Originally Posted by willk
It appears that the unit is what would be referred to as a "Leisure" engine as opposed to Commercial. As such, it is rated for limited hours per year and can only be run at max rpm for limited periods (expressed here as a % of run time). Typically, more HP will be squeezed out of a leisure engine than from a similar commercial block.
Correct but very very few diesels are rated for continuous use. You're probably only going to find them in trawler type displacement hull - definitely not a rib!
Mechanical Cummins 5.9L blocks produce anywhere from 140 hp (non-turbo, non aftercooled) to 370hp (turbo charged and seawater aftercooled). The 180/210hp rating is turbo charged but doesn't have an aftercooler. Cummins newer 5.9L common rail engine (QSB) has been tweaked to make up to 480 hp.
The US Navy gets 99% of their Willards with the 180 hp rating which allows a little more WOT use (50% of the time allowed at WOT versus 25% of the time for the 210hp):
"Medium Continuous Rating: This power rating is intended for continuous use in variable load applications where full power is limited to six (6) hours out of every twelve (12) hours of operation. Also, reduced power operations must be at or below 200 RPM of the maximum rated RPM. This is an ISO 3046 Fuel Stop Power Rating and is for applications that operate 3,000 hours per year or less"