Did you happen to notice the fuel usage in those boats by chance?
Edit OK, I just re-did my install, all-a-fresh now, and took a Tambor out in a new career December 9th departure Pearl Harbor to recon Eniwetok. Once out of Pearl, running at Ahead Standard for a few hours, I asked what my range would be and was given "18,000 nm", which is a bit long. It should be lower. I'm wondering if that's what the trouble is, jldjs? I did burn 5% of my fuel getting to Eniwetok, which is about right for the ratio involved, but not quite an accurate figure, so we're looking further into that now also...