Controlable Pitch Prop low rpm efficiency question

Just to have a practical example, here you are a performance graph of a Ellis 36’ semisplacement Lobster Yacht (440 hp yanmar engine)
This are its claimed performance (in attachment):
RPM MPG MPH
1200 23 8
1500 20 10
1800 10 11
2100 7,5 13,5
2400 7 18
2700 6 22
3000 5 27
Which could be the estimated fuel consumption with a CPP at different RPM?

 

Attached the power/ torque curves for this engine.
Regards

Guido

 

A related question is what if you installed a higher pitch propeller such that max rpm was 2500? This provides almost the same max output (310 vs 325 kW) but reduces engine wear and decreases fuel consumption. But I don't think it would run that way without a CPP (or gears) - the prop power curve would attempt to cross the max output curve at lower rpms.

 

We don't know what the ambitions of the posted design are. If you want to make a comparison, lets start with a system that was pursuing the same goal to begin with. And lets give them a comparable budget to work with as well.

If you had several different performance plots for different conditions of loading and seaway, then you could see more or less at a glance the fuel economy savings potential. At least you could put an upper boundary on it.

The motor curves shown would appear to be a setup for fixed prop and a large amount of versatility. It looks like it would be very comfortable and responsive to the helmsman over a wide range of conditions. If you went with a CPP, I doubt you would want to run that engine anyway. I'm thinking something with less torque droop would be in order. Or as Adhoc had mentioned, you could extend the life of this motor and it's maintenance intervals considerably if you put a lot of hours on at higher power levels. Neither of which are comparing apples to apples.

Realise also that the prop curve and tables would be for a perfectly clean hull and new prop. Both the thrust required to move the boat and the torque needed to provide any particular thrust will increase with time. It's all the things that are missing from those graphs which justify a CPP. A fixed prop can handle the graphs pretty well. A CPP's job is more to map the real world onto those pretty plots rather than skew them in search of fuel economy. If you want fuel economy, just back off the rpm. If you are concerned about real economy, do the engineering. If you just want to have fun, the example engine looks like it would do that in spades.

 

The post design ambition is to give the possibility to operate with the boat in a wide range of speed deciding to go faster/ slower depending on sea state, hurry and fuel econom that the owner want to pursue.
I agree with you that the comparison should be done baese on major comparable factors, but just to be sure that I've understood a CPP potential fuel economy, I've tried to plot manually for the same engine an hypothetical CPP power curve (green). So my question are:
- Could the green curve be realistic (I've drawn it a little bit lower than the crankshaft maximum power curve because I think that would be impossible to perfectly replicate it for some cpp inefficiency reason)
- if the green curve is realistic, can I say that with a cpp I can go at 18mph at 1800 rpm (for fixed propeller at 2400 rpm!) with 10mpg ( for fixed prpeller with 7mpg!)?

Look at image attached

 

Yes with if... It's also possible to overload the engine with CPP at any rpm. There's no ineffiency reason, just use of pyrometer to find out when the loading is ok..

 

No you definetily can't say that. MPG comes mainly from your boats resistance curve.

If your boats resistance would be constant and also your propulsion efficiency (from fuel to propulsion) would be constant, you would have constant MPG at all speeds.

Your boat will have very low resistance well below hull speed (say below 8 mph) and much higher at semi displacement speeds. That is the main reason for the MPG curve, which you should plot against speed not against RPM.

At same speed you will need the same propulsive power regardless of propeller and rpm. Then your shaft power depends on the efficiency of the propeller at that operating point. CPP may have clearly different efficiency at 18 mph (not necessarily better). And finally the fuel consumption for the needed shaft power depends on the efficiency of the engine, which is a complex function of rpm and power. If you take the same power at 1800 rpm or at 2400 rpm, you will likely have better efficiency at 1800 rpm, but without the actual map you don't know that and especially you don't know how big the difference is.

