Torque and fuel effiency

[TollyWally]

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 :( ?

[Guest625101138]

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.

[TollyWally]

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.

[Guest625101138]

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.

[TollyWally]

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..

[Guest625101138]

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.

[TollyWally]

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.

[Guest625101138]

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.

[powerabout]

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

[TollyWally]

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! :)

[powerabout]

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.

[TollyWally]

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.

[Guest625101138]

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.

[TollyWally]

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.

[Guest625101138]

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.

[TollyWally]

Thanks Rick,
As you suggest I have been getting operating points with my existing prop. I have attached the records for a timing change I have been playing with recently. Your guess that optimizing for 15 knots might work is encouraging. I need to measure my clearences for a bigger wheel. While not as effective as increasing diameter changing pitch would be easier to accomplish given the physical constraints.

[Guest625101138]

I have not worked through all the numbers in the table but it seems the drag peaks around 15kts and then drops a little. This would indicate is it getting up and planing around the top end speed.

You can use this calculator for planing to check on the value of the of drag:
http://illustrations.marin.ntnu.no/hydrodynamics/resistance/planing/index.html
The more you can tie up in terms of modelling the more accurate you can predict outcome of changes.

I determine that drag levels out around 8000N. The only way I could get more accurate would be to know the chord of the blades.

The important number as far as fuel consumption goes is nautical mile per gallon.

Rick W.

[TollyWally]

Rick,
I fooled with the planing calculator and have attached a shot of it. I faked several of the figures, just guesses really. I don't know what the catagories eps,a, and f represent.
The chart was somewhat puzzling. I am assuming thrust is being measured in kilograms. The horsepower rating seems very low. What is meant by hk?

When measuring drag you mentioned 8000N. What unit of measurement is N?
Regarding the chord of the blades the horizontal measurement of my existing prop is 6 1/4 " The vertical component is 7 3/8" the hub is 2 3/8"
the root of the prop is 3 3/4"

One of the fascinating things about using my flowmeter is learning how relatively broad the fuel burn envelope is. I average roughly 1.25 kpg from 8 to 15 knots or so.

[Guest625101138]

There was nothing attached!

The thrust is given in kg or more accurately kgf. There are 9.8N in a kgf (say 10 for easy calculation). There are 2.2lbf in a kgf.

So the 8000N I was showing would correspond with 800kgf on the Savitsky chart.

There is a green "figure" link in the text on the page that goes to an explanation of the input data.

I get a little confused with the way you use knots in economy calculations. Are you meaning nautical mile? Knot is a unit of speed not a distance.

Rick W.

[Carioca]

TollyWally,
First of all I find your optimization study fascinating, specially due to your final goal and the use of simple instruments to attain it.
What flowmeter are you using ? A Flowscan ? I have a pair of turbo-diesels on my 32 ft wooden motor-cruiser, so the metering is a bit more demanding (go and return lines).
I´ve been trying to followup on your efforts since you first post but I would need you to kick-start me and show e how these simple measurements you take while underway, tally with the hairy variables like engine torque, hull resistance etc..

Rick,
You are no amateur for sure ! Pleasure to observe you coaching TallyWally till he meets up with his target.

Regards

Carioca

[TollyWally]

Rick,
I most humbly stand corrected. I am indeed guilty of the common error confusing knots and nautical miles. I mean of course nautical miles per gallon etc. etc. I have incorrectly used knots per gallon to differentiate between nautical and statute miles.

We can add not having a firm grasp of attaching screen shots etc to messages posted here to the list of embarrassments. I thought I had attached an example from the Javaprop

My intial horsepower curves on the Javaprop seemed ridiculously low. What does hk mean on the thrust and horsepower side of the graph?

Carioca,
I am using a Navman flowmeter, a Furuno gps, and a pencil. Does your flowscan unit take into consideration the diesel return and give you a final real time burn number? The flowscans I've seen in the old days were analogue, needles and dials and I just didn't pay much attention to them.

All I do when I am running a fuel test is bring my motor to a certain rpm, and let it run a while at that speed. When everything settles down I log the rpms. speed off the gps, vacuum, and fuelburn off the flowmeter. Then bump it up 250 or 500 rpms and repeat. I try to do it on calm days and slack tides. If the tides are running I will make runs in two different directions to minimise current deviation.

