Jeff,
Iām going to suggest, and Iām sure I will either be corrected or confirmed, that the technical definition of whatās a safe rpm to cruise at is a more complex question than what Cummins is giving you with their rating of continuous duty and recreational use. Cummins is throwing those terms as a measure of engine warranty with a big helping of marketing.
What your interested in boils down to the simple question of what rpm (max boat speed) can I use and expect to have reasonable engine life. That depends on a whole set is assumptions including health of your cooling system, water temperature, engine loading at that rpm, design of your exhaust system, etc, etc.
If you look at what Tony is saying – 160 thermostats, idle pulley upgrade, coolant alarms, and PROP ADJUSTMENT – ask yourself what do these things have in common and what do they mean to how I operate? Simple answers. It means ensure you can get heat out of the engine and if there is an issue I want to know about it before the heat gets high. These things are of utmost importance when you are asking 100%+++ from your engine.
High flow 160 thermostats will help keep the engine operating warm enough to be efficient, get the heat out fast, and have a margin if something goes wrong. That means ~30 degrees of rise before the 195 alarm goes off and you pull back. Thatās plenty of margin to not do damage to the engine. So what can go wrong? Look at those idler pulleys and do some reading, they are a weak link and if one fails your cooling system goes to zero and heat rises immediately so we remove that liability with the upgrade. I still recommend at least yearly to remove the belt and give a spin to everything to make sure all is smoooth with no side play etc (idlers, alternator, pump, etc.).
Now for the prop adjustment and this is key. Think of going up a hill on a 10 speed in high gear, you can hardly turn the pedals, your working hard, slowing down as you go, getting hot. Now shift to a lower gear and it becomes easy to pedal, you start to accelerate, your feeling better and getting in your rhythm. This is why we prop down, we want that engine to work easy, not overheat, and have the energy to accelerate to its potential, and do it all safely. This is why you need to get your props adjusted.
Your question of what is max rpm you can cruise at continuously, Iād suggest that without a EGT guage there will be lots of suggestions but we canāt know for sure. Why? Because every exhaust design is different and that affects the EGTās. High EGTs will affect your turbo life. These diesels are just heat pumps when you boil it all down.
BUT, by doing all the things Tony is suggesting we can be reliably ensured that you are operating safe at ALL RPMs. That doesnāt mean go out and run 100% full time. Most boats have a comfortable cruise speed in your example 6-8 knots. And on occasion if you want to run at 2100-2200 you can do it safely.
Finally your CCV issue can be managed but donāt underestimate the importance of draining your oil pan 100% bone dry, adding 12 qts EXACTLY, and using that as your mark on the dip stick.
Let us know how it goes, we are all here to help!
Phil
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