Torque and fuel effiency

I do not really have a very firm grasp of all I am about to discuss, hence the appeal to those that do, for a little illumination. LOL, I could be wrong!

I have a 28 foot cabin cruiser powered by a BBC, straight shaft, 1.51:1 velvet drive, 17 x 15 wheel.

With the help of a gps, flowmeter, extensive notes in my log, and a rudimentry spreadsheet program I have in essence a crude floating homemade dynometer.

As I understand it, my motor will basically only create enough power to overcome the load placed upon it. What I am tracking is my real world, real life, prop curve.

Again as I understand it, what my prop curve is showing is the resistance of my hull going through the water expressed as fuel consumed. By using the standard formulas I break out various estimates of horsepower, torque etc.

The window of performance I would like to optimise for is cruising between 10 and 15 knts. This is operating in a range of 80 to 120 horsepower and requires 250 to 325 # of torque at the prop spinning between 1400 to 2000 rpms as close as I can figure.

Looking at the performance curves for a Cummins 5.9 I can't help but notice that it can providemore than enough torque while burning an almost insignificant amount of fuel.

The somewhat fuzzy fantasy has me repowering and cranking in vast amounts of pitch into the prop to optimise for my target speed. I am very much aware of the danger of overpropping but what happens if you overprop but don't run in the higher ranges?

I'm not really considering repowering but if fuel keeps going up who knows. One good thing about higher fuel prices, you can rationalize quite a bit if you only divide the price into fuel tank units. or should that be ?

 

I went through the Cummins data here to see if the idea had merit:
https://www.gce.cummins.com/mce/mce_6b/assets/fr90210.pdf

What really matters is the specific power (for fuel rate). That means how much fuel do you burn for given horsepower. You really want to know where the engine is most efficient.

The data set is limited because it is only given for the maximum torque at any rpm. Irrespective I was surprised with the results. The most efficient engine speed at full available torque is 1000rpm. The specific power is 49.2HP for each gal/hr. At full speed the efficiency is much lower with specific power dropping to 19.5HP for each gal/hr. So burning 2.5 times the rate of fuel for each HP. Actual burn rate is 10 times more.

Problem is that 1000rpm will not deliver you enough power based on your figures. You would need to go to 1400rpm to get the 120HP.

There is also another problem with just altering the pitch. The prop efficiency drops from around 64% to 57% so this will offset some of the benefit. If you make the gear ratio 1:1 then efficiency gets back up to 62% at 1400rpm on the prop. If you can go to a bigger prop, say 20", and spin it at 1400rpm then efficiency climbs to 67%.

NOW remember that this is only looking at the engine at peak torque for given rpm. Cummins will know where the most efficient operating point is and you should ask the question if you cannot find the data. This is the point you design for but there are a lot of factors to consider. Most important is some allowance so you can make way in heavy weather. If you swing a large prop it could bog the engine in heavy going and the motor just makes smoke and noise - not much progress.

Rick W.

 

Rick,
I appreciate your input. I wonder if you could quickly explain some of the basics. What little I know comes from reading various sources. It's hard to get everything together at one place and one time.

As I understand it, until higher hull speed creates too much drag, the larger the wheel and the slower it spins the more effiecient it is . Increase pitch until the torque required to spin the prop overtakes the capability of the engine.

How did you crank out the differences in effiency between different prop sizes pitches etc. 10/57 difference or 17% is a pretty healthy swing.

I'm interested in understanding the concept of overpropping. My boat is capable of cruising at 18 to 22 knts. 15 is as fast as I like to go and I would be perfectly happy to optimise for that.

The conditions that would need more power are going to slow me down anyway so I don't see a real big downside if the motor isn't being strained by running it too hard. While I don't have a pyro, between a vacuum guauge flow meter and gps I have a pretty good awareness of what my motor is doing.

In any event I guess I am telling you what I think and asking you what you know. Anything you can explain to me about this whole subject will be quite helpful. Painting with broad brushstrokes is more than adequate. Thank you for the time already expended and for any more insight that might be available.

 

The simplest way to get a grasp of what is happening with the prop is to get comfortable with using JavaProp:
http://colaco.freeshell.org/mhepperle/javaprop/jp_applet.htm

It was intended primarily for aircraft but it can be set up for use in water by changing the parameters on the Options page. To mimic your prop I used the MH 9.8% 500,000 airfoil. This is close enough for comparative purposes. The Re# is a bit low but the foil performance does not change much from this to the range your prop is working in. I use my own calculator that can work over a wider range but JavaProp is no more than 1 or 2% different.

If you open up the Applet then I can take you through the settings. There is a very good analysis page that will allow you to asses if the engine will be overproped.

