Cummins QSD 150 hp / R-25

OK brats, not trying to start an arguement here, i"ve never owned a boat with a diesel, but i have been operating heavy equiptment since i was old enough to reach the pedals & levers, and can personaly gaurentee you that all that engine RPM talk is mostly a bunch of bull. i own & have owned many different diesel engines with THOUSANDS of hours on em , most machines i "ve bought used with well over 1000 hrs on them . many times in winter i"ll start up say /dozer / excavator/back hoe......... and one may idle for 6 or 7 hrs & get used for 1 or 2 per day and they run fine , a good yanmar example is a small john deere tractor loader i use all the time i"ts small & gets used mostly to squeze in tight spots like in foundations....etc. because it"s only a yan 30 (low power) it basically has lived life at 2 rpms wide open or idle and has done so for well over a grand and still will for many more hrs of abuse ,. Most of the big horse power stuff sits on the pump limit (governer) -(wot) whatever you want to call it or chugs along a bit above idle for days , weeks ,months on end / run just fine. yea you have to clean - em out - rev -em - up get -em hot once in a while - maintain -em . but its kinda funny to hear everyone talkin about such & such a rpm so& so hrs bla bla bla turbos factory manual says ........ ha ha ha..... trust me them diesels will take more abuse than any other elect. gas. or plastic junk we have on our boats. THATAWAY - i dont think they did all that much to squeze new 150 into the 25 mabey motor mounts , doubt they changed shaft, rudder, or probably even the prop for that matter, but who knows , they could do same for 21........ john
 
I took delivery of my R25 a few days before the new 150 Cummins showed up at Wefing. I had been staying in my new boat at the local marina for a week when Marc brought the slightly newer boat out to check everything out. Our boats were both red and caused alot of interest at the small marina. Mine has the 75hp Yanmar (11 kts is too fast for me). The new Cummins is much quieter than my Yanmar and actually looks lighter. In fact when I compared both boats in water the newer boat set higher with the main exhaust completely out of water. Of course I had a full load of fuel, water, and God knows how much weight. My experience is that either of these engines will outlive most of us. I love diesels.
 
Another plot for discussion. I took some data points last summer while running Snug Tug at various speeds, full fuel, full water, three people on board but not fully loaded with cruising gear. (top speed with cruising gear added might be another mph or two so slower) Below is a plot of speed (MPH) vs MPG (statute miles) for the R-25 (Snug Tug). Speed measured using GPS. Gallons per hour (GPH) used with speed to calculate mpg from Yanmar (110 HP) engine propeller loading curve showing GPH vs engine RPM. (As they say on the automobile stickers: "Your mileage may vary!")

Speed+vs+MPG+++(R-25,+Yanmar+110+HP).jpg

Seems like once you get past 9 miles per hour (7.8 knots) the miles per gallon is pretty flat from that point on. There doesn't seem to be a "hump" in the curve where the MPG increases just after the boat goes on plane. Maybe my data is not so good or maybe the drag from the keel, which is always in the water even when the boat is "on plane" prevents the boat from acting like a true planing hull. It is a semi-displacement hull after all.

Some have talked about the boat being "on plane" at speeds of 15 knots or more. Doesn't the sharp break in the curve at 9 MPH (7.8 knots) indicate that the boat has left the displacement mode and started to plane? (at least as much as a semi-displacement can achieve a plane) I've got lots more data, but am not sure how to interpret it.
 
