Max Extended Cruise RPM

S

spierp

Well-known member
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Jan 19, 2015
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Fluid Motion Model
C-30 CB
We have an R29 with the D4-260 with a max RPM of 3,600 RPM. I've found myself sometimes cruising at 3,300 RPM, which I guess is a little over 90% power. The closer I get to WOT the more the boat levels off and gets up on plane, so it's tempting to run at these speeds when we're trying to get somewhere.

I've read some posts saying they don't like to cruise for extended periods of time over 80% power. Is there any science or volvo or ranger data that is driving that 80% number? I've also seen posts suggesting that it is advisable to run your engine at WOT periodically. Does anyone know what kind of safety factory is typically built into the max RPM. Will running close to WOT for an hour or two potentially damage or reduce the life of the engine?
 
Not positive about the Volvo but most of these light weight, relatively high rpm engines are built for "80 percent service factor". That means they are designed to operate continuously at 80 percent load (i.e.HP) or at an average load of 80 percent over the course of a "day". WOT is intended for short term/emergency use only.

Those who promote running at WOT occasionally to "blow out the carbon" are behind the times. These modern, electronic injection controlled engines burning ULS fuel run clean and don't need "blowing out" like old diesels unless they've been idling for hours and hours. Many mechanics still recommend it but they are slow to embrace change 😀 And especially don't listen to a car/truck mechanic about how to treat a boat engine. Grossly different loads on the engines.

If you peruse your Volvo manual it likely indicates the service factor somewhere.
 
Dan, I might comment that even with a modern electronic diesel engine, when operating at low power for hours on end the 'experts' (whoever they are) recommend a few minutes of high RPM every hour or so. Not necessarily full throttle but lean on it for a few minutes.
There are a number of cogent reasons for that - diesels, due to laws of physics, run best when run at or near the designed continuous output, the turbocharger bearings benefit from being spun up at frequent intervals, lower cylinder temperature leads to more internal soot deposits, yadda, yadda, yadda,
Put on your reading glasses and be ready for extreme excitement whilst reading this:

http://www.diva-portal.org/smash/get/di ... TEXT01.pdf

:mrgreen: OK, I don't expect many of you to actually read it. And Dan is right on that modern common rail engines with electronic fuel injection tolerate low rpm, low load, operations quite well.
Still "blowing it out" before you shut down is recommended - by me at least.
 
The Volvo's or our D-3 at least has a variable pitch turbocharger. I was told to make sure the turbocharger gets fully exercised from time to time. This to make sure that the vanes move freely and free themselves of what ever kind of build up that may occur. Every couple of hours I will run the engine at different boost levels to insure the above happens. I also from time to time make sure the engine will hit the max RPM target which in our case is 4000 rpms.
Not for long just to insure everything is as it should be. We cruise at either 7 or 8 mph or 14 to 16 mph. Speeds in between show worse fuel economy as the boat is not up on step. I usually adjust trim tabs so the forward wash from under the boat comes out just forward of our helm seat. Just my opinion here. I think limiting long term rpms as recommended to 80% of max has merit. These engines have relatively light blocks and are different from heavy industrial or truck engines.
 
Quote from page 61 of the D4 Operating Manual "For best fuel economy operations at full speed should be avoided. We recommend a cruising speed that is at least 10% below the maximum engine revolutions at top speed (full throttle)".

The manual goes on to acknowledge that various configurations result in different "maximum engine revolutions"; how, the 10% seems the recommended number. This is also confirmed by Ranger / Cutwater.

So according to these sources, if your maximum engine rpm is 3600, maximum recommended cruising rpm would be 3240.
 
thanks all for the responses, this is very helpful.
 
I was told by the dealer that Volvo is happy for their engines to be run at WOT indefinitely (80% recommended by Yanmar). The 90% recommendation specifically mentions fuel economy, so if you need to get there fast and money is no object WOT seems to be the way to go.
 
Regarding fuel economy, since the MPGs are nearly the same for any RPM above around 2500 RPM (at least for our boat). If you place equal relative value on time (1hr) vs. gallons (1 gallon) you would run at either 1000 RPMs or WOT (or 90% of WOT if you prefer) and not in between. In between these RPMs for each gallon you save you lose more in hours for an equal distance.

Of course there is more to boating than time and dollars and when the price of fuel goes up it may change the balance point. For me, noise is a huge factor as I like to listen to the stereo and/or talk comfortably while underway. The water conditions also play a huge role. We like 3500 RPM for a very comfortable balance of factors if the water is relatively calm and we are trying to get some where. If the point is just to be on the boat enjoying the sights and weather we just go slow.

See chart below for R27.

 
thanks @redraven, Knots multiplied by NMpG is an interesting metric that I had not thought of or seen before. I suppose it does provide some indication of optimal cruise speeds if placing equal value on hours and gallons as you say.
 
