My recently purchased GH37 was repowerd with brand new 4JH57 hp Yanmars with 2.32-1 reduction gears to swing the 24-13 props . I effortlessly do 6.5 knot at 1750rpm with another 400 rpms getting me another 1\2 knot and fuel burn combined going from 2gph to 3gph.
Currently the diesels will only max out at 2750 of the recommended 3000rpm .And though I will rarely need to operate in the 2500-3000 range I will have the props modified to achieve recommendations per manufacture.
Over propping and Lugging even if your only running it 3/4 max rpm most of the time will slowly kill any diesel, if your diesel or even gas motor will not Max out at manufactures recommended rpm it is over propped.
Many vessels especially planing or semi planing come from the factory propped correctly to run max speed for engine manufactures recommended max and minimum rpm. Anyone who has ever unpacked a vessel after Selling knows how the additional weight piles on over the years as non comes off and that adds to the symptom of over propping.
Displacement vessels diesels will suffer from excessive drag or if they were improperly over propped especially in the smaller displacement or turbocharged diesels as they build excessive heat from being lugged.
I'm a trucker and own a old rig with a 435 hp N-14 liter in line Cummins , perhaps one of the best engines ever built , but if I l dont split or drop a whole gear on a long hill climb to keep the motor revving and breathing my exhaust Pyro Temps will climb and if not addressed will cause damage . This scenario is no different than a over propped vessel.
Years ago i was in the commercial Parasail business for too many years and when I converted over to the small diesel fad from BB gas my headaches began. They were over rated and had a small peak torque range ,mainly the early KD Volvos , never mind the short lived 6.9 Mercruiser Navstar .
Switched over to Yanmar and thought life was going to be great when when they came out with the 6ylp 300hp which was a marinized and proven Toyota excavator propulsion .
Stated on the engine plate the recommended rpm range 3400-3800rpm so I underpropped it because of the high drag load of a 38' parasail loaded with 450lbs.
Got 1600 hours out of it before it had soo much piston ring blowby that it only had 70%of torque anymore.
Replaced it with the same motor now rated at 315hp but recommended operating range had been changed from previous motors 3400-3800 to 3800-4200. I had killed my previous motor per the manufacturers recommended rpm range, and no they would not acknowledge that change . The following 315 had upwards of 3000 hours on it when I sold it .
The lightweight high hp turbo diesels are not a 10, 000 hour motor more like 4,000 at best unless its that little 5.9 Cummins, and are the most effected by over propping.
But its not just small diesels, many 60-70' high speed cruisers with big 1000-1400 hp Manns in them can suffer from the same fate. In the past Many were built and had Arneson surface drives to minimize drag but ended up later being converted over to conventional inboard shafts . This was because the diesels couldn't take the high load torque differentials the surface drive demands throughout the speed range of a large heavy planing vessel.
Oh and service your engine and cooler zincs and keep your exchangers and intercoolers cleaned out as well as replace your exhaust mixing elbows in a timely manner .
