Controlable Pitch Prop low rpm efficiency question

I'm not saying anything like that.. 25% of from 260g is ~200g about (195g) And remember we are comparing the same thrust with different props, different pitch, blade area and so on including a different rpm if we find that's better with CPP becouse that is the whole point of CPP..
But what I said earlier to get accurate numbers?? So no point nitpicking ballpark fiqures..

 

OK, I thought you were talking about higher output situation, since you said "running CPP max torque rpm".

That 260 g/kW/h was at 17% of max power and 1400 rpm (max 2550). There is no torque curve, but looking at the power curve I think max torque is clearly above 1400 rpm. And this is a turbo charged engine, thus max torque location depend mostly on charging pressure settings in software.

With CPP adjusted to higher pitch, the same 17% of max power would be at lower rpm. Maybe 1000 rpm? I don't think it is likely to find anything close to 200 g/kW/h at such a low rpm. The earlier linked map of VW 1.9TDI shows about 220 g/kW/h at 17% max and it is a very low consumption engine advertised for 197 g/kW/h (at 67% of max output).

So for this engine, I think the efficiency would go from 260 to maybe to 230 g/kW/h at 17%. That is about 10%. What happens to propulsion efficiency? Isn't a CPP a bit less efficient in the first place due to bigger hub and bigger shaft (most likely exposed in this case)? Then working outside nominal pitch would make it even worse. How much? Not much is needed for increasing the power output by 10%.

 

Not enough info to determine.. and a lot of quessing.. what you wan't me to say? We don't have complete power curves for that engine, nor we have resistance curve of the boat..

From Wikipedia:
"A fixed pitch propeller (FPP) is more efficient than a controllable pitch propeller under a specific rotational speed and load condition. At that particular rotational speed and load, an FPP can transmit power more efficiently than a CPP. At any other rotational speed, or any other vessel loading, the FPP will not be more efficient, either being over pitched or under pitched. A correctly sized controllable pitch propeller can be efficient for a wide range of rotational speeds, since pitch can be adjusted to absorb all the power that the engine is capable of producing at nearly any rotational speed."

 

True. Is there any paper on comparing in detail CPP vs. FPP for a real case anything close to this case (not a ship)? I did some searching and could not find any real numbers. I would like to see a graph of fuel consumption vs. speed from almost zero to full speed. Most you can find only deals with ships and operating close to full power and even those do not give real numbers.

Why is CPP so rare in leisure boats and all smaller boats?

 

What I can think of there's a couple of reasons. Outside shipping there's only scandinavian fishing fleet where it's used largely so there's no traditon elsewhere. Leisure boats generally don't have so much engine hours in their lifetime that it would be worth the costs. Most of the rest might have so limited speed range there's no point.

Some Danish or Norwegian Technical university or collage might be a worth of try. The companies making CPP's won't give their own studies out.

 

CP propellers are most beneficial in a situation where there are multiple operating points such as heavily loaded vs light loaded (i.e. tanker in ballast vs full, fishing boat pulling nets) or a significant deviation in the resistance curve from a v^2 relationship. In the case of a catamaran, I doubt that there is any significant change that would result in such a condition. If you were to look at j (Va/(n*D) over the full range of speed, it would be pretty constant, thus the propeller efficiency would also be constant. The only gains would come from specific fuel consumption of the main engine. This could probably only be changed a bit without adversely effecting the propeller performance. So it is doubtful that a CPP could provide any measurable performance improvement over a FPP.

 

> gains would come from specific fuel consumption of the main engine

IMO, this could be substantial (like 2x) in cases with a mix of high and low speed operation. It would be interesting to see real data.

 

jonr, we seem to have a difference of opinion.
You think a x2 improvement is possible and I think it's a nonstarter.

Take the fuel map plot you posted. Lets drop a fixed propeller power curve onto it and say it crosses the 10hp line at 280g/kW-Hr. How are you planning on doubling that?

That map does have one interesting feature. If the CPP is used to drop engine rpm down to 1250 from 1700- the BSFC goes from 280 to 250. If one engine is used along with feathering props, the BSFC also goes from 280 to 250 if you map a cubic curve off the 1700 rpm to the 2150 mark.

 

Pick a case with enough range in outputs and the right engine and BSFC can range from 250 to 500. Agreed, that may not fit Red Dwarf's 10% case, but johneck was talking generally.

 

One thing to note is the mfg. starts the power graph at 1200rpm.

I would not use any prop to load the engine below this RPM.

Some engine power graphs start at higher RPM , again I would use the graph as advice about the lowest cruise RPM.

 

I was talking generally, but it is based on 20+ years of experience in looking at these situations. As is generally the case, if you have a total dog, you can get big percent gains, but if the existing system is reasonably well designed (as in this case), you can't.

 

What's a typical BSFC at idle? At trolling speed?

Even 15% gains in fuel economy can be substantial $.

 

I'm not an expert, but based on what I've read here, it seems that CPP could be an optimal choice for a semidisplacement pleasure boat whose speed range is quite wide (7-22 knots for a 43-45').
What could be the expected % of fuel saving for such a boat?
Any example/comparison beetween FPP and CPP for this kind of boat?
Regards
Guido

 

It has less to do with speed range than in the variation in torque required at any given speed do to variation in seaway, winds, displacement, or operations such as towing or trawling. (And it is probably best to view the variation with respect to the rated torque of the driveline, not the average seen in operation.) Engine power takeoff could be another factor. A fixed prop can do well over a fairly wide range of speed if none of the above ever change. Also, CPP's are somewhat favored by low powered craft compared to the example you gave. It would make more sense in the same boat if it had 1/4 the power and ran 7 - 11 knots. Now you can actually use the rated power the majority of the time and therefore get the greatest benefit from the smaller CPP. It's better to save 5 percent at full power than 10 percent at 1/4 power. The CPP is less likely to pay for itself if the power to it is only 1/4 of rated power. It's the total fuel savings, not the percent fuel savings that matters.

 

> It's better to save 5 percent at full power than 10 percent at 1/4 power

Unless you spend 90% of the time at 1/4 power.