[TorpX]

I must go to the Japan-luzon Straits with my Porpoise.

Ah, that explains it.

Cruising at ~ 9.0 kn. is the most fuel efficient. fireftr18 is right that battery charging will use up more fuel, but I wouldn't freak out about it.

With ISP, you will be able to get better battery performance than you would otherwise. I was able to patrol all day at 3 kn. submerged, surface, charge batteries, and move 30 nm overnight using about 1% fuel per day. That in a S-boat. I don't remember exactly how much fuel is used by charging, but it is approximately proportional to the amount of depletion. So, it you run submerged at a low speed, say 3 kn., you should not use more than about 30 % of your battery. This can be recharged without an enormous quantity of fuel.