The rest is just entering the data into a spreadsheet. I use the standard rule of thumb of 10 horse per gallon per hour for burning gasoline and 20 horse for diesel. This gives me a base horsepower number to use and the rest all comes from plugging that value and the tach readings into the basic formulas to arrive at torque, shaft speed etc.

It is my thinking that I am measuring the horsepower and torque at the prop with this rather crude set up. It is also my thinking that a motor will only produce horsepower in an amount equal to the load placed upon it and the amount of power need to rotate the internal assembly. Further more it is my thinking that the load placed upon the motor by the prop is a reflection of the drag and friction created by pushing the boat through the water and I am measuring this drag indirectly through fuel burn and expressing it in torque required to overcome hull resistance. I use my flowmeter all the time. I use it to find favorable current and back eddies etc.

I preface all of these statement by "my thinking" because LOL, that's what I think. More importantly though, it is so those here on this forum more knowledgable than myself can see where I am coming from and correct me where I am mistaken. I am, after all, sort of faking my way through all this. I've read a few books on the subject but it hard to find everything in one place at one time. Plus as I've alluded to in the past, you can't ask a book a question to clarify something.

Feel free to use the spread sheet if it will work for you. The values you input are the rpm, speed, fuel burn, and vacuum. You can change the price of fuel and the gear ratio on the comment page. I'm not sure if you can copy the example I posted. If you can't, give me a private message and I will email a copy to you. If you have any questions please feel free to ask.

LOL and you other guys, if you see some flaw in the thinking or formulas etc. LET me know!
After all In my former life I was a fisherman not a physicist! :)

[Guest625101138]

Forgot to add that hk on the linked page I believe is a typing error. It should be hp.

This calculator gives reasonable results. You have to remember that it is for planing boats and your hull has not achieved full plane at 15kts as the drag is dropping off a bit after that. It is usual to get a hump in the drag curve as the boat lifts onto the plane.

The results are always lower than what you get in practice because there is no allowance for prop efficiency, appendage drag such as rudder and prop strut and hull windage. These can all be determined but the prop is the most important factor because the are usually low efficiency.

Another factor that often gets underestimated is the actual loaded weight including all removable items. It is amazing how much junk a boat can accumulate. Every bit costs something in terms of extra power.

The prop will be the best indicator of actual power and thrust providing it is clean and in good shape. I should be able to get close to actual drag now that you have provided the maximum chord length.

Rick W.

[TollyWally]

Rick,
What are good working definitions of being on full plane? What is it in my figures that gives you the information to determine that?

At 15 knts my transom is clean and my wake is crisp. At 10 - 12 knts not so much. The figures on the sheet I attached are the results of an experiment with advancing my timing. The mileage seems a little better. I've been remodeling and refinishing my interior and haven't been taking my boat out enough to really be able to make a determination about improvement. I need to make a few more runs and let things average out and then compare records.

My understanding is that semiplaning is a less efficient mode than full planing. My milage in the past has been pretty constant from say 8 to 13 or 14 knots. It falls off noticeably over 15 knots. Now I think my secondaries kick in right about there. And while the faster I go the more lift I get from planing but that balances out against the resistance from going faster I imagine. LOL, so many variables, so many questions.

[Guest625101138]

By using the information in the table you supplied regarding engine rpm and boat speed I can determine that the drag reduces. I could not be sure of the absolute numbers because I did not have the blade size but using the SAME theoretical prop I can get relative drag for each point you record.

Your fuel consumption data also bears this out because the consumption at 15kts is higher than when you are going a little faster.

I will work right through your table tonight our time and interact with JavaProp to calculate the power absorbed and delivered by the prop. I was a bit confused with your use of knots instead of nm.

Transition from displacement to full plane is a topic discussed elsewhere on the site. With a hull that is intended to plane you get good results from the Savitsky model once it gets over the drag hump - probably around 16kts on your hull. Not all boats have a hump but typical beamy boats do. So it just gets down to what is the best tool for analysing the drag to see what makes sense. The transition is the hard part as no model that I know of fits well. If the numbers you determine for drag from the prop data vary markedly from the Savitsky model at say 18kts then you need to look for causes. How clean is the hull? What appendages are there and how well are they faired? Has the prop got any damage? Is the boat heavier than it should be? And so on?

Getting economy gets down to nailing the losses and doing what you can to reduce/eliminate them. When you use human power for motoring, as I do, you get very sensitive to losses and what you can do to avoid them.

