21' boat will need either 1 3 liter based motor(300x,new 3.2 green engine) or if you want a real fast boat 2 2.5 (like 280 off shore or drag 300) engines.boat with 2 engines will be hard to insure and lots of maintenance-2.5s are race engines.a 21 with new 3.2 300 merc might make 100+ mph with good setup and prop.and get good fuel milage and run smooth.your wife wouldnt even thinks its a fast boat until you floored it