892TIs on the 56', and I misspoke earlier, our ideal cruise speed is about 15/16kts. with a top end of about 19 kts. (We haven't tried to wind her up yet though). That one boat tops at 21 kts I believe.
Here's some good info about the Hatteras 53's in a review from David Pascal. A lot of the comments are about the YF but they share designs with the MY . http://www.yachtsurvey.com/boatreviews/Hatteras53.htm