And finally you don't know (without measuring) that you are on the "propeller curve" with your boat and propeller. Most likely you are not. The curve given in the engine specs is just a guess for a some generic boat and propeller. To get a rough estimate you could calculate the power from MPG curve assuming constant engine effiency.

 

A different method of having a prop absorb more HP when the engine is run slow, and inefficient is a ZF 2 speed transmission.

While this would not have the control a CPP could have , it would be easier to retro fit into an older boat.

This would be great solution for the oxymoron "Fast Trawler" where a 200-500HP engine is usually operated at 40 hp.

Still might not be enough to stop slobbering tho.

 

Or maybe an automotive transmission?

To properly answer some questions, you need a fuel flow meter and something to measure torque on the drive shaft.

 

TeddyDiver, Fast Fred, Joakim, philSweet and other posters of this thread, I would appreciate very much your opinion on my "semidisplacement/low planing speed boat project" thread and its web poll.
Regards
Guido

 

Hi Red,
I am also facing the design of prop configurations in my project. My project is a 60 ft twin mats sailing catamaran ply(could be nanno wood) /epoxy. I am looking at what i have to have anyway.
My conclusion was to embark my car and change the alternator. The car motor will be the genset and the propulsion will be hybrid electrical. (the car motor will charge batteries and push electricity ac to emotors at it's most efficient rpm ) We do not plan to go around the world, it is a sailing cat for Bahamas and Florida shallows, mostly parked in kiteboarding shallows paradises. The advantage of the electrical propulsion is that you can manufacture that power, it is hard to refine oil on a boat ... also , fuel cell (the H in H2O) is abundant in boating world and relatively easy to store it.
As you know, elet. motors are easy to work with at low rpms (props or turbine) , they dont require heating and reverse is easy, with the sails out, you can regen also.
Depending on the type of motors you choose ac or dc , you can play with the number of screws in the water depending on your speed.
Having been around shallows at some time, jet drive is not out of the picture for me either.
Hobiecat came up with a tandem kayak with pedaled fins instead of prop. We feel it is efficient and it adds to the speed at sailing mode, cant really be in very shallows.
For very shallows. air prop is probably the way to go if algeas is a problem.
Going back to jet drive, water intake and breaking is to be carefully designed.
The idea is to use the same emotor for air and water propulsion, we have some options.
So your last sentence is the way to go in my opinion.
If you dont really need a car , you will likely look at diesel genset. my car is a gas VW Cabrio, one can use a diesel car also obviously. I dont know if you can add H to diesel carburators...
Good luck.

 

The green curve is improbable. You would need to consult with the applications guys at the mfg. In my mind it represents too great a change in pitch to be viable. You have the same diameter, so the slip will be the same. If you figure the same slip at the original and reduced rpm, what is the change in pitch that is implied? What is the pitch range needed to cover the entire green line you drew?

Even if it were reasonable, the charts you showed say nothing about fuel economy. What makes you think there would be a fuel savings? If you go to the fuel map that was linked to and scale your probem on to it you find about a 15% gain in efficiency. Not a 43% gain. Where did you get that 7mpg to 10mpg idea from? Based on the torque droop, I'm guessing the case at hand would be less than 15%.

 

Those numbers are from the table of rpm vs. mpg. 1800 rpm -> 10 mpg and 2400 rpm -> 7 mpg. Guido seems to have thought that these values would be independent of propeller and speed.

 

I'm not able to answer you about slip/pitc/diameter issues. I hope that some of this forum users could help us.

Regarding to fuel gain my 7-10mpg is based on consumption curves by the Ellis yacht builder (I attach them again for your convenience), they seemed to be substantially confirmed by engine's chart (nearly 9 GPH at 2400 rpm and 5 GPH at 1800).

 

The 9 and 5 GPH consumptions are based on the propeller curve, which means the engine is outputting about 70 HP at 1800 rpm and about 140 HP at 2400 rpm. If you change the propeller/gearing so that engine outputs 140 HP at 1800 rpm, the consumption will change as well.