The engine efficiency comes into play at your top end and you can bet the efficiency drops near full throttle.

There are three things to get a handle on. The hull drag, the prop performance and the engine performance. The aim will be to see if there is a sweet spot that can be improved by getting the gearing to match the biggest diameter prop you can reasonably fit.

Rick W.

Rick W.

[Guest625101138]

I looked at the data sets for the two runs. They are inconsistent. I did eventually notice that run one was against the tide so reported speed is probably low.

The best data fit for run one was the MH 9.8% prop with AofA set at 2.2 degrees. This give the correct chord for the pitch but it varies somewhat from point to point.

For the second set of data the best fit comes from the MH foil with AofA set a 1 degree. Again it is not consistent at all points. Both sets of data show the drag hump but the second set shows a big rise in drag. I think the last rpm reading in RunTwo might be incorrect. The power looks too high.

The specific fuel consumption shows around 300 grams per kW. This is high from my experience. I have seen figures under 200g/kWh. It makes me wonder about the accuracy of the fuel measurement or the condition of the engine.

The prop efficiency ranges from low 50s to low 60s.

The lowest specific fuel consumption comes at 3000rpm in both cases and it is 255g/kWh RunTwo and 294 RunOne. The 255 figure is probably closer because RunOne data needs the current offset. This is not too bad.

So from an engine perspective it would seem you would like to set it to run at 3000rpm. From a boat perspective 15kts does not seem the ideal speed because you are hitting the peak drag.

You really do not get decent prop efficiency unless you go bigger diameter. The loading is heavy for a 17" prop. Shifting weight around could reduce the drag hump. I am thinking the boat is heavier than designed.

Rick W.

[Carioca]

TollyWally,
No I do not have a flowmeter of any sort on my diesels, although I believe they are expensive due to the forward and return fuel metering requirement. Yours is for a petrol engine, so it is a lot simpler.
I am at the end of a major refrubishment of a 1962, 32 ft woodie cruiser that I plan on putting into the charter trade. I shall post some photos when I am done. Understandably, I would like to run it as efficiently as possible, but as this refurb project ends, you understand that I am looking for another that taxes my brain, and not my pocket so much !!!
You are not just a physicist, but an ex-fisherman instead ?! No wonder, but on a more serious note, I am sometimes dumbfounded by the way that some people are able to address tough problems - and solve them using relatively simple instruments ! I have seen physicists addressing an solving one-of-a-kind transformer-like problems, that would leave enginers standing by in awe ! Back to basics, as you are doing !

The best I can do for the moment is to try and follow what you and Rick are churning out and reserve this thread for degustation as a time as when my boat is finally on the water !

Cheers and keep on the good work !

Carioca

[TollyWally]

Rick,
I can't begin to thank you for all the work you have done on my behalf. The drag chart was very informative. I believe I can easily see the drag hump you speak of. That is where the curve drops radically at between 13 and 14 knots or so, right?

Your observations regarding specific fuel consumption are worth my exploring in greater detail. I have always been aware that I am trying to paint a detailed picture using broad brush strokes. My specific fuel consumption measurement vs horsepower or KW is pretty straight forward. I am just plugging my numbers into a very basic formula of 10hp. per gal. per hour.

Tidal conditions certainly skew the results. I am logging tidal conditions, that is the reason for the sheet being set up for two runs. But I can see the value in setting up a 3rd run that averages the two. Summer is approaching and I will make an effort to collect as many sets of data as possible. I can see the value of adding a averaged run into my standard sheet. Also perhaps I will make another sheet that averages the data from all the runs together to see if that reveals or disguises different burn anomolies.

Boat weight is a variable that I can resove nect time I have her hauled. It kills me that I did not note the weight last time I had her in the slings. The published test weight for the boat is 6500#. I usually use 8000# for my calculations. A couple of my buddies run a boat yard, (LOL where I failed to note the real weight on the travel lift). Thier guesstimate of boat weight was 7500# based on thier experience shipping boats here and there. So I added another 500# for crew weight to be conservative. The boats being shipped are actively used boats with what ever fuel is in them when they pull up to the slings so I just let that part slide.