As far as efficiency goes there are a lot of factors that come into play. Typically big slow props are most efficient but if you are diameter constrained then other factors come into play. The efficiency is strongly related to the aspect of the blades - narrower, longer blades are better than short, wide blades. The most efficient water prop would be what you see on an airplane but there are practical limits on the diameter and material stress for a boat.

If you get JavaProp up and running I can explain some of the tradeoffs.

Rick W.

 

Thanks Rick,

I've looked at that before, probably on a suggestion from you to somebody else, if memory serves me. I'll put some time into trying to get a feel for it later in the week. I'll get back to you when I am better able to follow along.

Your example of an airplane prop was quite helpful. I can "see" what you mean about size and stress coming to play in both the thicker environment of water as opposed to air and the location of the propeller etc..

 

To get you started in the metric environment.

For Options
Density 1025
Viscosity 0.0000013
Velocity does not matter

For the Airfoil
Select MH 9.8% 500,000 with AoA of 0 at all points

For design page
Number of blades 3
Diameter is 0.425
rpm is say 1400
spinner - I guessed 0.06 for the prop hub
Speed was 7.7m/s (15kts)
power was 89000W (say 120HP)

Then you just press design and see if things look right. You can adjust the rpm to get the actual prop pitch or it could mean your power estimate is wrong.

You can look at the blade geometry to see how closely it matches what you have.

This should get you started. You can play with whatever number you like to see what efficiency is possible. With a large slow prop you could get maybe 80% but it will be a monster I suspect. You could increase the number of blades. This improves blade aspect if the diameter is constrained. However the blades could be too small to take the loads.

It should enable you to see how the variable interplay.

Post some results and I can see if it agrees with what I have.

Rick W.

 

Wow Rick,
thanks for the heads up, I was just starting to break out the calculator and puzzle through converting into metric world. I will fool around with this and have a better understanding I hope by mid week.

I hope you understand that most of this is theoretical. In all likelihood I won't really repower. I've been quite interested in learning how it all works.

It all really started with my flowmeter. Coupled with my gps I use that thing all the time. We have some fair tidal activity here and I use that flowmeter to find the sweet spots in the current when I'm running.

I saw another boat similar to mine repowered with one of those Cummins and once I saw the huge amounts of torque available at such ridiculously low fuel burns my mind has been racing.

At your leisure, if you could explain a little more about how things work it would be great. I've read a bit about all this but it is hard so put it all together. I've run my fuel burn numbers through the basic rules of thumb and formulas to extrapolate horsepower and torque etc. But I'm sort of faking it all as I go along. I think I know, but I don't know I know if that makes any sense.

No need to reply immediately, I've already taken up much of your time. I am quite interested however and will post again after I fool with that prop calculator a bit.

 

You could shoot a request for information to Cummins regarding the engine you have or are thinking of getting and ask where it achieves peak efficiency and the fuel rate at that point. That becomes your design point. My guess is it will be around 2000rpm at about 70% torque. This is the key information. If this is in the range say 80 to 120 HP then you can select the design point for the prop.

It will also pay to know how large the prop can go.

You can then play around and see if there is a case to change based on reduced fuel consumption on what you want to do with the boat.

You do see an increase in the number of cats being used as they can be very economic around the 15 knot mark. So that is the other option to repowring or reproping.

Rick W.

 

TollyWally

I think you have worked out that hull resistance curve, prop load curve and engine torque curve are all different.
I know that in some installations you can get way with overproping for a more economical cruise if the engine load is very low at that point( low hull resistance)
You can do this on petrol and diesel installations BUT you can kill your engine if you run at a higher speed.
I know some dealers are altering the elctronic diesels after they overprop them to prevent full fuel where the owner is only concerned with cruising.

Multi speed gearbox or variable pitch prop or variable displacement engine or cruising engine and high speed engine is your answer..maybe

 

Poweraboat,
Thanks for the insight. If I understand you correctly, you are saying that I have worked out the hull resistance curve, but perhaps not the prop load curve or engine torque curve. LOL you could very well be right!

All I am really doing is recording fuel burn at certain rpms and logging it along with speed, and vacuum. I crudely figure horsepower at 10 horse per gal per hour. From that figure I extrapolate out engine torque, shaft torque, shaft speed, knots per gal, etc. using the standard formulas.

It was my thinking that an engine only developes enough power to keep itself rotating under a load. I thought that the prop curve was measuring the horsepower and torque at the prop under the load developed by the hull resistance.

By nature I am a curious person. I have read various books but a lot of the time you are putting together a little bit here a little bit there. So that was my thinking. I greatly appreciate and indeed seek what ever clarification or explanation the folks here might be willing to provide. After all it's damn hard to ask a book a question if you don't quite get something!

