Controlable Pitch Prop low rpm efficiency question

Red dwarf- it is not practical to operate a propulsion system at 10% of intermittent rating because all the transmission losses are pretty much fixed in terms of torque and are usually expressed as a percentage of the RATED hp. Thus many of your losses while cruising will be 10 times as great percentagewise as they need to be. Likewise, maintenance costs will be far higher than needed. By setting up to cruise on one engine you're only five to 1. Actually, when you figure losses, it's closer to four to one. This is reasonable number.

I don't think a CPP is really going to help you any at all. You would normally increase pitch at a slower speed and lower the engine rpm by more than you lowered speed. But you will already be at the bottom end of practical rpm even with a lightly loaded fixed prop.

Cruising efficiency is a funny number to get your head around. As you throttle back, most of the drivline torque loss is constant. Accessories drop with RPM. Alternater load will try to keep up with demand. The BSFC figures of the motor aren't worth worrying about below about 1/2 throttle because they just don't tell you whats going on on your boat. And whats going on on your boat involves at least as much nonlinear behavior than the motor's fuel consumption.

The issue is to keep the shafting and tranny as small as possible for cruising and don't drag any extra stuff around. Plan to use feathering props if you are considering single sided running.

Consult the motor mfgs about your predicament. The choice of turbo setups could realistically be something you could play with to get the most in the direction you want to go.

If you must have a fifty-400hp range and the low end is the bulk of the hours, I think you need a pair of 50-75 hp straight drives and maybe a pair of waterjets for sprinting. Good luck cramming that in a cat.

 

Answers to most of the questions and doupts are answered here:
http://www.boatdesign.net/forums/props/controllable-pitch-propeller-summary-30695.html
It would be beneficial to continue the discussion in that thread if there's questions arised after reading it first ..
BR Teddy

 

I had read that before and I just read it again (not all the links). I didn't find any data of CPP efficiency at different pitch settings nor efficiency comparisons to FPP (except vague "CPP is more efficient"). Can you show us data of real comparison (calculated or measured) of an optimum FPP vs. optimum CPP at wide speed range.

Red Dwarf: What is the estimated speed at 2*25 hp and 2*200-250 hp?

 

That's case sensitive.. Ideally to find out take a boat, calculate the resistance and a estimates of usage with different speeds. Send the data to CPP manufacturer to get a quote of a CPP setup for the best match. Of course if you design a fixed prop for limited rpm and power it's the most efficient for the job, but outside that ideal range they stink becouse what's more important than a few % difference in the propeller effiency is the the total effiency of the drive train.
Ideally when designing a boat it's also concidered in the spiral the benefits of CPP vs fixed prop, so there's no simple answer.
But.. basicly it boils down to the fact do you need ~90% of the time only "cruise" speed which might be about 65 to 75% of the max power, occasionally full power and occasionally something less or the engine hours annually are quite limited.. then it's better to stay with fixed props. If not, then it's good to consider a CPP instead.

 

I think for most leisure boats cruise speed is more like 20-50% of the rated power and in this case rather extreme 10% is the given value. In my case (35 feet sailboat + MD2020) I cruise at 2500 rpm while maximum is about 3400 rpm. Probably that is less than 50% of max power. Most similar boats have 50-100% more power than I have and still they cruise at about the same speed.

Still there are no CPP systems in normal leisure boats.

Lets take an example. Say Red Dwarf is using Yanmar 6CH-DTE3: http://www.yanmarmarine.com/theme/y...gines/Technical-outline-Drawings/PDF/che3.pdf

It has specific fuel consumption of 225 g/kW/h at optimum range (2200-2550 rpm, 65-100% of max power). Still at 1800 rpm (35% of max) the consumption is 235 g/kW/h, thus only 5% more. And at 1400 rpm (17% max) it is 260 g/kW/h (only 16% more). All this is based on rpm^3 propeller curve, which is rather unrealistic for most boats. A catamaran may have closer to rpm^2 and thus much more power is taken at 1400 rpm.

If you would use a CPP, you could adjust the propeller curve to be closer to the output curve. How much would that lower specific fuel consumption at 1400 rpm? Most likely 225 g/kW/h can't be reached at 1400 rpm at any power. And then you are taking more power -> even lower rpm at the same target power/speed. Now your CPP is quite far from nominal pitch and thus its pitch distribution is quite far from optimum. What would be the propulsion efficiencies of the FPP and CPP at that very low power? Typically FPP on a displacement leisure boat has it's best efficiency far below max power due to high shaft rpm and propeller loading at max power.

Then if you would choose Yanmar 4HCE3 from the same datasheet, the optimum specific consumption would be 252 g/kW/h thus about the same the as 6CH-DTE3 had at 1400 rpm! And at the same 20 kW it has 290!

Thus from these engines the bigger one is more efficient at any load. But it should also be noted that 4HCE3 has 40% higher specific fuel consumption at 1400 rpm compared to its optimum.

 

Looks like those Yanmars are optimized for full throttle. What I'm used to are curves like Westerbeke 71C. It has it's best torque rating at 1950rpm where the consumption is 4l/h and max power around 45hp when fix prop only delivers 19hp to propulsion, full throttle at 3600rpm the same values are 15l/h 71hp, quite the opposite... As I said, case sensitive..

 

Just out curiosity, what kind of boat you have? Skerry? Sounds like you don't need an engine at all, just oars Is it still able to run 3400 at full throttle?

 

Sorry Joakim, just noticed something we both missed. In those consumption curves the calcs are made with fixed propeller so in reduced rpm the consumption is accurate only with that setup, with fixed prop using only a fraction of the power what the engine is capable of. Running with CPP you get better engine loading and the consumpion/hp get's better, much better..

 

It's a Bavaria 35 Match, which is a very typical modern Cruiser Racer (5800 kg, 10 m LWL). MD2020 has its maximum power at 3600 rpm. My setup is able to reach about 3400 rpm (7.4 knots) and cruise speed is about 6.4 knots. The boat came with 16x11 VP 3 blade folding, but now I have 2 blade Gori Racing 15x10.5. These two propellers are equal at 2500 rpm, but the VP had 0.1-0.2 knots better top speed (same rpm). I have a sail drive with 1:2.47 gear ratio.

 

Yes I know they were made with rpm^3 propeller curve. Wouldn't that curve go close to the best specific consumption point at some rpm? Where is the room for improvement, if you never get worse than +16%?

 

That's with fixed propeller. Running CPP max torque rpm you should have 10-20% better economy than full rpm when the same % of engine loading is used at both speeds. That's the difference to opposite direction so we are talking about 25 to 30% saving compared to fix prop..
But you have a saildrive, so CPP is out of question anyways.

 

So are you saying that engine has 25-30% lower specific fuel consumption at some point. So it would be ~160 g/kW/h, which would be a very good value for a huge low speed diesel engine. I don't think so! 200 g/kW/h would be excellent peak efficiency for that type and size of engine and this one probably doesn't have much better than the advertised 225.

 

You could have a small engine driving the same shaft (say with a chain drive, a freewheel and lower gear ratio) but I expect that it would be custom work.

 

Regarding the shape of the fuel consumption curves- two different curves are being compared. One is fuel consumption, the other is BSFC. You have to divide by Hp to compare the shape of the second curve (Westerbeke) to the first (Yanmar).

 

For discussing engine efficiency, this is the best type of graph:

http://ecomodder.com/wiki/index.php/File:ALH_BSFC_map_with_power_hyperbolae.png

Note that on this engine, you can get reasonable bsfc down to about 12% of peak output (@1000 rpm, 50% load), maybe down to 10%. Call it 20% of cruise output.