Torque and fuel effiency

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.

 

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.

 

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.

 

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.

 

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.

 

zeus drive....maybe?

 

"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.

 

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.

 

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.

 

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