Admittedly, I made it up. It was a way for me to help decide what speed is the best choice economically givin some value time. Note that the units come out to nM(squared)/gallon hours which is kind of odd but the key is the denominator (gallon hours). MPG isn't a satisfactory metric for me as I value my time and our 1 day cruising range (which we usually are limited to with guests) would be quite limited at 6 knots. The fact that we stay at relatively the same MPGs above 2500 told me the value curve was goIng up in that range. I created the metric to see where the bottom of the curve was and to see if it went back up at lower RPM ranges. It does but I was surprised to find how low the RPMs to go to be more efficient (per gallon hours) than top speed. I try to stay away from extended cruising at the bottom of the curve unless time is not a factor.

Bottom line. It makes me feel better about going fast all the time! 😀
 
Red Raven":pphtsxto said:
Bottom line. It makes me feel better about going fast all the time! 😀
Click to expand...

As a friend and fellow Ranger Tug owner says, you pay for and/or service your engine based on hours used, not miles cruised. So miles per engine maintenance dollar is another interesting metric and/or rationalization 😉

Cheers,

Bruce
 
Frankly this discussion is taking a disturbing direction. There hasn't been any consideration given to beverages consumed per mile traveled. I realize some of the younger folks out there can chug them down at 15kts. But some of us older folks have to sip our coffee/beer/wine etc. But regardless of rate of consumption, for a given individual the beverage/nm parameter is inversely proportional to speed. Just additional food for thought (pun clearly intended) 😀
 
I have no problem with the chugging the beverages at 15kts. It's the mixing that is the challenge! 😱
 
@Bruce. I like it! Go fast, more miles per engine maintenance! 😀
 
Incidentally, I never understood the 55 mph limit on the freeway we used to have. Yes, saved fuel per mile, but at what cost to the economy (and our life) in time? I suppose that is why they axed it! Bad metric! :twisted:
 
This is good information , nice job on the chart. We've owned our Cutwater 26 for 3 months boating on Lake Michigan seldom calm water most boating has Been in 1'to3' seas . My speeds and fuel consumption calculations may be better in calm waters. I have been keeping a log on every cruise we have taken to get a true fuel consumption to RPM and speed . My numbers are close to your 27 ranger with you actually performing better in all areas . I have a 220 hp D3 and the stepped hull, I thought that higher speed fuel economy would be a little better then the tug and that was one of the reasons for going with the Cutwater over the tug . Not so. We plan on cruising the Great loop in 2 years and fuel consumption is a major factor in a couple of areas on the trip . As your chart shows and what you have answered is slow 7kt's is the cut off for best fuel economy or cruise range . This is close to what I have logged also . If cruise range is not a issue for us when going to a destination I cruise at 3220 rpm average speed 12kt, 7.2 Gph ,2 mpg , which is slightly under 80% . If I knew that the D3 longevity would not be compromised by 90% running I would run there because I get the same fuel economy and increase approx 5Kt in speed . While cruising the rivers and ICW I expect to be mostly running 7kt and enjoying the sights and increasing cruise range , but on open waters it would be preferred to get up and go . Knowing what other Ranger/ Cutwater owners have experienced is good information . It would also be good information to get factory feed back on Volvo's recommended use .
 
BBMarine, The performance numbers in my chart are numbers from the factory in 2014. They were what I had when I created the graph and the general shape showing the optimum speeds is the same. See my photo album for all the factory charts I have (Tugs only). The performance numbers for our tug are not as good as the factory numbers. (1-2 knots less at each RPM) we are typically loaded up with a lot more weight and have bottom paint so that probably explains the difference. We are still very happy with the performance. I will start collecting some more numbers to build charts specific to our boat.

The Cutwater is designed with a tilt toward speed vs. the tugs economy. I would think the performance is best at higher RPMs. Let it go!
 
BBMarine,

If you create your own log of Speed versus RPM and Fuel Consumption, I suspect you will find the numbers have GREAT variability based on wind, waves, tide, bottom paint growth, and weight (fuel, water, waste, people, & your stuff) on your boat. I say this based on my own data and charting which you can observe in this link:

viewtopic.php?f=5&t=5868&p=42866&hilit=nmpg#p42866

As you can see in my charts, my data points varied a lot, so that was why I used a polynomial fit to get a "picture" of what happens in general.

To make it easy to capture data points, I simply used my cell phone camera to take pictures of the Chart Plotter periodically as RPM, Speed, and conditions changed. Then I entered the data points into Excel and plotted it.

So, in conclusion, if you want to plan your boat's range for the Great Loop fuel stops, I suggest getting actual data points for your boat in various conditions, then plot it.
 
Take a look at the R25SC Fuel Consumption topic. None of the tested conditions are very significant, so the numbers are pretty close. The Standard Deviation of the datasets are such that the arithmetic average is appropriate. The are also log-normally distributed, indicating an appropriate commonality in the dataset and accuracy of the data collection. In my day job I do geostatistics...fun with numbers. "Liars, Damn Liars and Statistics" (Mark Twain). 90% of all statistics are fake and the other 15% are made up" (unknown).
 
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