Red arrows on the depth gauge
To tell you the truth, this one kinda has me stumped. We didn't use any such markings during the Cold War on the Darter. The fact that they vary greatly from boat to boat belies one all encompasing explanation.
So my answer is this: they probably denote a variety of things, which would be determined by the specific boat's crew depending on what visual reference reminder they wanted for the planesmen. I agree with Nuc in that they may mean operational depths for specific operations. For instance, 48 feet would be about the depth that the bridge went under, leaving just the shears visible above the surface. 67 feet would be about the depth that the head of the periscope would dip under. These markings may also have a relationship to calculated buoyancy, i.e. the depth that you would flood or blow the negative or auxiliary tanks to achieve positive or negative buoyancy
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