Absolutely true. I was only answering his idea that it should be better.

True. That said, you're only going to get six-to-eight hours doing that and then you'll have to recharge them. Since the engine doing the recharging has to run at full speed it will actually cost more fuel than if you just ran at 4 knots on the diesels. Unfortunately the game has this wrong and you can actually save fuel by travelling on batteries. Also they will recharge even if you are out of fuel. SH4 fixed this.

I was unaware of submarines using batteries for long-distance travel. Decks-awash surface approaches, yes, especially where silence is important. Maneuvering in port, yes. But travelling? I've not read those accounts.

According to U-Boat Far From Home. The epic voyage of U-862 to Australia and New Zealand:

"One shaft was kept running at half speed coupled to a diesel while the batteries were charged using the diesel dynamos. The remaining shaft was coupled to the motor with current from the batteries. With this arrangement the greatest range could be covered while proceeding at approximately 7 knots."

As far as I understood the most efficient (fuel vs.range) way to sail the U-Boot was using one diesel directly, making revolutions for about 8 to 10 knots.
With the necessity to recharge the batteries, they used your quoted setup for recharging, sacrificed 1 or 2 knots but kept the fuel vs. range efficiency as high as possible. The low charging rate may have been a welcomed feature either, to keep the overall lifetime of the batteries up.

Running the second diesel to recharge the batteries would have used definitively more diesel fuel.

Post under reconstruction...

http://www.abload.de/img/under-construction-si41kk8.jpg

Do you remember the dialog between Kriechbaum and the Kaleun about the bad weather conditions? Kriechbaum explains they are making 'no speed over ground' any longer and the Kaleun orders to dive?
This is the reason why the U-Boote had a better over all range using the e-motors in combination with the diesels.

Just switching regularly between the two propulsion methods to enlarge the range doesn't make any sense, because there is a certain energy loss in transforming fuel into current and back to kinetic energy to power the propellers - even with German machinery. :)
But under certain - more or less regular appearing - circumstances, this deficit is overcome by the fact to travel at all instead of only burning fuel.

Thanks for the posts and thoughts. I only added the bit above just as a point of interest. Hope people didn't take it as a shot across the bows :salute:

Wow, great answers -- both the easy answer, of "Can't do it in SH3" and apparent "undocumented features" that allow charging on the surface with no fuel ;)
Lots more discussion than I anticipated, and interesting to see some background. Steve's mention of hydrodynamics and the link made me wonder, since it's related: for surface running, since these boats weren't designed to do any planing, will setting the depth such that the longest possible section of the boat is at the waterline result in the greatest possible speed (at least before the disproportionate amount of engine power needed to overcome the design hull speed)?
I'm really enjoying seeing how the game dynamics both mimic and diverge from real-life boat ops, too.
Thanks, all.
Zygoma
May the number of your surface operations exceed your number of dives by one, at the end of your career.