No craft in the annals of yachting centers about itself such features of general interest as the yacht Defender. Perhaps the greatest series of races in these annals is the one that represents in its sphere the great fact of modem times-the greatest fact, it may prove to be, of all time-the rise of the New World, a rise to greatness that has reached, and in some respects has passed the point of successful rivalry with the Old World.
Among the yachts connected with the series of races for the America's cup, Defender is in many respects unique, standing head and shoulders above all the distinguished number.
The last races came at a time of high tide in yachting interest both abroad and in the United States, when rivalry was keenest and the evolution of the racing craft seemed to have reached its maximum. The determination to win back the lost trophy was made years in advance, and one can trace the course of deliberate, scientific, tireless effort to this end, till at last it seemed, with great reason, that invincibility had been reached in Valkyrie III.
It was immediately recognized in America that the defense of the cup could not be left to any yacht yet built; moreover, the time after challenge would be too short to permit experiment. Valkyrie III was the successor to a dynasty that reached its climax of perfection before the challenge was sent; while the American champion might be expected to follow the line of its predecessors, and might be expected to be the climax of a sequence of representatives, yet there was but one chance of producing the climax
The sense of danger aroused the American yachtsmen to the necessity of co-operation. Effort combined and centered upon producing a single craft. The order was placed with Herresboff Bros. of Bristol. R. L, and cost and all other considerations were subordinated to the one object, speed. The yacht built must win.
It was natural, under these conditions, to expect boldness, for the builders, though sure in execution, have been characterized by boldness in design.
The shape this bolc1ness took for realizing a maximum of power was along the liue of weight distribution, for lowness of center of gravity, rather than along the line of maximum metacentric radii, or power of form. The advantage of excessive lowering of the center of gravity, instead of the raising of metacenter, becomes apparent, without further explanation, from reference to the fact that power of weight, determining the position of the center of gravity, doe& not incur, as does power of form, determining the position of the metacenter, the increase of head resistance and frictional resistance that sets in when the vessel heels or is in a seaway.
The method adopted for lowering the center of gravity consisted not only in placing a maximum weight of lead on the deep fin keel, as found also in the Challenger and other yachts, but also in reducing to a minimum the upper weights, the saved weight appearing in the form of additional lead on the keel, being equivalent in effect to a transference of weight downward through a great desistance.
The reduction or economy of weight, particularly of high weights, which thus constituted the characteristic feature of the yacht, places her construction alongside of marine construction in general, and of naval construction in particular, where weight of hull and fittings affects intimately the limit of the military qualities themselves.
This coincidence of purpose alone would make the yacht an interesting study for the naval architect, but the form the purpose took enhances the interest, makes it general and intense, for it adopted a new material-aluminum-which, from its extreme lightness, has been offering great hopes to the naval architect, but which, from its corrosion in salt air and salt water, has Checked, if it has not effectually shattered these hopes. Moreover, the conditions were peculiarly such as to constitute a severe test of the virtues and failings of the new metal.
In view of this similarity of purpose with naval construction, particularly the bearing on torpedo-boat construction, and of the value of the experiment with aluminum, the Navy Department directed an inspection of the yacht and a report on her "method of construction." This report is given in Part I practically as it was made after inspection at Bristol just before the yacht left the builder's hands.
In August last, more than a year afterward, a second inspection was made at New Rochelle, through the courtesy of Oliver C. Iselin, Esq., to determine the conditions of preservation and the conduct of the new material in the face of the deteriorating conditions of service. Since it is on the question of corrosion that the use of aluminum for marine purposes hinges, an effort was made to throw, if possible, some additional light upon the subject, particularly at the present time, when, notwithstanding the advantage it offers, aluminum has been unfavorably passed upon, both abroad and in this country, upon grounds that appear to be incomplete. Samples of corrosion taken from the Defender, also a sample of a corroded aluminum plate, together with a sample of the salt water in which, in a closed vessel, the corrosion took place, the latter furnished by Professor A. H. Sabin, were sent to the chemist at the Navy Yard, New York, with directions for a qualitative and a quantitative analysis, with a view to determining the phenomena of corrosion in the particular case, the attacking agents, the soluble and insoluble products.
