Max RPM for Cummins 150 hp motor

[NorthernFocus]

So why not just change out the transmission to a different gear ratio?
Just a old farm kid asking! Bob

Cost. Which if cost is not a consideration you could also go with a variable pitch propeller.

 

[BB marine]

...If the engine can not turn the prop at the recommended WOT rpm it is over propped for all RPM ranges.

I'd be very interested in expanding my understanding by having access to the technical references that are the basis for this statement. Please share. While there's no argument that sizing the wheel for design RPM at WOT is best practice I'd like to understand how not doing so means that the engine is always operating outside it's design window/intended duty cycle.

Dan, You asked this question early on and answered it many times in full agreement of my statement. My statement stated " If the engine can not turn the prop at the recommended WOT rpm it is OVER PROPPED FOR ALL RPM RANGES. I believe you agreed with this statement !

Thanks for making my point. Your test procedure is reflective of how most people operate boats. They are fixated on a given rpm and run there no matter what. Of course if you run at a fixed rpm the engine will run harder/hotter if it is "over propped" relative to optimal WOT sizing. No argument. Never has been. But run your same test at percent of WOT for the two props and you'll get different results. Or ignore rpm completely compare exhaust temperatures and boost pressure at the same fuel burn rate with different props.

In my statement I was talking about boat owners that is what the forum is full of. Most boat owners are fixated on rpm. In my Hypothesis I gave the example of 3 different RPM ranges. The engine when over propped was loaded differently in all RPM ranges. The manufacture sets the WOT RPM recommendation based on the designed load that they feel gives the engine the best performance and longevity. The manufacture also knows that most boat owners are not Mechanical engineers and know one thing. Speed and rpm. The general statement to someone propping a boat, make sure that it is Propped to the manufactures recommendation. This takes the guess work out. Propped correctly WOT at recommended rpm I can operate my engine anywhere I want all day long. With the exception of WOT. I have operated my boat similar to you by using fuel burn has my reference to engine load. The Volvo D3 does not indicate engine load on the EVC. I now have a direct injection diesel that does not give fuel burn stats or load percentages. Old school larger displacement 4 cylinder diesels. I have to do about a years with of log book entries to get fuel burn averages at different speeds to figure out what my exact fuel burns are at different rpms in different different sea conditions and with different loads on board. OR confirm that my Max RPM is at or 100 rpm above the Manufactures recommended RPM to confirm I am loading the engine at the engine manufactures design. If I don't make full rpm, I am over propped thru out the full rpm spectrum. Boat owners, Boat Builders, Boat mechanics , Marine engine manufactures have to use squiggly lines to get from point A to Point B. It works!

And people, PLEASE, stop comparing how a boat engine runs to cars/trucks. And if you can't figure out why then ask yourself why cars are sold with 200hp engines when it takes 20hp or so to cruise down the highway at 60mph. And why don't electric cars have gearboxes?

Most boat engines are car engines. Car engine that were designed to operate at 10 to 20 % load. Then they are put in boats and operated at 80% load. This is more reason to confirm it is propped correctly to maintain the expected longevity. The D3 Volvo that was installed in my Cutwater was a engine that was put in a Volvo S40, S60 and SUV. Great little engine that spent most of its time at 2000 rpm. Now it is put in a 8000 lbs boat and operated at 3400 rpm hours at end or "80% load" burning 9 gallons an hour. When over propped this engine is loaded above its design from 700 rpm to 3400 rpm when the throttle is pushed down. You can say when I get it up to my speed I will be below 80 % load so I'm still loading it properly even though my RPM is 400 below what the Manufacture says it should be at 80 %. My fuel burn is 80% of max fuel burn my RPM is 400 under what it should be but the engine is loaded correctly. That is above my pay grade to figure out! My pay grade is I tell the boat owners that I know and have dealt with the same thing that I have stated in this thread. Prop the engine as per the manufactures recommendation. If you do not the engine will be over propped through out the full rpm spectrum.

I hope you had a good Flight! I am enjoying this discussion too! As a squiggly line technician 😀

 

[NorthernFocus]

I'm actually still in transit heading overseas.

We fundamentally agree on this issue just approach it from different directions. In the middle part of your post you hit the nail on the head. In addition to optimal design WOT prop sizing is to protect people from themselves. Or actually to protect engines from boat owners.

Also it finally occurred to me that the whole point of the truck running uphill is just to get across how heavily loaded a boat engine is relative to a road vehicle. Bringing gears into the analogy led me to over think it.

I've just been trying to make the point that if the boat is operated with knowledge being over propped or over loaded doesn't have to be detrimental. But no argument sizing the wheel properly is best practice and should be the starting point.

I wrote and delivered a lot of operations and maintenance training over the years and always tried to focus on why not just what. I can't help trying to educate rather than just telling people do this or that. Fishing rather than fish.

Boarding next leg. Cheers.

 

[dgiles]

Biggest point is if the boat is overpropped, you will never achieve max performance. If you are a trawler or want to cruise at trawler speeds, cool. If you want your boat to perform at max performance at times or when needed, you need to be propped accordingly.