[OKO]

As you all know, XVIII wasn't made for pacific theatre, and they were short legs for this wide area.
Simple problem to solve using larger fuel tanker on this big ship... but simply a bad theatre for them, historically ...

Here is the way I use the XVIII : it's like boom and zoom => cruising 11 knts to go to patrol area (11.000km range), then patroling at 1/3 (7 knts) on the desirated zone, until I have a proximity radio contact or, even better, until one of my sensor reveal me a potential target.

If it's a radio message for an incoming (not outgoing) contact, I plot an interception path to meet the target, only if I could do that without expending to much fuel to hunt.
This mean, no much than 200km (say 100NM) ( at full (17knts), if i couldn't intercept the target within 100NM, I simply forget it and wait for another one.
If it's easy to go to an interception point, I do my best to go there with minimal fuel consomption : If I can go there to 11 kts it's nicer than to go at 17 knts, so you simply have to plot a good path when it's possible.

Once a target pop up to my sensor, I immediately go to Walter propulsion.
If a target is caught by sonar, I have around 20 miles before watching it, so, moving at standard, surfacing, at 23knts right ahead to the target position, reading, during the way in, the right or left defilement on the sonar to lead the path of the target.

Once I see the smoke (using PE here) I finalize the approach, staying enought away to avoid detection (remember to send a "convoy" message to the base when you could do, it will help you and give you more objectives, increasing the immersion).
During that time, I manage to position myself, very easily with walter sytem, ahead of the convoy path.

So the final word is this : only use the walter propulsion when you already have caught a contact on you sensor, never before.
This way you could make around 4 patrol with your H2O2 (reloading 2 or 3 times).
This is actually my first patrol with XVIII, just finished to use all of my torps and half of my fuel, still 84% of H2O2, 45.000 tons in 10 days, south of Java for patrol area.
Stock SHIV (XVIII is implayable with RFB) only external view (for the show)

Fortunatly, there is lot of refueling point, but XVIII is really short leg in fuel for pacific theatre.
Amazing boat anyway.
With 1/4 of type IX torpedo as reload, once you found a convoy, he is in very bad trouble, whatever the escort, all major ships are condamned.

It's harder to find them than to sink them, but usual uboat commander will be really surprised at the speed all things will occurs, about 3 to 4 time faster than usually.
That's need to be pretty accurate and to think 3 to 4 times speeder than in a conventionnal WWII submarine.
But this is not because you have a so good weapon, you become a terrific commander, this ship need to be properly handled to be used correctly.
It's simply not a "magic boat", and you have to learn to do the job in 3 to 4 time less time than other submarines.
Here, experimented submariners will do a much bigger difference than average ones.
Even if it's much more easier to evade problems, it's also harder to engage, as the window is shorter to fire at targets.

This XVIII / XXI are the first modern submarines, in term of performance, the way they are built, and doctrine used with them.
Americans, russians, english and french used this technology at the end of war to built new generation submarines.
We could say XVIII (more likely XXI) completely changed the face of submarine warfare after the end of WWII.