The value of an intimate knowledge of the phenomena is evident, serving as it would as a basis for research and experiment to find preventative preparations.
A sequence of heavy pressure in the chemical laboratory has Prevented as yet the analyses, and effort to hasten them since the writer's detachment from the New York Navy Yard has been of no avail. Only an incomplete, preliminary analysis of the corrosion from Defender has been reported, an analysis so incomplete as to be of no material value.
It has therefore been decided to give in Part II the result of the study of the subject of the adaptability of aluminum for marine construction, without the original data hoped for on the question of corrosion.
PART I.—THE METHOD OF CONSTRUCTION OF THE YACHT DEFENDER.
1. Preliminary Description.
The boat was examined while afloat at anchor. Her general form is indicated in Figs. 1 and 3. Her approximate hull dimensions are as follows: Length over all, 122 feet 3 inches; length on load water line, 90 feet; maximum beam, 23 feet; beam at load water line, 22 feet; draught, extreme, 19 feet.
The idea that gives the distinguishing feature to this advance type, as will be seen below, is the realization of extreme sail carrying power from a great metacentric height, initial and under inclination, realized from the disposition of weights. Though realizing a high position of the metacenter from an elevated position of the center of buoyancy and long metacentric radius, the extreme is reached in the low position of the center of gravity.
The great metacentric height and consequent sail-carrying power is derived more from the element of weight than from the elements of form.
The method adopted in realizing the low position of the center of gravity is that of reduction in weight of hull and fittings and the addition of weight to the keel, weight being taken from the upper portions and added to the lowest point.
The method of realizing a reduction of high weights is the use of light materials, of light scantlings, with a light method of construction and fastenings.
The reduction of frictional resistance and of liability to deterioration are sought in the use of bronze, manganese bronze, for water-washed portions.
The features of the distinguishing methods thus identify the construction, in the objects sought, with the construction of vessels of war, particularly torpedo vessels and torpedo-boats.
2. The Method of Construction.
Referring to the sketch of the midship section, Fig. 3, the construction is as follows:
I. THE SKELETON.-A steel angle, 4" x 4, closed to the angles of the water lines, from Yi." to /8"thick, forms the stem and binds the ends of the shell plating, with double riveting, rivets of bronze for the bronze plating, diameter 5/8", and of aluminum for the aluminum plating, diameter 72".
A bronze casting, 4" deep by 20" wide, of the form indicated in sketch, forms the keel. On its under side is attached the lead, shaped to the form indicated, secured by bronze tap bolts 8" long by 34" diameter. The bronze shell plating extends to cover the lead, reducing the frictional resistance, and giving additional support, securing to the lead by bronze taps 5" long by 1/2" diameter.
The beams of the upper or main deck are aluminum angle bulbs, 5" x 2", spaced 40, fitted to only alternate frames, the frames without beams ending without fittings under the deck stringers. The beam arms are secured to the frames against which they sit back to back by three rivets of 1/2"diameter and by a single strip of aluminum plating 24" long, 4" wide and 3/8" thick, forming a triangle with the frames and beam arm, secured by three rivets at each end of 1/2". The two beams between which the mast is strapped are of steel, 4 1/4" x 2 1/4".
A bilge stringer, steel angle bulb, 3 1/2 x 2", extends throughout the entire length on each side, at the height indicated in the sketch. A corresponding inverted steel angle bulb, 3 1/2" X 2", extends under the main deck beams in the position indicated in sketch along the whole length on each side about 4' from the central line.
The bilge stringer angle bulb and the inverted angle bulb under beams on each side are connected by 14 struts made of iron piping, 1 1/2" amidships and 1 1/4." forward and aft, spaced five frame spaces. These pipes have forgings welded in their ends and flattened to lie on the backs of the angle bulbs, to which they are secured by three rivets of 1/2" diameter.
A deck stringer plate of aluminum, varying from 28" to 24" in width and 5/16," thick, extends over the beams from end to end. On this stringer plate, along its outer edge, extends an aluminum angle bulb, 5" x 2 1/2", which receives the upper edge of the shell plating and forms the upper ending of the hull at the sides. The deck beams are strapped in addition by diagonal plates of aluminum, 2/8" thick, varying in width from 5" aft to 7 1/2" forward, the four converging on the mast being 10" wide. The length of these strapping plates and their disposition are indicated in sketch, Fig. 4.
