We have gone there a few times and it is a great sail both directions. Last time was on an Island Packet 465. On a beam reach we made 8 knots with wind of about 15 kts and it took about 5 hours. On a cat you should be quicker. I would not do it in the absence of wind. A day and Buck Island and another day for an island tour was fine for us. There are also two golf courses should you wish to play.

Look up posts by "BardJ" from a few years ago. He has a trip report from a visit and it was helpful to plan an anchoring location as well as stuff to do. Make sure your GPS is functional as for a an hour or so you will not be able to see St. John or St.Croix..