I have to agree with Aquabelle. The drawing shows the "coupling" to have sort of a nipple shape at the junction with the stuffing box. Hard to see how the flange at the other end of the coupling can serve to compress the packing in that arrangement.
That said, several questions some to mind. It seems the diagnosis at this point is that there is come point where the shaft and tube have insufficient clearance to allow water to enter. From the drawing, hard to visualize where that might be. Normally, the only point at which the shaft is in near contact with the shaft is at the front of the adjusting nut. From your drawing it appears that you have an adjusting flange rather than an adjusting nut.
1. Is there water between the cutlass bearings? If all packing is removed, is there free flow of water at the collar (coupling?). It sounds like you have free water flow up to the stuffing box and none past that point. That would seem normal to me (other than the requisite desired "drip").
That said, several questions some to mind. It seems the diagnosis at this point is that there is come point where the shaft and tube have insufficient clearance to allow water to enter. From the drawing, hard to visualize where that might be. Normally, the only point at which the shaft is in near contact with the shaft is at the front of the adjusting nut. From your drawing it appears that you have an adjusting flange rather than an adjusting nut.
1. Is there water between the cutlass bearings? If all packing is removed, is there free flow of water at the collar (coupling?). It sounds like you have free water flow up to the stuffing box and none past that point. That would seem normal to me (other than the requisite desired "drip").