Cummins QSD 150 hp / R-25

[henrym]

Cummins QSD 150 hp / R-25

2 persons and 2 dogs on board, full fuel and ¼ water
Speed = GPS GPH = Cummins SmartCraft display
Square Root of LWL 22.5'± = 4.743 X 1.34 = 6.35± knots hull speed

1200 RPM 4.8 knots .6 GPH = 8.0 MPG
1400 RPM 5.3 knots .8 GPH = 6.6 MPG
1800 RPM 6.0 knots 1.4 GPH = 4.3 MPG
2000 RPM 6.3 knots 1.7 GPH = 3.7 MPG
2500 RPM 6.8 knots 3.0 GPH = 2.3 MPG
3300 RPM 15 knots 5.5 GPH = 2.7 MPG
3750 RPM 18.2 knots 7.4 GPH = 2.4 MPG

Max. continuous RPM = 3800
Max. RPM 4000
The QSD engine is so quiet that it does not sound like a diesel engine!

[journey on]

Those are interesting numbers, and I added two new columns. One is NMPG and the other is RPM/MPH. The first allows a comparison between the C-25 and the R-25. The second shows propeller efficiency as a function of speed (displacement vs planing?)


MPG---RPM/MPH
8.0--- 250.0--- 1200 RPM 4.8 knot 0.6 GPH
6.6--- 264.2--- 1400 RPM 5.3 knot 0.8 GPH
4.3--- 300.0--- 1800 RPM 6 knot 1.4 GPH
3.7--- 317.5--- 2000 RPM 6.3 knot 1.7 GPH
2.3--- 367.6--- 2500 RPM 6.8 knot 3 GPH
2.7--- 220.0--- 3300 RPM 15 knot 5.5 GPH
2.4--- 208.3--- 3750 RPM 18 2 knot 7.4 GPH



I would assume that the R-25 takes a lot of power to plane, but the propeller certainly is more efficient once its on plane. Or something.

Sorry about the formatting. If anyone can tell me how to post a table, I'd be thrilled to learn it.

Boris

[thataway]

Interesting numbers--and Boris's ads more "information"...for comparison.

This certainly seems like a much better combination than the Yanmar. You will be probably be actually cruising at the 3300 RPM--which is what it appears to take to get on a plane. It will be interesting to see how this holds up, as you fill up the water tank, put on more cruising gear and supplies. But it looks a lot better than the 110 or 125 engines. I think that validates the 150 hp choice over the 130. Just that few extra HP at the mid range will make all of the difference in getting on and staying on the plane. Looking at the Torque and HP curves of this engine, it is well suited for the boat. Above 3300 RPM, you start to loose a little torque. So that the 3300 to 3400 should be a good speed to run the engine on a plane.

Thanks!

[Dene]

Definitely the 150 hp would be worth the extra money, especially considering Cummin's reputation and how quiet they are.

Question....can one run this engine at 3300 rpm's for extended periods of time without damaging it?


-Greg

[thataway]

According to what I see on the Cummins site, the engine can be run for extended time at the 3300 RPM. Although they give a 3800 contious rating--I would be reluctant to run that full time--but that is a personal opinion.

[journey on]

OK, to show the comparative economy between a C-25 and an R-25, here's a chart. The C-25 MPH figures are from a Honda test. The R-25 figures are from the numbers for Little Toot, posted above. They seem to show the difference between a lighter planing hull and a displacement hull, at the higher speeds. First time gas is more efficient than diesel. Any explanation?



Here's another for Anna Leigh and Daydream, just for a check (both C-25's, with Hondas.)



Boris

[henrym]

Very simple - WEIGHT..

Little Toot weighed in on the scales at 7,500#. (trailer not included)
Full fuel, no water, and no gear. But, she has just about every option on the list.

FYI - When Ranger was demonstrating the boat at Miami, she had
6+ S.O.B. and could not achieve the her potential speeds. Not to
mention, there was a pronounced sucking noise coming from the fuel tank.

Henry

[Sea Wolf]

[thataway]

It seems as if the Ranger is over proped slightly--at least for that load. Dropping the pitch 2" (1.5" if you are having the prop changed by Prop Scan) would achieve the WOT. Fuel consumption would be about the same--and the boat might come onto a plane faster. But these are things which have to be worked out.

I am a little surprised that the C Dory 25's are getting over 3 miles a gallon at the higher RPM's. It looks almost like a flat curve--with no difference of fuel consumption at 4000 to 5000 RPM. What speed does each RPM range equal?

