72 AN ANALYSIS OF THE ISHERWOOD SYSTEM 



of the deck longitudinals, especially so since the fiber stresses due to longitudinal 

 bending of the ship as a unit are unduly small. 



This is, however, a question pertaining to the particular design under consid- 

 eration rather than to the principles of the Isherwood system of construction. 



MISCELLANEOUS STRESSES. 



Torsion. — The question of torsional strength would probably have escaped at- 

 tention had it not been for the fact that the inquiries, which will be referred to 

 later, elicited the information from a European shipping firm, who request that 

 their name be not used in connection with the statements they make, to the effect 

 that the two Isherwood ships they own and operate showed a torsional weakness. 

 To quote their letter, in free translation : — "The ships seemed to be looser, allowing 

 an undue amount of twisting. This made it necessary to stiffen them by means of 

 additional bracketing, which successfully overcame the difficulty, but which 

 also partly wiped out the saving of weight above referred to. But, as already 

 mentioned, these were some of the first longitudinally framed ships ever built, and we 

 understand that the later vessels of this kind are proving very satisfactory. * * *" 



Since the torsional strength of any body is proportional to the polar section 

 modulus, and since it is evident that the increased number and areas of the 

 longitudinal members in the Isherwood ship mean an increase in the polar section 

 modulus (cf. the section moduli for horizontal axes), it is rather difficult to under- 

 stand how an Isherwood ship can allow "an undue amount of twisting." The only 

 explanation that presents itself is that the shell and deck longitudinals were not 

 properly connected where they were cut at the watertight bulkheads, a suggestion 

 that appears to be a probability when it is remembered that additional bracketing 

 successfully overcame the difficulty. But even if the bracketing were originally 

 faulty, the shell and deck plating should have remained intact, giving these vessels 

 practically the same polar section modulus as they would have had if they had been 

 built to the transverse system, and they should consequently not have shown any 

 torsional weakness. 



It is not impossible, however, that the question of torsion is not fully under- 

 stood; it may be that the transverse stiffening of a hollow body has an effect on 

 the torsional strength that is not represented in the polar section modulus, but it 

 seems probable that this effect, if it does exist, would be slight. It appears, then, 

 very unlikely that a properly designed and constructed Isherwood ship should be 

 weaker in torsion than a transversely framed vessel of the same type and 

 dimensions. 



Stresses Due to Vibration. — It is contended, and is probably true, that vibra- 

 tion is less in Isherwood ships than in transversely framed ships. But it should 

 be remembered that the question of vibration becomes serious only when there is 

 synchronism between the vibration period of the hull, the vibration period of the 

 machinery, or the vibration period of liquids in tanks or liquid cargo, or between 



