84 AN ANALYSIS OF THE ISHERWOOD SYSTEM 



butt straps are designed to give the same longitudinal strength as the longitudinals, but even 

 so, there is always danger of slipping at the strap. If the classification agencies are willing 

 to allow this, we have an indication which will warrant the belief that the brackets mentioned 

 do not seriously reduce the strength of the longitudinals. 



When I first started this work I realized that the brackets were more or less open to 

 question. If I had had the time and money available, I should have preferred to conduct 

 a series of tests on them. I do not know of a machine built by means of which we could 

 imitate the longitudinal stresses induced in a ship, but it would have been well if some sort 

 of tests could have been made to show just what the effect of the brackets is. However, lack- 

 ing these tests, the best I could do was to give the analysis in its present form. 



As to the weakening at the hatch openings, Mr. Hiatt fails to mention that the trans- 

 verse coaming at the hatch end is stiffened by 10-inch by 3j4-inch by 74-inch channel face 

 bar for the second deck and a 10-inch by 3)^-inch by 56-inch channel face bar for the up- 

 per deck, a face plate being fitted in each case. The deck longitudinals Avhich stop the hatch 

 are bracketed to this transverse, and consequently the transverse may be regarded as a sim- 

 ple beam bent horizontally. Some of the stresses in the longitudinals will be transmitted 

 to this transverse beam and from there to the fore-and-aft coaming, and to the longitudinals 

 at the other end of the hatch. What percentage of the stresses are thus transmitted I do 

 not know, but I do not think it is fair to throw out the entire amount of steel in the longi- 

 tudinal members between the hatches. I should like to emphasize further that the calcula- 

 tions are meant to be comparative only, so that no injustice was done since the steel be- 

 tween the hatches was included for both ships. Unfortunately our knowledge of stresses in 

 ships or in other structures is not thorough enough to enable us to draw conclusions with 

 any real assurance as to the exact stresses that will be present, and how they will be distrib- 

 uted. If we had such knowledge, we could proba;bly lighten our ships materially and say 

 to the financier: — "Here is a ship. It is absolutely safe, and I can prove it to you here and 

 now, and no need of tests." Let us hope that some day we shall arrive at that stage of de- 

 velopment. 



As to Mr. Sperry's and Mr. Reid's remarks, I wish to thank these gentlemen very much. 

 They were very illuminating. 



Mr. F. M. Hiatt (Communicated) : — The remarks offered by the writer in the 

 discussion of this paper were apparently received by some as a criticism of the Isherwood 

 system of framing. The intent, rather, was to question the sufficiency of the evidence so 

 far submitted in proof of superior longitudinal strength. 



Attention was invited to the fact that in a measure, at least, the Isherwood system 

 of framing has already passed the test of time. This fact is recognized, but, of course, 

 has no bearing on the relative strength of that type of framing as compared with the trans- 

 verse. It should be understood that the weaker of two ships may be strong enough, in which 

 case the stronger might properly be allowed a reduction in scantlings. 



The transverse system of framing has been built up through years of experiment 

 in seagoing vessels. At present it is the best criterion available as to the requirements 

 which must be met in a seagoing hull. It is therefore eminently fitting that new systems 

 of framing" should be analyzed and compared with the transverse system as a standard. It 

 is at the same time of especial importance that such analyses should be fair and just, 

 otherwise self-deception results. 