Boris, do you really mean GPM?--if that is a really big "sucking sound".

But 7 gallons an hour does not sound high to a lot of people when you are up at a planing speed with that size of boat.

Definately the C Dory planes easier, with less HP.

[journey on]

Well, Bob, isn't GPM gallons per meter?

Sorry, indeed it's Miles per Gallon, MPG, and I'm not going to change the chart annotation. Used to have a professor in Fluid Flow who would give a F if you got the units wrong. Guess I wasn't his best student though I did learn the difference between the weight of a cubic foot of water (62.4) and 2g (64.4.)

As to the flat MPG curve after it's up on plane, Joe, I'm just the data processor. The boats were probably nearly empty.

As to weight, Journey On weighs ~7000 lb. after Judy loads it. 2.5 MPG is about the worst I've seen, bucking wind waves. 3 on smooth water at 15 knts.

boris

[tpbrady]

Bob,

I see a similar fuel curve on my 22 where fuel consumption in MPG is virtually flat from 24 mph up to WOT at 31 mph. I think what is happening is drag is decreasing as more of the hull comes out of the water in response to the motor being trimmed up. The amount of HP required to generate velocity stays somewhat linear.

Or at least that's my guess.

[thataway]

Tom,
If you look at the vast majority of boat tests, as a boat speed increases, once it climbs over the bow wave, which is basically the defination of comming onto a plane, the miles per gallon decrease after a certain point. This point is considered to be were the boat is fully on a plane and is usually the most effecient speed. You are correct that seems to be little variation in many of these C Dories when they up on the higher speeds (you mention 24 to 31 mph)--but in the graphs in this thread, there is little difference between 3000 and 5000 RPM--that does not fit most other usual boat tests. I have about a dozen magazines in my room currently and they have about 35 boat tests amoung them. Typical is this
best speed--gives the best range and miles per gallon, and then gradually decrease in the upper ranges. Some are flatter in the upper range as you suggest, but none are flat like these curves in this thread.


Drag forces, include water resistance, wind resistance, wave making (eddy) resistance and any chop resistance, assuming anything but dead calm conditions. There are stepped hulls, hydrofoils and prop ridding boats which do have significantly less surface area.

See David Gerr's Nature of Boats.

For the Boat test.com (agree that they are optomistic)--
For the 22 with a 90 carburated Honda, the boat is on a plane at 14.2 mph and gets the best "mileage" of 4.51 mpg@ 3500 RPM. At 19.1 mph it drops to 3.81; 24.7 mph=3.55 mpg; 27.4 mph = 3.38 mpg, 30.9mph, =3.41 mpg, and at 32.9 mph (6100 RPM) = 3.45 mpg. Although the upper range is relitatively flat, there is a dramatic drop after the most effecient speed.

For the CD 25 with the 150 Honda: the best fuel economy occurs at 3500 rpm: 15.4 mph= 4.05 mpg; 18.7=3.6 mpg, 23.4mph = 3.71 mpg; 27.2mph=3.36; 31.3 = 2.85mpg; 32 mph = 2.44 mpg

In the graph above, Anna Leigh only gets 2.5 miles a gallon at 3500, where and then the miles per gallon increase to over 3.2 at 4000, and slt better at 4500 and 5000. Where as Daydream gets the 3.0 miles a gallon at 3000 RPM, and goes up to over 3.5 at 4500 and 5000 RPM. They do not show this one dramatic place where the fuel mileage is exceptionally better--and I suspect it relates to weight, engines, and propping of the various boats. Definately the Ranger tug shows a big hump--which is much less in the C Dory

[IT-SEA-BIT-C]

at what speed does the 25 tug start to break over on plane? what is min. plane speed? thanks john

[Wefings]

Another interesting factor , especially for the diesels [probably more pronounced than with the gas outboard, easily planed hulls] is that they are propped and tested in the relatively cool, moist ,dense air of Northwestern God's Country and when they arrive in Southern God's country they don't seem perform quite the same .
Marc

[Plan C]

>For the 22 with a 90 carburated Honda, the boat is on a plane at 14.2 mph >and gets the best "mileage" of 4.51 mpg@ 3500 RPM.

I'll mention that with the new fuel injected Honda 90, I have gotten a high of 6.5 mpg at mostly 4200 rpm. Or at least it made me feel a little high after I calculated that. On another extended distance and less broken in, I got 5.5 mpg. Nice engine.

Dave