The ability of your crew at campaign start should NOT be high, which also means your whole sub systems and, in your case the crew's ability to spot far away targets, all these should be reduced. If you click 'I' and go to the crew window you should generally read something like '56% trained...' meaning there is a lot of room for them to improve. You improve them by earning points with good performances and accomplishing your missions right. It's then important that you distribute these points (whithin the same crew window), this doesn't go automatically.
If instead you mean the enemy's ability to spot you, well, it's most likely as in real-life: persons have different sensual abilities to observe. I think the game code does model that. So you may get lucky to be unspotted at short range, but for your sake just assume it's not going to happen...One method is to only leave your periscope out for short times (at low speed, or its trail becomes visible!), use it in intervalls. And of course the enemy will always spot you after you spot him, that's obvious, but you can get closer if you present the right aspect to the enemy, which is you are facing it at right angle and low speed so your trail is less visible. A cautious calculation is :
You are spotted at 1x critical distance with full broadside, 2/3 critical distance with right angle to enemy.
Cheers,
