364 F. H. Todd 
living spaces, and buoyancy tanks, the external cargo spaces being of very light construc- 
tion since the liquid pressures inside and outside could be equalised. It has been claimed 
in fact in a study conducted in Japan that this could result in an actual reduction in steel 
weight. But can we indeed accept this conception? There is always the possibility of a 
power failure or other mishap which would force the ship to surface at sea and possibly in 
bad weather, and it would seem that under these conditions the submarine would need to 
possess longitudinal strength comparable with that of any surface ship. This would seema 
hazard particularly to be catered for in any pioneer submarine tankers, because in these 
early ships there will doubtless be many precautions taken to ensure a shutdown of the 
reactor in the event of any unforeseen or unacceptable faults in the equipment. It may even 
be that a shutdown could follow a mere fault in the controlling instrumentation. Moreover, 
with the very large draft of such a vessel, she may in many cases have to load or unload at 
a deep water terminal in a relatively exposed position where the conditions of the sea may 
be quite rough as compared with those normally existing in harbour. Also in approaching 
port when the water depth becomes less than some 400 or 500 feet such a submarine would 
undoubtedly have to surface and do the last part of the journey as a surface ship and 
therefore subject to the effects of rough weather. Taking all these possibilities into 
account, it seems extremely doubtful whether the conception of a very light structural hull 
for submarine tankers would be acceptable, at least until considerable experience had been 
gained in their operation. Fora cargo ship there is no question but that a pressure hull 
would be necessary throughout. The Mitchell Engineering Company have carried out an 
investigation into such a cargo submarine and have come to the conclusion that the hull 
would need to be made of mild steel up to 3 1,/2 inches in thickness amidships in way of 
the machinery and compensating tanks, and vary down to 1 inch in thickness toward the 
end. On this basis they estimated the deadweight/displacement ratio to be 0.56 for an 
ore-carrying submarine of 50,000-ton displacement [5]. There are many other problems 
involved in the construction of a hull of such dimensions. As has already been pointed out 
when discussing the resistance qualities of different designs, the circular hull submarine 
leads to very large drafts such as could not be catered for by any existing docks or terminal 
ports. On the other hand, the circular hull is the only economical one from a structural 
point of view and any departure from exact circularity will mean the provision of extra stiff- 
ness and greater thickness of plating [6]. Although liquid cargoes could be handled very 
easily from a submarine, other types of cargo could be very awkward in their demands for 
large hatches which would have to be pressure resisting and which would, in fact, form 
another weakness in the strength qualities of the hull. It is probable from a strength point 
of view that the only reasonable cargoes would be bulk ones such as iron ore or similar 
high-density materials. 
Two of the most important qualities in such a ship would be its directional stability 
and controllability. If we consider a submarine some 800 feet in length travelling at an 
immersion of 400 feet and built to resist pressures up to twice this depth, we are asking 
the crew virtually to fly the vessel in an atmosphere whose depth is only equal to the 
length of the submarine. This would probably give any air pilot nightmares! Since in 
order to compete commercially on a power basis with the surface ship the submarine would 
also have to travel at some 30 knots or more, it is easy to see how by the accidental 
occurrence of even a moderate trim by the bow the ship could very soon exceed her safe 
diving depth. The question of providing adequate directional stability and controllability 
is therefore a paramount one in the whole design. 
In order to obtain adequate transverse and longitudinal stability, it will be necessary 
for the submarine to carry permanent ballast, as is done in military craft. It is also impera- 
tive to have some surplus buoyancy when on the surface, and in any commercially operated 
craft the amount of such reserve buoyancy would almost certainly be controlled by 
