Originally Posted by jonp
I own a 550 pro with a Yamaha f70 and have no problems reaching 6000 rpm 32 knots with 3 divers and 6 tanks etc. My engine is now mounted 2 holes higher than the dealers had set the boat up from new and I've changed from a 13x15 inch alloy prop to a 13 3/4 x 13 Solas stainless.
My friends Pro 550
4 divers, cold water kit (drysuits)
Mercury 70hp, 13 pitch aluminum prop, 5800 rpm or so was about 27 mph.
For dive boats, I strongly encourage rigging at the max engine rating.
The downside is that the 5.5m is a fairly short hull, so in rough conditions, you're not going to need the power anyways because you're going to have to throttle back. Offshore you could probably get away with a smaller outboard because of that. Great Lakes region, definitely go with max hp.