I do have additional drag on my hull. The original owner installed twin tracking keels on approximently the quarter buttock line. They are about 4 feet long or so, maybe 5 inches deep and an inch wide. The edges are rounded. The leading edge is angled but not greatly. The aft edge is a hard 90 up to the hull. You can see two very small "roostertails" in the wake behind the boat. Most people are unaware of the "roostertails" but I can see them because I know they are there. I can only assume that they were installed to help the boat track. In that regard they are a resounding success. I can lock the steering wheel and walk away from the helm with great confidence. I only occaisonally need to adjust the wheel. I would be loath to sacrifice this feature.

The motor condition seems fair. Compression is good, plugs look good. It has a 4 barrel spread bore carb. The secondaries don't kick in until after the drag hump you observed. I have been corresponding with a good motor guy who has made some suggestions about fooling around with the jets and power valves. I am working my way into getting the courage to do that. I have been thinking about getting an additional carb so I can keep a control carb and a one to modify one for experimentation.

You indicate that the existing prop is heavily loaded. I keep thinking and mentioning increasing pitch. You don't touch on this. Since you understand this stuff and I don't I would assume my thinking about this is flawed. Increasing pitch appeals to me because It is relativley easy to do. I have a spare prop that I dinged last summer. It has to go to the shop anyway and I could have them repitch it for not much more money than I am already going to spend fixing it.

Are you using data from the Javaprop to make the drag curves you attached? I need to get myself up to speed on that thing. It would be great if I could learn how to creat drag curves like the ones you sent me.

In closing I would again like to thank you for all you have done on my behalf. I have learned a great deal. And after all, in a very big way, that is the point of all this. I am a curious person and find almost all nautical things fascinating. I enjoy my boat very much and always feel better when I am out on it. The upside of higher fuel prices is one can rationalize chasing better mileage! :)

[TollyWally]

Carioca,
LOL perhaps you misunderstood my post, I am NOT a physicist. If I was, I would have a much better handle on all this. It is most frusterating trying to get information and theory from the sources available to me.

The flow unit I have cost me about $150.00. I know that they make them for diesels, they are more expensive but not terribly so. For me it was a very worthwhile tool. You should be able to get performance curves for your motor that will be helpful in figuring out what speed to run. Gas engines are much harder to get the curves for.

I got data the first hour with that flowmeter that would have taken me a decade to figure out by dipping the tanks etc. Further more the flow meter monitors your fuel level quite accurately. In almost evry boat I've ever been on the fuel level gauge is either suspect or broken.

You could get by with one meter and two transducers or just one meter and extrapolate for the other. I use mine to detect current also. You can see the numbers change as you nose in and out of currents and eddies. It's often subtle at first, but once you get the hang of it it is easy to see.

In any event good luck with your project. Show us a picture! :)

[Carioca]

TollyWally,
No, I did not misunderstand your post, and if you insist so much on not having been trained in physics, well you have just demonstrated that having come as far as you have, you could probably do without it altogether, at least for this application ! I mean this sincerely.
Do you know who manufactures flow-meters for diesels ? Floscan is in the over-500-grand category, for each motor, that is. Any others ? Have you ever used the RPM sensors/instruments that measure 'pulsation' on the fuel lines ? I believe they are available for diesels, to attach snugly over one of the fuel-injection lines and connect up with a digital display on the boat´s dash. There was a thread on just this technology in 'Diesel engines' on this Forum some time ago. RPM take-offs on alternators are prone to error, belt slippage etc...
I´ll post a photo of my boat, in the next couple of days or so.

Cheers

Carioca

[SaltOntheBrain]

TollyWally,

I've scanned through this thread and thought I'd throw in a couple of things to think about if you haven't already.

What's the weight difference between the BBC and the 6BTA?

Overpropped diesels are very prone to running hot.

Other than that, it looks like you and Rick have covered everything.

Lance.

[TollyWally]

Hi Lance,
Appreciate the input. The Cummins and the chevy aren't too far apart. 1300# for the diesel and maybe 1100# for the Chevy. I think the Cummins is longer being a 6 banger so that would be a bit of a complication and probably add more to the balance issue than just the increased weight.

I got turned on to the Cummins by accident. I saw a repower that swapped from BBC to Cummins. When I learned a little more about it I was floored by the torque available at such a miniscule fuel burn.

Now I am just a babe in the woods and I am asking much more than telling but it seems to me if it takes 300# of torque to move my boat at the speed I want and the Cummins will supply that at 2 gallons an hour then somehow I could manipulate the shaft speed and the wheel to achieve the impossible:)

But it is important to realize that I am just feeling my way in the dark here. Rick has been extremely helpful leading me along down the path. A more realistic goal is to optimise what I have.

