Yeah I know.. in game sunrise seems to be when the whole sun is visible.
I've yet to do the sunset...

Been a long time since i did that real nav stuff, cannot remember :arrgh!:

Not when I watched it. Halve of it.

sun is for landlubbers
 

Ha! Sun...

By 1943 you never saw the sun, except maybe through the periscope lens.
The only real clock on board was with dark of night and 'light bulb'.
This was why routine aboard a u-boat (or any sub really) was so important, because it gave the crew some kind of 'time anchor' to stay grounded with.
There is one thing the game (and so far I have never heard anyone mention this as a mod, or included in a mod) does not model, and that is the effects of 'caneurosis' (ger: blechkrankheit or blechkoller -lit. 'tin disease'). A wartime expression for the state of violent hysteria, induced, in the case of u-boat crews, by extended nervous strain/stress combined with the unnatural conditions in which they lived.

Ukonig, you actually bring up point that I've been wondering about lately. I'm in late 1941, and I usually am on the surface during the day and spend a good amount of time underwater at night. The reason I do so is because I assume that the ability to spot ships by the watch crew is diminished at night. Do you know if this is the case in SHIII? Or is the ability of the watch crew the same, no matter day or night? I figured that since they can't see that well at night, having your sonar man at hydrophone station would fill the gap, so to speak.

I dunno, looks like I'm doing this wrong too! lol...

I meant that historically, U-boats lost a lot, if not all of their surface attack ability by 1943. At that point in the war, the allies had enough of the tools and the talent to send u-boats to the bottom in record numbers. The u-boat was obsolete. And yet it was all the Germans had to offer. The type 21 was still years away. And when they did finally get them on stream only 4 of the 140 boats built ever made it to operational status. The 21 was built straight from the drawing board with no time for prototypes or testing. Anything that went wrong was going to have to be corrected as they went.
Which meant the rest of the U-fleet was working with outdated equipment. The snorkel solved some of their problems, but without one your chances of survival were not great.
Historically, if you play with high realism, this is what your campaign might look like.
1939 - you can attack on the surface, night time preferred but daylight is an option, under certain conditions. Aircraft cover is scant. Not a lot of destroyers available for anti submarine duties so convoys will be lightly guarded. And the guards are inexperienced so they make more mistakes, like, charging off in the wrong direction from where the attack came from, thus leaving the flank of the convoy wide open for your torpedos. Or missing their chance to drop a load of charges on you because their sound guy lost contact. Remember, the u-boat is not so much a submarine as it is a torpedo attack boat that can dive. In 1939, 1940 it is still very much a surface ship.
1940 - much as 1939, except now there are a few more escorts on convoy duty. The merchant skippers don't always do stupid things like scatter, or cut across each other causing a 'traffic jam'. You can still attack on the surface night or day (depending).
early 1941, getting tougher. The tommies have radar on most of their ships now. Night time is no guaranty of stealth. Use surface action carefully. Be more watchful of aircraft.
late 1941-1942 America is in the war. Now it just got real. Depth charges lost their 152m depth maximum and can now go to any depth needed to hit a submerged boat. More air cover. More radar. A few new improvements on the U-boat front as well, but not nearly as much or as well as the allied equipment. Keep in mind that the allies know the strength limits of the uboats they are fighting. They know how long your batteries will last. They know how long your air will last. They know they can outlast you.
1943, summer. Your surface action days are pretty much over. HK units travel with convoys, sometimes they have MACs or merchant aircraft carriers sailing with the convoy, they are aware of your uboat signals and are even reading Donitz's mail. The odds are stacked very heavily against you. You spend most of your time underwater (during the day so ships/planes won't see you) draining your batteries, using up your O2. Then your chief engineer tells you that you must surface to recharge. You surface. You're upside for maybe 5 - 15 minutes when your radar warning starts going into high gear. Crash dive to avoid aircraft. Wait a bit, try it again. Crash dive again after 4 more minutes. How much charge did we get? Not enough to make a difference. Surface. Aircraft. Don't dive, fight. A few passes. Hit by aircraft, bombed, sinking, U-123, never heard from again.
So you do your battery charging runs at night. Always on the lookout for coastal command to drop bombs on you. Even though that with radar small enough to fit on aircraft, night is just as clear as day. Its also why they removed the deck gun off most uboats after 1943, as you rarely spent time on the surface anymore, it was now dead weight. Engaging surface ships in the face of such air assault was an act of suicide.
1944 - the allies sunk 237 uboats last year. Mostly by aircraft. It will be a repeat in 1944. There is nowhere in the atlantic that a plane cannot get to. The allies can direct their convoys away from the uboats, and marshall enough HKs to where the uboats will be, to make easy work of them.
If you've made it this far you are either really skilled or really, really lucky.
I don't know if the game makes much distinction between watch detection ability from night to day. I do know there is a difference when fog rolls in, but I just assume they will see what they need to see night or day.