The IJN subs are claimed to be based on the WWI U-155 Deutschland merchant sub. That carried 250 tons of its cargo external to the pressure hull:
And this article on cargo subs in general, and WWII German cargo sub design in particular, starts the topic with a statement that most cargo should be carried externally:
The IJA also built cargo subs. Notice the prominent "saddle tank" bulges:
All this leads me to thinking perhaps the bulge on this sub is then a part of its cargo carrying space? No idea if this is true, nor why the IJN would use a keel bulge when others used saddle tanks. (Perhas as some sort of trim measure?)
As I titled this post..."Wild speculation." Nothing more. Hope we get an authoritative answer from someone. I am curious myself!
: Can anyone tell me what is this stange round
: thing underneath the bow itself on a:
: IJN I-361 class transport submarine
: Sensuikan), also called Type-D submarine
: I have got both the: Aoshima 1/350 I-370 -
: and I-365.
: Most of all it looks like a torpedotube -
: but it is not. Maybe a sensor of a kind?
: I have never seen anything like it on other
: I can't find any info about it, and no photo
: of the submarine-class below waterline seems
: to be available either.
: Any info will be very welcome