In the game it doesn't matter. you can raise it at test depth/flank speed without damage. If it is light enough you may even be able to see things like mines.
IRL there as a seal that would leak and take damage at high speed and the scope would vibrate to the point of being useless. The max usable speed is around 3kts but varied some as modifications to the shears during the war. Typically they would raise it somewhere between 50 meters and the surface and rotate looking to the surface for shadows or any sight of danger. Upon breaking the surface the man on the scope would call out that it was breaking the surface and when it was clear. This gave the diving officer verification of his depth indication. He would then scan 360 in low power quickly announces whether there were any close contacts. The a high power sweep and reports of ANY contacts.
Rip
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