I've spent some time on the phone with several cummins techs and gotten some interesting feedback from folks on the baotdiesel forum. This is what I have learned so far. Basically, throw the mercruiser manual away. I was told the manual was being revised. Cummins techs told me to prop the engine to run 4600-4800 wot with a max load then never run it there again. Biggest problem for this engine is to over prop (over stress) it.
My boat came with a 15x19" pitch prop that will just pull 4100rpm. That's just fine according to the manual where it states the engine should turn 4000-4400rpm WOT. Where the problem comes in is that the engines rated "high idle" (zero load on the engine) is 5000rpm +-50. Mercruiser manual has us proping the engine to turn 80-90% of it's rated rpm. Cummins says that's too low and can seriously over stress the engine. I went to a 17" prop that now turns 4400 with a decent load. That is still on the ragged edge according to cummins and they recommended having it repitched to 15 or 16". They really want me to see at least 4600 rpm and preferably 4800.
Where the real problem comes in is when we are running an over stressed (over propped) engine that has to bleed off some boost and then has a sticky wastegate. Engine over boosts and breaks. Having the engine propped to turn 4600-4800 then never exceeding 4400 will keep the engine from reaching the overboost condition in the first place and should really never see the wastegate open. Pay close attention to the wastegate hinge pin. If it rusts and sticks you may blow an engine. Just about every engine failure I've been able to track down has been directly related to a failure of the wastegate to function properly. Best solution is to keep boost (stress level to the engine) down to the point where it doesn't even try to open the wastegate. I have not found this part (wastegate) on my engine yet but plan to do some looking SOON. Second is to get a boost/egt gauge and watch it closely when under a heavy load. Boost/egt are the blood pressure and pulse rate of a diesel engine. We keep those under control and we're less likely to give the engine a heart attack. Best gauge would have an alarm for over boost.
Problem is apparently worse for lighter boats like RIBS who may have propped the boat for speed empty then add a load of people and dive gear that may add 50% to the load in the boat. The large % change in load possible in smaller craft is definately something to consider when selecting a prop. Best solution is to have at least two props. One for speed when you are running a light load and one with a lot less pitch for those days you load it down to the gunnels!
This is all just my opinion/observations and I'm not recommending anyone do anything different than the manual suggests. I am just passing on what I have been told. I do suggesting you contact cummins by phone or email at wave.master@Cummins.com
for their recommendations. If you own one of these engines I strongly recommend you contact a cummins tech who has a clue about these engines. Be sure to get his name and number or better yet get an email reply for reference in case anything goes wrong. Beware of Cummins too, they have very few techs with any REAL experience with these engines.
Hopefully I can relate some meaningful numbers as soon as I get the gauges installed and try some different props. Arlon