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== == == == Most large mass-market vessels (Carnival, Royal Carribbean, etc) have top speeds around 21-24 knots. Smaller vessels and boats designed for long distances can go a few knots faster (the Queen Mary 2, for instance, can do 29 knots which is highly unusual for a large cruise ship).
This does not mean they always operate at full speed-- generally they will go as fast as necessary to get where they are going on a given day, so on a short hop between nearby ports, they may go much slower. Figure 14-20 knots is a pretty typical range for most cruises.
A Knot roughly equals 1.15 MPH. So that gives us 16-24 mph. Again, this is just the ballpark. The reason for the low speed is the rapidly increasing resistance of water against the submerged hull. Ships designed specifically for high speed (warships generally) will also result in a bouncier ride, which isn't something most cruisers are interested in.
