Torque and fuel effiency

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.

 

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.

 

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.

 

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.

 

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

 

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!

 

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.

 

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.

 

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.

 

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.

 

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

 

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!

 

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!

 

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

 

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.