 

Tollywally
Some other items to add to your calculation,
Your BB Chev willl start to use more fuel per horspower once you are over about 3000 rpm as the internal friction starts to go way up and over about 4000 the velvet drive also starts to suck up lots of power.
Remember peak torque (peak VE) is almost always lowest bsfc which is one of the reasons why a diesel will beat your petrol at lower speed as its torque peak is lower
I also have used the rule of thumb of 10% of badged horespower is US gph at wot.

 

Well...I found some time and fooled with that java prop thing. How humbling, I just don't get it. I'm going to puzzle over it and see if some clicks.

 

I have attached some screen dumps from JavaProp using your prop and power information.

1. First SET the Options page.
2. Next SET the Airfoils - I selected MH 9.8% with Re# 500,000 and at 0 angle of attack. This is the closest foil available for a typical prop working at moderate speed.
3. Now go to the design page and SET the parameters you gave me for your prop and power estimate. PRESS the Design It command and it will give you the prop detail. With 1400rpm at the prop the pitch needs to be over 20" to get the speed. This means the revs are wrong or the angle of attack is wrong.
4. I set the rpm to 1830 and this gives the correct pitch.
5. I then went to the geometry page to look at the blade shape. The maximum chord is a good indicator if you are in the ballpark.
6. You can use the Muli-Analysis page and the Single Analysis page to look at the prop data. This gives things like the actual Re# and the peak power that the prop can absorb for the nominated revs. This would be useful in assessing if you have overpropped.

You will see the the actual Re# is higher than 500,000 but once you get up to this sort of level there is little difference in lift and drag coefficients compared with the numbers you get for your conditions.

You can see that the thrust for the condition is 7093N. I went back in to see what power an efficient prop would need to generate this thrust at 15kts. I tried 1m diameter running at 500rpm. Power demand drops from 89kW to 64kW. So this gets back to the original idea of a lower revving prop but you can see it needs to be extremely large diameter to get high efficiency. I doubt that such a large prop could be used practically. Also the blades have a maximum chord of 100mm so they would be marginal for strength.

So this is the sort of thing that JavaProp allows you to do.

Hope this gets you going a bit further.

Rick W.

 

Rick,
I appreciate your efforts. I initially set the varying fields as you suggested in your post earlier in the week. I am having some difficulty understanding the results. I am going to print out the thumb nails you attached and try to puzzle everything out. Again, I appreciate your efforts.

It was my understanding that diameter has more of an effect than pitch on absorbing the rotative power of an engine. In my situation diameter is constrained so I thought that pitch could be increased to try and make up the difference.

I have rather crudely figured that my hull currently requires roughly 325 # of torque to push it through the water in the manner I desire. I was excited by the ability of the Cummins to produce such vast quantities of torque with such modest use of fuel.

I sense you have given me a valuable tool with the JavaProp I hope I can puzzle it out enough to gain some insight from it.

 

Torque is only half the story. Power is what matters. Power is force times speed or torque times rotational speed.

So if we say your boat requires 7000N to do 15kts (7.7m/s) then the power output from the prop is:
Power = 7000 * 7.7 = 53.9kW

The input to the prop to achieve this is what JavaProp allows you to determine. So if you set the conditions for water and the foils as before you can then set the thrust to 7000N and see what JavaProp can do for different types of props.

I looked at an 18" (0.45m) prop spinning at 1900rpm and it requires 77kW. It is still not very efficient but it is better than the 89kW required with the existing prop.

So what you do is get a few operating points for your existing prop. This requires accurate measurement of speed (using GPS in no current) and accurate prop shaft speed. If you are confident with the engine tacho then this will do and you can use the gearbox ratio to get the prop rpm.

You should find that you can fit all points for boat speed and prop rpm using the same basic prop data. This just confirms that things make sense.

Now you play with the things you can play with in JavaProp to see if you can get a more efficient prop. Can you fit 18"? In your case 4-blade reduces required power by about 1kW over a 3-bladed. Alter the AofA across the blade to see if this improves - this alters the chord width and will automatically occur if you get a prop with that chord. If the chord is too small then the prop will not have the required strength.

There is one variation that comes into play with typical boats that JavaProp does not take into account and that is shaft inclination. When you use efficient props you really want to avoid shaft inclination because the blade loading becomes cyclic.

We happily tolerate a lot of inefficiency in boats because we have power to burn so prop efficiencies around 50 to 60% are quite common.

Once you have worked out the best prop you can fit and you can find one on the market, you now need to match the engine to the prop. You need to know the most efficient engine speed and set the gearing to achieve your boat speed at the optimum engine rpm.

I guess if you get it all optimum at say 15kts you might be using half the quantity of fuel used if it was designed for higher speed initially. It depends on what you have to play with. Prop diameter and engine gearing are the big ticket items in terms of giving benefits.

Rick W.