The US Coast Guard uses 251s for main propulsion on two classes of cutters. The 210 foot cutters have a pair of 16-251s, I believe rated at 3200 HP each, while the 270s have a pair of 18-251s, rated at 3500 HP each. I've had the pleasure of standing between a pair of the 18 cylinder engines, running at about half speed. The Polar class icebreakers (Polar Star and Polar Sea) each have SIX 251s. I believe the US government and some of the utilities are the largest purchasers of new 251 engines and parts at this time.