Thus the skeleton consists of a steel angle stem, a bronze keel, with lead underneath, steel angle bulb frames, with floor plates of alternating heights, of bronze at the bottom and steel above the bronze, with steel angles binding the frames on the two sides, wooden deck beams for the lower deck, and aluminum angle bulbs for the main deck beams, fitted only to alternate frames, steel angle bulb bilge stringers, one on each side, inverted steel angle bulbs on under side of main deck beams, one on each side, two rows of inclined struts between bilge stringers and inverted angle bulbs under main deck beams, one row on each side, spaced every five frame spaces, aluminum deck stringers and angle bulb at sides of main deck, and diagonal deck strapping fore and aft.
2. THE COVERINGS.—The shell plating is on the raised and sunken system, with liners behind frames under outside plating, secured to frames by a single row of rivets of 1/2" diameter. All butt straps are double riveted.
3. CONCLUSIONS.—The method of construction as above described presents three prominent features, namely:—
- Simplicity and directness of construction.
- The use of new materials and their combination with steel.
- Lightness of scantlings.
The complex nature of the forces to be resisted, the combination of pronounced twisting and a heavy thrust of mast with the usual forces, would lead to the expectation of complexity, whereas examination shows an entire absence of redundant parts, and even of parts found in usual construction.
The examination of the parts in view of the forces to be resisted shows a singular directness of purpose for each part or piece that enters the construction. These characteristics, simplicity and directness, are so evident from the sketch of the midship section that they need not be pointed out in detail. Notice should be taken, however, of the use of angle bulbs instead of. simple angles or combinations of angles. The sectional moment of inertia being greater for the angle bulb, there is a gain in stiffness, or a saving in weight for the same stiffness. Notice should be taken also of the simplicity of fastenings, the economy of rivets.
Special attention should be given to the system of struts, which, though exceedingly light, give to the transverse sections a girderlike nature hitherto unattempted afloat. Special attention should be called also to the disposition for insuring continuity of resistance in the main deck, to the longitudinal angle bulbs under the deck beams, and the combination of angle bulbs and wide deck stringers at the sides, but especially to the diagonal strapping which opposes itself directly to the twisting forces referred to above, note being taken also of the fastenings of deck planks to each other.
It may be added that the characteristics in question appear in full relief when the boat is conceived inclined under the great localized forces; when the parts are conceived distributing these forces and transmitting them to the resisting forces without, the structure will be seen to present remarkable stiffness of form, remarkable resistance to deformation, combined with the simplicity of structure described above.
In sum, the boat is an architectural departure and is an interesting study for all architects. For the marine architect the study is of practical value, for the departure is along the line of his constant effort to realize a maximum of resistance to complex forces with the minimum of weight.
The realization of this main object, however, is not confined alone to the disposition of the material, but extends also, with even more marked characteristics, to the selection. The lightest strong metal known to architecture is used wherever characteristics other than weight do not prevent. The shell plating of the top sides, the main deck beams and strips securing them to the frames, the deck stringers and top side angle bulb, the deck strapping plates, and various fittings, enumerated above, are of aluminum. The builders claim a gain or saving of ten tons over steel construction as used in the yacht Colonia of nearly similar build. The transfer of this weight from the top sides to the keel, through a vertical height of about 21', causes the distinguishing characteristic referred to at the beginning, that of a lowness of center of gravity which places this boat apart from all other products of marine construction.
This use of aluminum presents particular interest from the effort it represents to realize not only a gain in weight, as yet moderate in quantity when compared for simple strength with steel, but also the more pronounced advantage afforded by a more efficient disposition of metal, a section of greater inertia, and consequent greater stiffness, than can be realized with the smaller quantity of the heavier and stronger metal.
In addition, in view of service and deterioration which sets in from the surface and which may be looked upon as penetrating readily a certain distance and then ceasing, the thicker plate of the lighter metal offers decided advantage over the thinner plate of the stronger metal, assuming equality of strength when first built. These features are important ones in maritime construction, particularly in naval construction.
The last feature is of particular importance in torpedo-boat construction, which requires the use of plates of extreme thinness.