Your comment about overpropping a diesel is spot on. As I understand it and please (anyone) correct me if I'm wrong the diesel runs hot if the torque load from the prop exceeds the available torque from the motor. My Chevy has a pretty decent amount of torque available at lower rpms. It is my thinking that I might be able to overpitch the prop to optimise my performance at lower rpms. I could run into trouble if I tried to run the motor too fast. But I would be willing to live with a lower top end if I could get a more economical cruise. Several people have already pointed out the valid idea of leaving some power in reserve for tougher conditions. Since I already slow down in conditions where I might need more power I think that would take care of itself.

Rick has pointed out that my prop is already heavily loaded so perhaps my overpropping idea won't work. Again, I certainly appreciate your input and look forward to any observations you might make.

[Guest625101138]

I used JavaProp to create the drag curve. I selected prop rpm and boat speed for one of the middle range points in the data. Adjusted the power and AofA of the foil until I got Javaprop to match the pitch and chord of your prop.

Once I had a prop design that matched yours at one point I set the various values of rpm in the design page and, with the same prop geometry, used the Multianalysis page to determine the drag that aligned with the speed for that rpm. It was tedious - probably took 15 minutes for each data set.


I have my own software for doing this but it would take me a while to set it up for your boat.

I was thinking you had a diesel. The specific fuel rate I get is probably OK for a spark ignition engine.

You are correct about the power reading being quite a lot higher than the Savitsky model gives. I do not have all the hull data so I am guessing some but the numbers would be close enough.

When there are big differences between the theory and what you measure from the prop then you know there are other drag factors.

The prop could be a real dog. The leading and trailing edges are important from an efficiency perspective.

The keels should ideally have 4" long wedges front and back.

I did play around with the pitch looking at improving the prop efficiency but could not get any dramatic benefit.

If you take the data as accurate then it would seem there could be benefit in running the engine at 3000rpm at your chosen economic speed - probably around 12knots. This would require reduced pitch. However without proper engine curves I cannot say if running at the higher revs with lighter load would actually improve consumption. It might just happen that at 3000rpm the motor is close to maximum torque so it gets peak efficiency.

Problem with going to a bigger pitch prop is that the engine may not have enough torque to get over the drag hump. This is when you are overpropped and speed will be stuck around 12kts unless you have a following sea.

The blunt nosed cut-off keels will be costing you almost 5kW at 20kts. If you placed a 4" wedge on either end their cost reduces to about 0.1kW. It is little things like this can make a difference.

It is possible to get anal about these things but I just like to know what things cost. You are then in a position to decide whether it is worth the price.

Rick W.

[TollyWally]

Rick,
I feel better about my fuel consumption now that the gasoline diesel mix up has been resolved. This thread started out with me pondering the Cummins diesel. My figures with my gas engine had lead me to believe that it took about 325# or so of torque to achieve my desired top cruising speed.

I was fascinated by the Cummins ability to produce so much torque at such low fuel burn rates. While I would need almost the opposite of a reduction gear it occurred to me that if the math worked out I could get 325# of torque from the Cummins at 1000 rpms at 2 plus gallons per hour. I would possibly need a trolling valve to lower the speed enough for docking etc.

But then we sort of headed in the direction of my other more realistic goal of optimising my existing motor. I have attached (I hope) a set of performance curves for a motor quite similar to mine. It is has been marinized by another company but the block is the same and the internals are similar.

Please don't do anything tedious with the data. You have already done so much. I am merely including it to perhaps make a better guess about the overpropping issue.

I am quite interested in your comments about the twin keels. If I understand you correctly you are saying that by angling the keels fore and aft I could reduce the drag in a measurable way.

I could quite easily do that the next time I haul it out. You said 4". The dimensions I gave you were from memory. Is there a specific angle that might be used in case my figures are wrong? In case the fasteners are in the way of bobbing the keel would a faired extension in front or behind be a worth while project?

LOL I wish there was something I could help you out with. :)

[Guest625101138]

Engine torque is only half the story. It is power that counts. That is why specific fuel consumption data for the engines is so important. As I recall the little Bukh diesel I had burnt 180grms/kWh at its best speed. Diesels are usually quite a lot better than spark ignition.

