures—Thermometers and
Barometers—Chemical Physics—Boiling Points, etc.—Solubility of
Substances—Methods of Obtaining Specific Gravity—Conversion
of Hydrometers—Strength of Solutions by Specific
Gravity—Analysis—Gas Analysis—Water Analysis—Qualitative Analysis
and Reactions—Volumetric Analysis—Manipulation—Mineralogy—
Assaying—Alcohol—Beer—Sugar—Miscellaneous Technological matter
relating to Potash, Soda, Sulphuric Acid, Chlorine, Tar Products,
Petroleum, Milk, Tallow, Photography, Prices, Wages, Appendix,
etc., etc.

_The Mechanician_: A Treatise on the Construction and Manipulation of Tools, for the use and instruction of Young Engineers and Scientific Amateurs, comprising the Arts of Blacksmithing and Forging; the Construction and Manufacture of Hand Tools, and the various Methods of Using and Grinding them; the Construction of Machine Tools, and how to work them; Machine Fitting and Erection; description of Hand and Machine Processes; Turning and Screw Cutting; principles of Constructing and details of Making and Erecting Steam Engines, and the various details of setting out work, etc., etc. By CAMERON KNIGHT, Engineer. _Containing 1147 illustrations_, and 397 pages of letter-press. Third edition, 4to, cloth, 18_s._

_On Designing Belt Gearing._ By E. J. COWLING WELCH, Mem. Inst. Mech. Engineers, Author of ‘Designing Valve Gearing.’ Fcap. 8vo, sewed, 6_d._

_A Handbook of Formulæ, Tables, and Memoranda, for Architectural Surveyors and others engaged in Building._ By J. T. HURST, C.E. Thirteenth edition, royal 32mo, roan, 5_s._

“It is no disparagement to the many excellent publications we
refer to, to say that in our opinion this little pocket-book of
Hurst’s is the very best of them all, without any exception. It
would be useless to attempt a recapitulation of the contents, for
it appears to contain almost _everything_ that anyone connected
with building could require, and, best of all, made up in a compact
form for carrying in the pocket, measuring only 5 in. by 3 in.,
and about ¾ in. thick, in a limp cover. We congratulate the author
on the success of his laborious and practically compiled little
book, which has received unqualified and deserved praise from every
professional person to whom we have shown it.”—_The Dublin Builder._

_Tabulated Weights of Angle, Tee, Bulb, Round, Square, and Flat Iron and Steel_, and other information for the use of Naval Architects and Shipbuilders. By C. H. JORDAN, M.I.N.A. Fourth edition, 32mo, cloth, 2_s._ 6_d._

_Quantity Surveying._ By J. LEANING. With 42 illustrations, crown 8vo, cloth, 9_s._

CONTENTS:

A complete Explanation of the London Practice.
General Instructions.
Order of Taking Off.
Modes of Measurement of the various Trades.
Use and Waste.
Ventilation and Warming.
Credits, with various Examples of Treatment.
Abbreviations.
Squaring the Dimensions.
Abstracting, with Examples in illustration of each Trade.
Billing.
Examples of Preambles to each Trade.
Form for a Bill of Quantities.
Do. Bill of Credits.
Do. Bill for Alternative Estimate.
Restorations and Repairs, and Form of Bill.
Variations before Acceptance of Tender.
Errors in a Builder’s Estimate.
Schedule of Prices.
Form of Schedule of Prices.
Analysis of Schedule of Prices.
Adjustment of Accounts.
Form of a Bill of Variations.
Remarks on Specifications.
Prices and Valuation of Work, with Examples and Remarks upon each Trade.
The Law as it affects Quantity Surveyors, with Law Reports.
Taking Off after the Old Method.
Northern Practice.
The General Statement of the Methods recommended by the Manchester
Society of Architects for taking Quantities.
Examples of Collections.
Examples of “Taking Off” in each Trade.
Remarks on the Past and Present Methods of Estimating.

_A Practical Treatise on Heat, as applied to the Useful Arts_; for the Use of Engineers, Architects, &c. By THOMAS BOX. _With 14 plates._ Third edition, crown 8vo, cloth, 12_s._ 6_d._

_A Descriptive Treatise on Mathematical Drawing Instruments_: their construction, uses, qualities, selection, preservation, and suggestions for improvements, with hints upon Drawing and Colouring. By W. F. STANLEY, M.R.I. Fifth edition, _with numerous illustrations_, crown 8vo, cloth, 5_s._

_Spons’ Architects’ and Builders’ Pocket-Book of Prices and Memoranda._ Edited by W. YOUNG, Architect. Royal 32mo, roan, 4_s._ 6_d._; or cloth, red edges, 3_s._ 6_d._ _Published annually._ Eleventh edition. _Now ready._

_Long-Span Railway Bridges_, comprising Investigations of the Comparative Theoretical and Practical Advantages of the various adopted or proposed Type Systems of Construction, with numerous Formulæ and Tables giving the weight of Iron or Steel required in Bridges from 300 feet to the limiting Spans; to which are added similar Investigations and Tables relating to Short-span Railway Bridges. Second and revised edition. By B. BAKER, Assoc. Inst. C.E. _Plates_, crown 8vo, cloth, 5_s._

_Elementary Theory and Calculation of Iron Bridges and Roofs._ By AUGUST RITTER, Ph.D., Professor at the Polytechnic School at Aix-la-Chapelle. Translated from the third German edition, by H. R. SANKEY, Capt. R.E. With 500 _illustrations_, 8vo, cloth, 15_s._

_The Builders Clerk_: a Guide to the Management of a Builder’s Business. By THOMAS BALES. Fcap. 8vo, cloth, 1_s._ 6_d._

_The Elementary Principles of Carpentry._ By THOMAS TREDGOLD. Revised from the original edition, and partly re-written, by JOHN THOMAS HURST. Contained in 517 pages of letter-press, and _illustrated with 48 plates and 150 wood engravings_. Third edition, crown 8vo, cloth, 18_s._