RE: Replacing a Ford Lehman SP135
Yup, some commercial fish boats do indeed suffer early engine demise (maybe after 15 years) , at near*idling RPMs for hours on end as they pull in and drop off nets and lines.* A properly set up fish boat has an engine big enough to move a load of fish in big seas that may well add 50% or more of the empty weight of his vessel. Most, not all, are set up with internal oil heaters to maintain crankcase temps above 200 during these long periods of very low RPM.
I met one happy fisherman this summer who replaced his very high hour 25 year old DD with a JD that weighed 1/3 as much - more room for fish and no oil drips in the bilge.*Many commercial*boats sit idle for*months on end in the PNW, so "winterizing" or having a buddy run them under load every month*is a must.
Charter fishing boats often have a kicker so they aren't forced to idle their big diesels. A boat I fish on has a 9.9 Merc*with the twin 180HP Volvos*shut down for the 4 to 10 hours per day of trolling.