The wedge I am talking about for the keels is a "V" in plan view that is 4" long and 1" wide - very gentle taper. It sort of knifes into the water and allows the flow to streamline behind the keel rather than leaving a void. Think of how much effort it takes to hold your hand side-on down in the water at 20kts. That is the sort of force the keels are experiencing.

I will be interested to get information on the boat weight when it is weighed. The more information you gather on these things the better you are able to narrow in on the losses. You just keep applying the theory until you have accounted for enough of the things that matter to get good alignment between the theory and reality.

One interesting thing that I have learnt about typical props is that blade section does not matter that much. You can have a perfect foil section and get crappy efficiency with it and then using a crappy foil section like a flat plate, that is just a bit bigger in diameter, achieve the same efficiency. You can test this yourself with JavaProp. I have tested it in practice as well.

It still pays to have nice edges on the blades though.

I think this is a useful thread as you are able to provide some good test data from actual operating. It just helps build a knowledge base for people to build on. Some find the idea of collecting this sort of data not worth the effort.

Rick W.

[Guest625101138]

I looked at the engine data. See attached.

The result is very interesting because it shows the best motor efficiency at full throttle and much lower power occurs at 3000rpm. The best specific fuel consumption at full throttle is identical to what I got from your RunTwo data at 255grams/kWh. At lower power it is even better (It looks too good to be true).

I guess this means that if you have a nice speed say 10 to 12 knots and you pitch to run 3000rpm at this speed then you should get the best economy.

Taking say 12kts and using RunTwo data at say 85kW, the required prop pitch would be around 13". So this is opposite direction to your original proposal. If you do this though the boat will not reach its current top speed.

Rick W.

[powerabout]

zeus drive....maybe?

[TollyWally]

"So this is opposite direction to your original proposal"

LOL Rick,

All I know is what I don't know, which is everything. So I guess I know everything! :)

If I am understanding you correctly you are suggesting that my optimum engine operating speed looks to be about 3000 rpms.

From the drag chart it looks like running between 10 and 12 knots is operating below the "hump" of maximum resistance.

That reducing my wheel pitch to perhaps 13 inches will benefit me at 10 to 12 knots but reduce my current top end presumably because the 13" pitch won't give me enough thrust to overcome the increasing resistance of running the hull at higher speeds.

Thus the lower top end would not come about by a failure of the engine to develop enough power but by the wheel being unable to deliver enough power. So I would be taxing my motor less throughout it's operating range.

Instead of being cautious about running my motor at too high a speed with an overpitched wheel and placing grave strain upon it, I will need to exercise caution that I do not let my motor run at an excessive speed that the lower pitch will enable it to pull.

Which is as you point out the exact opposite of my initial idea to achieve optimisation by increasing the torque load at lower rpms with greater pitch. Diameter being more efficient and effective but more limited by physical constraints.

Wow, a question or two...

Is a motor's torque a valid metric for thinking about a boats resistance to moving through the water? What I mean is, does it take X amount of pounds of torque to push a boat at Y speed?




I think I will continue to concentrate on engine tuning and building up burn data with the constant that I currently have.

I am going to put time into that javaprop thing and see if I can get a firmer grasp of that tool.

The next time I pull out I will examine reshaping the twin keels. If I am understanding you correctly, you are suggesting tapering the keel fore and aft. That you mean with a 1 inch thick keel I should taper it back it back. Shaping it from say a 1/4" at the leading edge, to full thickness 4" back.

I wouldn't want to get much thinner that 1/2 or 1/4". It seems like I would need some meat for strength. Would it be profitable to shape aluminum extensions for and aft to achieve a thinner leading and trailing edge.

I think without more and better data we have just about beat this horse to death. I need some time to digest all of this and reflect upon it. I would like to revisit this in the future perhaps with crisper data.

Besides a lot more carefully conducted fuel burns and getting an accurate real world weight what other information should I gather?

Besides a few clarifications perhaps we are close to bringing this thing to an active close. I would again like to thank you for all the information you have so generously shared and the considerable time you have spent on my behalf.

[Guest625101138]

Motor torque is only half the answer. You need to consider rpm as well. You could use half the engine torque at twice the engine speed to get the same result. From the prop perspective you just set the gearing to match the engine to the prop.