The advantage of taking weight from lower portions and putting it on the keel is less pronounced than for the case of the upper portions. Further, a special bronze alloy, stronger than steel, though heavier than aluminum, offers, when polished, pronounced advantage over all painted or varnished surfaces, according to the experience of the builders, in smoothness of surface.
The water-washed portions of the shell plating are of manganese bronze, from which results of ultimate strength are realized about 12 per cent. in excess of the results of steel used for the same purpose. In view of the additional fact that the under water parts are for the most part on the compression side of the hogging girder, and will consequently be less called upon for resistance within tensile elasticity, and of the fact that bronze offers advantages in maintenance of smoothness, according to the builders, and decided advantages in resistance to deterioration, this feature is one of particular interest in naval construction, particularly in the case of torpedo-boat construction, where deterioration is most injurious.
In sum, the construction of the boat seeks to realize the advantages offered by two materials that may be considered new, and presents for study all the interest that attaches to the experiment of these materials in the new field of hull construction.
But in seeking to realize the advantages of the new materials the builders have not lost sight of the advantages still offered by the old. For the frames and floor plates and for special fittings like the tie plates around the mast and the chain plates and, in general, the parts where the advantages offered by aluminum and bronze are less in evidence, and where special strength without special stiffness is desired, use is made of steel, with evident advantage. It should be noted that the lower parts of the floor plates, 12" in height, which are liable to corroding effects of bilge water, are made of bronze.
In sum, the boat seeks to realize the advantages offered by all three of the materials, aluminum, special bronze, and steel.
Though aluminum boats have been built abroad, and though bronze has been previously used by the builders in conjunction with steel on the Vigilant, the combination of aluminum, bronze and steel has never been attempted. In material, as in architecture, the boat stands apart. But, further, simplicity and directness of construction and new materials and new combinations of materials are not the only characteristics of this remarkable boat.
An examination of the scantlings as described above, and as seen on the sketch of midship section, shows throughout a lightness that is remarkable when it is remembered that the boat is essentially a structural experiment on untrodden ground. This lightness of scantlings is most marked in the region of the mast. It is true that here are found the maximum dimensions of pieces, the heaviest scantlings of outside plating, that the two beams between which the mast passes are of steel instead of aluminum, that the deck strapping plates are increased in width, and that a special steel tie plates binds up the four beams, and the ends of the four strapping plates, but even with these special dispositions the provisions do not appear commensurate with the local strains, particularly on the weather side, that the structure will undergo by reason of the narrow spread of the stays, introducing an enormous thrust of the mast and crushing force between the sides due to the enormous spread of sail and the great height of the center of pressure. Notwithstanding the special provisions enumerated, it appears not improbable that a heavy wind, particularly if applied suddenly, will cause the structures in the region of the mast to spring, if not to give way, it being assumed that the mast is sufficiently robust not to give way beforehand. In view of the experimental nature of the boat, this feature of lightness, particularly in the region of the mast, is second in its striking nature only to the features described above, and offers an interesting field for the calculation of the strains to which the materials are liable to be subjected. This feature has not been included in the present examination of the "method of construction," but a summary investigation leads to the conclusion that the scantlings are such as to bid fair to afford a conclusive, if not crucial, test of the strength of the new materials.
But the test of strength is not the only test in store for the new materials: the test of endurance will also be conclusive. The materials are in a combination that will produce serious effects of galvanic action, if such action is liable to take place under the conditions of service. The aluminum and bronze are not only in contact with each other, but are both in contact with steel, and this contact is most intimate in the case of rivets passing through two or more different materials, as is the case of rivets between frames and shell plating. It is to be noted that methods have been used to obviate as far as possible the liability to this action, such, for instance, as the avoidance of the use of aluminum below the water, the use of bronze for the bottom of the floor plates where liable to be in contact with bilge water. The conditions of service will test the efficiency of these methods.
The test will extend to the use of bronze for the water-washed surfaces, to the extent of the advantage it offers for the resistance to deterioration and to fouling, and, if possible, it should be made to extend to finding the extent of the advantages it offers in frictional resistance, the increased smoothness compared with painted and varnished surfaces.