A little hard for my old eyes to read your curve information. But what your first curve shows to me, is how effecient the boat is at low speeds (to be expected). The resistance markedly increases at 7.8 knots. Assuming that the LWL is 20 feet, The sq root is 4.47, the max effecient hull speed is 6 knots, and at 7,8 knots you are at 1.7 x sq root and starting to approach the speed where the boat begins to climb over its bow wave. (definition of a plane is when the boat has fully climbed over its bow wave and the wake is cleanly breaking aft). Semi dispalcement boats can be pushed even up to planing speeds with enough HP. But in this case, you are limited to 13 knots. (15 mph) Or only 2.9 x sq rt lwl--at the high end of the semi displacement speed. If you look at David Gerr's The Nature of Boats --a book I highly recommend--the number for plane for a heavy deep V would be 2.9 to 3.1 x sq root LWL. But the Ranger takes more than that because of the keel and its lines. (of course there are many factors in each hull form, so there is no precise universal number). But with a semidisplacement hull, you may not see a real "hump" or it may be broad, since the boat does not leap on a plane. In fact your curves look very similar to Gerr's for a displacement boat. I suspect that even with the 150 hp you will not see a true hump, as you do with a typical planing boat. There are also known calculations for HP vs weight: for example a boat weight of 7000 lbs with 110 hp would struggle to get on a plane, although with a very effecient hull it would be possiable. For example the CD 25, weight at 6500 lbs, and 130 hp is a bit underpowered--and the C Dory hull is a very effecient planing surface.

Many planing boats will be on a plane in 5 seconds, and it is difficult to hold the boat between the beginning of the hump and until it is on a plane. You have to apply a lot of force during this time. Once the boat is up on a plane, you back off on the throttle and the fuel consumption drops. This is not the case with the Ranger tug. It is application of HP and the speed gradually accelerates.

So I don't see any surprises there, and do not conclude that the boat is planing. Thanks for posting the interesting data!
 
Thanks for the additional background. I'll have to get a copy of that David Gerr book. It looks like something dramatic occurs with the R-25 at around 7 to 8 knots (8 to 9 mph). If I understand what you say, then above that speed it's in a semidisplacement mode but not yet on plane until somewhere over 15 knots. Maybe somebody needs to put a 200+ hp motor on one of these babies to see what it does across a wider speed range. Then it would be easier to see if there's a "sweet spot" in the speed curve and what hp is needed to get there. Of course the variation in weight and other conditions makes it hard to compare one test against another. Manufacturers typically use empty boats with near empty tanks to get the performance figures they publish. I don't know what premium Ranger charges for the new 130 hp and 150 hp engines, but if they're quieter, then that in itself might be reason enough to use them. As for me, I'm already committed to the Yanmar 110 hp. Given the recent trend in diesel fuel prices I may have to stick to 6 knots anyway!
 
The dramatic thing is that once you exceed 1.34 (give or take) times the square root of the waterline, it starts using a lot more fuel (HP) to go faster--and that is the point at which the boat starts to climb over its own bow wave. I haven't done the numbers but probably you will be using 10 hp (.8 gal/hr) at 6 knots in smooth water.
 
During the sea trials the R-25 did about 6.25 knots at 1800 rpm which on the Yanmar engine data sheet shows 20 HP and about 1 gal/hr. That gives just over 6 nautical miles per gallon, Not bad mileage but s - l - o - w . Most of my previous experience has been with sail, so I'm acclimated to life in the slow lane.

Cruising: "The fine art of going nowhere, slowly and at great expense."

I ordered the David Gerr book from Amazon. It says it's for the "nautically obsessed," which is about right for me! Looking forward to studying it.
 
I am going to have to agree. I think it would be best to take a little cup out of that prop. I had run that boat at 4050 rpm at 21.0 kts with 20 gallons of fuel and 10 gallons of water on board. I think you should gain about 100 rpm to bring you in the 3900 range.
 
OK, now I'm confused. The comments directly above are for an R-25 with the Yanmar 110 hp engine. Max rpm = 3200. It hits 3200 at full throttle, so the prop seems to be pitched about right. Are your comments for a Cummins 150 hp powered R-25? We've been discussing hp vs speed of various engine combinations in this thread, so maybe it's getting confusing?? Some of the posts earlier in the thread gave info on the 150 hp boats. We're just trying to get a handle on performance vs horsepower expectations and experience. Anyway, thanks for the additonal info!
 
