[WhoisAlanGreenspan?]

I'm guessing you're on the right track; submarine ramming attempt on the prop. See post 15 for larger pic.

But I'd also suppose the best defence against submarine rammings in the middle of the night would be a navy destroyer escort. I think the message is 'you've been warned'.

[AppyPappy]

Iran has lot of mini-subs. They could be attaching a mine to the tanker. Insurance rates go up on Persian Gulf oil.