Your current gearing is virtually ideal for matching your motor to the prop and boat to get the best top speed. I looked at the prop efficiency at 12kts and 2000rpm is close to the best even with the lower pitch so gearing at 1.51 is still near the best. There is very little you can do other than increase diameter to improve prop efficiency. Reducing the prop pitch just means the engine will be in a more efficient operating point if your chosen speed is around 12kts.

There is one thing I cannot get a handle on yet and that is the drag hump is at a higher speed than it should be for a typical 28ft planing hull. I have not analysed your specific hull but you would expect the drag hump for a 28ft boat to be down around 8kts not 15kts. It bothers me when things do not fit. Any observations you make regarding trim and wave patterns as you get on the plane would be helpful.

With the keels, the closer you can get the leading and trailing edges to a point the lower the drag. Leaving the front with a tiny round will help it work for tracking. A really sharp point on the front would reduce effectiveness once off line.

Rick W.

[SaltOntheBrain]

I don't have the books for reference, but Lindsay Lord or Uffa Fox was a proponent of propping for touque peak even at the expense of top speed.

I'm confused about your target speed of 10-12 knots. What is your trim state at that speed? Wouldn't a 28 footer be plowing a lot of water at that speed? It couldn't possibly be planing at that speed. Right? That's way faster than hull speed.

How much do you use the boat? How much is the swap going to cost? Is the BBC due to be retired? How long will it take to pay off in fuel savings?

Don't get me wrong, I'm loving this thread, and I'm a big fan of CTD's, (I drive one, and I fished tournaments for years on a twin CTD 28 Rampage) I just hate to see a lot of money and time spent to ultimately lose money and time.

Lance

[Guest625101138]

I don't have the books for reference, but Lindsay Lord or Uffa Fox was a proponent of propping for touque peak even at the expense of top speed.

I'm confused about your target speed of 10-12 knots. What is your trim state at that speed? Wouldn't a 28 footer be plowing a lot of water at that speed? It couldn't possibly be planing at that speed. Right? That's way faster than hull speed.

How much do you use the boat? How much is the swap going to cost? Is the BBC due to be retired? How long will it take to pay off in fuel savings?

Don't get me wrong, I'm loving this thread, and I'm a big fan of CTD's, (I drive one, and I fished tournaments for years on a twin CTD 28 Rampage) I just hate to see a lot of money and time spent to ultimately lose money and time.

Lance

I agree with these comments. The drag hump should not be too far above the hull speed. However the prop data shows a peak in drag occurs around 15kts. The drag at 18kts is roughly the same as 12kts but of course the power required is 50% more because the speed is 50% higher.

Right now the best engine efficiency aligns with a peak in hull drag around 15kts. So it would not seem to be the best idea to sit at 15kts. The fuel consumption per nm actually improves a little just above 15kts.

The one thing that seems clear is that the engine has a sweet spot at 3000rpm.

The other thing that shows up from the prop data is that a maximum point occurs in the drag around 15kts.

So right now the hull is at its WORST when the engine is at its BEST. There is really no combined sweet spot.

If you want to reduce fuel consumption and cannot change to a bigger diameter prop then the best option is to repitch to 13" to get 3000rpm at that slower speed.

If you are not happy going at say 12kts and want to go faster at best economy then you could target 3000rpm at 18kts. This would require a pitch of 17".

The latter case will almost certainly be overpitched and the engine will not rev out. The only issue then is whether it will get on the plane. If the motor data provided is correct then it will have the power at lower rpm to plane.

In summary, the aim would be to repitch to get the motor and hull sweet spots aligned. For the motor it is at 3000rpm. For the boat it is at 18kts. This should give noticeable improvement in fuel economy - maybe 25% at 18kts.

So three choices - leave as is and live with it. Choose to pitch lower and target 12kts. Choose to pitch higher and target cruise speed of 18kts.

There are enough inconsistencies in the data sets to wait until you have more readings as well.

Rick W.

[FAST FRED]

Lindsay Lord or Uffa Fox was a proponent of propping for touque peak even at the expense of top speed.

Proping to cruise at max torque usually is a good idea for efficiency.

Overproping is common among folks that want the best diesel efficiency as if the Mfg spec to prop for max rpm is followed the cruising range will be lousey as the engine wont be working very hard.

The overproped boat will run with the engine working much better BUT the danger of overloading exists.

To keep from having overloading damage , after the re proping simply go to full throttle , and note thr RPM.

Most diesel engines will not overload if operated at least 10% under the maximum rpm.

15% is really safe.

FF