Thus, in sum, the Defender, while presenting a remarkable study of simplicity of construction and efficiency of distribution of materials, embodies two new materials in such light scantlings as to offer a test of their strength, and in such combination and disposition as to offer a severe test of their qualities of endurance, of resistance to deteriorating influences.
The objects throughout, that of realizing a maximum of strength with a minimum of weight, particularly in the upper parts, of realizing a minimum of skin resistance, are identical with the objects in naval design, more particularly in torpedo-boat design; while the methods adopted, the use of three materials, two of which are new, in a combination which seeks to realize the special advantages of each, and the means resorted to prevent the bad effects liable to the combination, are all steps across the border at which torpedo-boat construction has arrived into the fields beyond.
This interesting production has sought to realize the extreme of sailing advantages along the road that torpedo-boat construction must follow in coming developments of hull construction, and the conditions are peculiarly favorable for its cutting away many of the obstacles and indicating what changes in direction are best for entering the new ground.
To realize the advantages of the experiment, investigation should extend to cover the behavior of the new materials under the tests of strength, during stress and after repeated stresses, and under the tests of endurance and resistance to deterioration.
PART II.-THE USE OF ALUMINUM IN MARINE CONSTRUCTION.
A new material enters the realm of structural usage by creating new fields or else by outstripping the occupants of old fields.
Broadly speaking, the field of marine construction is now occupied by steel, wood, and the alloys of copper. Aluminum must, in consequence, wrest from these older materials the foothold and territory it is to occupy.
A part of this field, associated principally with the motive power, is shut off from aluminum by an impassable barrier of temperature. A fraction of the temperature at which bronze and steel remain unmodified will overthrow the physical properties of aluminum. Roughly speaking, aluminum is barred from temperatures where human life cannot exist. Not far beyond the boiling point of water it loses half its virtues of resistance, and above 400° F. should not be subjected to strain.
In addition, by reason of its softness, aluminum is debarred use as armor, which constitutes an important section of naval construction. Moreover, as will be seen below, its inability to extinguish sudden and violent dynamic forces excludes usage for armor supports and fastenings.
Of the remaining fields not thus cut off, principally hull construction and hull fittings, the vast bulk is occupied uncontestedly by steel. Wood and bronze (the term bronze being used broadly to signify the alloys of copper) have only special provinces, wood finding its principal use where stiffness, not strength, is required, and where the service does not require a hardness of surface, but facility of working; and bronze finding its principal use where complexity of form requires special properties for casting, or where special corroding agents are to be resisted, or where a wearing surface is desired to save the usure of steel.
Even the special properties of bronze and wood may be considered as possessed, to a greater or less degree, by steel. Aluminum then will be made to measure properties with steel, the special properties of wood and bronze being considered only incidentally where they appear in the comparison with steel.
The factors whose product or resultant determines or measures the adaptability of a material for structural purposes are strength, weight and cost, strength and cost being used in their broad sense. The measure of adaptability for strength is the approximation to a maximum, and for cost and weight it is the approximation to a minimum. In marine construction the problem of design in general is to realize within a fairly wide range of cost a given or required strength with a minimum of weight.
The underlying object in general is to realize a maximum military efficiency for each unit of weight.
In the industries, however, and in general for land structures, the purpose of design is to realize a given or required strength with a minimum of cost, a minimum of weight in general accompanying a minimum of cost. The underlying object in general is to realize, within a wide range of weight, a maximum return for each unit of cost or capital invested.
For the present purpose, therefore, the comparison of aluminum and steel will be for strength and weight first, then for cost.
The question of endurance, or length of life, which will be seen to be of capital importance, is essentially one of cost, though intimately associated with strength, since the rate or rapidity of dissipation of strength determines the length of life.
The figures used in the comparisons of aluminum are the latest ones of the Pittsburg Reduction Company, for their best 5 to 10 per cent. alloys. These figures are probably a little high for the general status of aluminum at the present moment, particularly when the comparisons of simple strength are made with mild steel, mild steel being used throughout, as it is the most regular, and best illustrates, as will be seen, the great contrast of the two metals in their resistance to dynamic forces; but they have been retained in view of the present rapid state of progress in the production and manufacture of aluminum, a state corresponding to the stage of steel about ten years ago, when still on a steep rise far away from the proximity of the maximum in the curve of progress.