IT-SEA-BIT-C":27jbtk13 said:
OK brats, not trying to start an arguement here, i"ve never owned a boat with a diesel, but i have been operating heavy equiptment since i was old enough to reach the pedals & levers, and can personaly gaurentee you that all that engine RPM talk is mostly a bunch of bull. i own & have owned many different diesel engines with THOUSANDS of hours on em , most machines i "ve bought used with well over 1000 hrs on them . many times in winter i"ll start up say /dozer / excavator/back hoe......... and one may idle for 6 or 7 hrs & get used for 1 or 2 per day and they run fine , a good yanmar example is a small john deere tractor loader i use all the time i"ts small & gets used mostly to squeze in tight spots like in foundations....etc. because it"s only a yan 30 (low power) it basically has lived life at 2 rpms wide open or idle and has done so for well over a grand and still will for many more hrs of abuse ,. Most of the big horse power stuff sits on the pump limit (governer) -(wot) whatever you want to call it or chugs along a bit above idle for days , weeks ,months on end / run just fine. yea you have to clean - em out - rev -em - up get -em hot once in a while - maintain -em . but its kinda funny to hear everyone talkin about such & such a rpm so& so hrs bla bla bla turbos factory manual says ........ ha ha ha..... trust me them diesels will take more abuse than any other elect. gas. or plastic junk we have on our boats. THATAWAY - i dont think they did all that much to squeze new 150 into the 25 mabey motor mounts , doubt they changed shaft, rudder, or probably even the prop for that matter, but who knows , they could do same for 21........ john
Click to expand...

John, there were several interesting tech articles in Passagemaker about a year ago in which Steve d'Antonio, the tech editor, explored this issue in detail. Long story short, it appears that engine speed is important to marine diesels. I'm no mechanic, so can't argue one way or another, but d'Antonio backs up his opinions with data that might be of interest to you.

Warren
 
I sucribe to passagenaker and Steve's articles are entertaining , and somewhat informative, just take most of it with a grain of salt . I 'm just trying to say if you maintain diesels well they are pretty darn hard to hurt at any RPM & will last longer than anything on the boat , cept mabey the anchor :lol:
 
I am going to have to disagree with it-sea. The older type of tractor engines which were low speed, non turbo were almost bullet proof. But they do have problems and require careful maintance: filtered fuel, oil changes in the injection pumps (Simm's pumps). Today most of what you buy are highly turboed, they live in a salt air environment, they are often not run for months and then run hard. Some marine engines are tractor conversions--but many are not. The engines as in the Ranger 25 are turboed, they run at high speed (Acustis notes he has run one at over 4000 RPM). These engines have to be run up to temperature and the turbo spooled up to prevent carbon deposition (and some other problems)
I have had personal discussions of Bob Smith of American Marine Diesel, who has been one of the largest distributers of marine diesels for many years about the longivity of the high speed, lightweight diesels, and they are not going to give the 10,000 to 15,000 hours without problems which the slow tractor engines have in the past. 2,000 to 3,000 hours before major work appears to be common--if they are not abused. If you look at many of the high speed boats running diesels often they have been replaced or rebuilt in a few years or a few thousand hours. The sport fishers who run high speed turboed engines; rebuild the turbos every few thousand hours on a regular basis. The boat diesel running a boat at semi-displacement speeds is like a truck going up a steep grade all of the time.

Yes, you can run the engines at lower speeds, but you have to bring the engine up to operating temperature, plus every few hours bring up to at least 85% of WOT.
 
As funny as it may sound I also have to AGREE , with Bob disagreeing with me.( I highly value your opinions Bob ) Ihave 3 newer machines with turbo's and i gotta tell ya between besides the salt water enviorment , you wouldn,t believe the the enviorment they live in , crappy old fuel , usually watter logged, dust plugged raditators,cold temps in winter, and the list goes on............. So thats what i base my posts on. believe me when i tell you it"s not easy keeping up maintance of a small fleet of equiptment, especially when the push is on. stuff goes way past due, looking back on most of my posts my fingers are not really gettin what my brain is trying to say on the screen, i"m realizin that i need to work on that or i"ll have to stop posting, cause it isn"t commin out right. mabey i should have just said todays diesels if maintained properly & operated correctly will be very reliable . in my exp.just the oppsite generaly occours & the motors are still unbelievably reliable. tryin to help but i"m thinkin about hangin up hat sorry brats for any bad advice it was never intended ............... John
 
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