I would assume it has the same bottom as a 47 Lightining? I'm running a 2002 triple 500 EFI's and it runs 82 GPS. Motors should run on 87 no problem with no blowers. If they are 525SC's you need 92+ and they will eat it up!
GPH is not good on a triple boat, but I tend to run lower RPM to cruise. Our boat has 28 bravo I's and I cruise around 2,500. It does almost 6 mph at idle.
Just wondering if you're talking about a 48EC? Not sure if they made a 47 or not. If so that boat is a bigger animal. Had a buddy with triple diesels and it ran around 60.