374 
FOREST ""AND STREAM. 
[Not. 6, 1897. 
N^otes on the Tacht Defender and the Use of 
Aluminum in Marine Construction. 
BY RICHMOND PEARSOW HOBSON. ASSIST ANT >AVAL 
CONSTRUCTOR, UNITED STATES NAVY. 
Reprinted by permission from the Proceedings of the United 
States Naval Institute. 
Copyrighted by the U. S. Naval Institute. 
{Continued from page S55.) 
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 physi- 
cal 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 vir- 
tues of resistance, and above 400° F. should not be subjected 
to strain. 
In addition, by reason of its softness, aluminum is debarred 
from use as armor, which constitutes an important section cf 
naval construction. Moreover, as will be seen b 'low, its in- 
ability to extinguish sudden and violent dynamic foic s ex- 
cludes its usage for armor supports and fastenings. 
Of the remaining fields not thus cut oflf, principally hull 
construction and hull fittings, the vast bulk is occupied uu- 
FIG. 5. Section of Equal Ultimate Teasile Strenerths. 
contestedly 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 stiff- 
ness, 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 sur- 
face is desired to save the usure of steel. 
Even the special properties of bronze and wood may be con- 
sidered 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 consid- 
ered 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 pur- 
poses 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 ma- 
rine construction the problem of design in general is to real- 
ize 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 struct- 
ures, the purpose of design is to realize a given or required 
strength with a minimum of cost, a minimnn of weight in 
riG. 12. Sections for Equal Bending: Moments, 
to Elastic Limit. 
Fiber Compres ed 
general accompanying a minimum of cost. The under- 
lying 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 pres- 
ent 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 re- 
sistance 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 ris' e 
far away from the proximity of the maximum in the curve 
of progress. 
A.— COMPARISON FOR SIMPLE RESISTANCE. 
1. In tension. 
(a) Ultimate tensile strength.— Steel, 60,0001b«. per square 
inch. Aluminum, 40,0001bs. per square inch. Ratio, 1 to l-.'S 
or 3 to 3. 
Cross sections for equal ultimate tensile strength, see Figs. 
5 and 7. 
Weights.— Weight of one cubic inch of steel, 0.2831b.; ot 
alunainum, 0.0941b. Ratio, 1 to 
FIG. 6. Section of Equal Elastic Strength in Tension. 
PIG. 7. Plates of the Same Length and Breadth and of Equal Tensile S'rength. 
FIG. 8. Plates of the Same Length and Breadth and of Equal Elastic Strength. 
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FIG P.- Sections of Equal Weight. 
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FIG. 10. Sections of Equal Weight. 
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FKt. U. §eotioa3 for E^aa,l Bja^iog Moneuts. Fiber UnJeriUitiwaCe Compressnre Streog^b ■ 
