Earlier in this Online Book, we discussed the fuel map gap, that annoying (and expensive) disconnect between an engine’s ideal operating regime and the torque curve of a fixed propeller.
What if we had a propeller whose pitch—and, therefore, torque demand—could be adjusted to the most efficient point for whatever operating conditions happen to exist at the moment?
This is the idea behind a controllable pitch propeller (CPP).
Hi Matt.
Thanks for making my understanding of this topic improve drastically. I like to gather this type of rather specialized knowledge, as it’s well suited for demonstrating that I’m an annoying know-it-all. 🙂
Motorboats in Norway from the sixties or older, mostly have variable pitch propellers, no gear box, just a clutch and a pitch lever, so I have used them a lot. Maneuverability and power adaptability are indeed, as you say, far superior. I’ve noticed that 50 years old quite primitive motors tend to be just as fuel efficient as brand new installations. Now I know why. Normal new engines have small fixed pitch propellers on high revs. Old ones have big diameter VPP props and are designed for slow revs. So the efficiency advantage of the new engine tech is lost.
The system I would like to have in a future “ideal boat” is pure electric. No generator, just solar, wind, efficient batteries, etc. Then the system gets much simpler. As you say, electric motors have almost the same torque at all speeds, and it can be run just as well in reverse, so no gear box needed either. I’ll have that on a catamaran, so I’ll lift the prop out of the water when sailing, unless I want to charge batteries, so a feathering prop is also not needed.
The batteries will be more expensive, but all else will be much lighter and cheaper, so in total I’ll get a lighter and cheaper system that is close to maintenance free and much more reliable. I’ll have to sail when there is wind, no lazy motoring, but I already do. I’ll also have to dump all time schedules in the trash, but that’s where they belong, so looks like a nice system. 🙂
Hi Stein,
While electric drive is certainly interesting, the practicality is pretty limited for most offshore voyaging profiles. We have an analysis that Eric Klem and I did here: https://www.morganscloud.com/2013/04/23/electric-or-diesel-electric-drives-for-voyaging-boats/
And Erik created a brilliant calculator that you can use to determine whether or not electric, or diesel electric drive is practical for your intended use here: https://www.morganscloud.com/2014/04/13/real-numbers-for-electric-and-diesel-electric-drives/
Sadly, for most voyaging usage profiles all electric is not practical due to very long recharge times (think 10 days to recharge from one 8 hour run).
On the bright side, I’m totally ready to buy a Tesla 3!
the lrc58 is such a cool boat – i wish Dennis would update us as to his travels in Koti – i last saw him in January knifing up Veronica Channel in the Bay of Islands
another advantage which was not mentioned, is the additional maneuverability gained through being able to ‘walk’ the stern both to port and starboard in reverse with a combination of forward gear but reserve pitched blades and vise versa
Hi Marcus,
Dennis and I are working on a fun post about the LRC that we will publish in the next couple of months.
“proven in decades of 24/7/365 service on thousands of vessels around the world” – on ships?
But what about in a typical sailboat use, which is tiny fraction of 24/7/365, often with long periods of being stationary? Furthermore, 50+% of the time underway is likely to be using windpower.
Autoprop seems a much simpler concept to live with, even if it is not quite as efficient.
Hi npf,
Just to clarify, Matt never wrote, or even implied that a CPP was the right choice for a sailboat. As you say, it probably isn’t. But we have plenty of readers with motorboats here, where the benefits of a CPP, for the right usage profile, can be compelling.
Further, as Matt explained in the first chapter, this is part of an ongoing series of chapters on efficient power train options and CPP’s are most assuredly part of that.
I’d like to see more CPP options for cruising sailboats, but they just aren’t readily available in those small sizes. It’s awfully hard to find CPPs smaller than 0.6-0.8 metre diameter, which is enormous for the kind of sailboats most of us would be considering.
This particular class of equipment is really more suited to long-range motoryachts (where the thrust requirements change quite dramatically as weather conditions change and as the boat gets 25-30% lighter from burning its fuel), and to larger sailboats – say, 60 feet and up – that frequently motor or motor-sail, and that can really benefit from a fully feathering, large diameter propeller.
John,
Article ahead if it’s time.
I’m very happy with our Hundested CPP installed on our 80′ Alum Cutter. Brilliant engineering.
Gc
Hi Carlos,
Good to hear. What kind of boat is it, and what are your cruising plans. I’m interested in the usage profile that made the added expense worth it.
Thanks, Matt.
A very informative and helpful article.
I’ve been evaluating putting a Brunton’s Autoprop on our boat for some time, and believe it will improve motoring [and sailing] efficiency over our current fixed prop. [And greatly reduce reverse prop-walk when not desired…] It might even prolong engine life. [One of my rationalizations…]
Unfortunately, we have other boat jewelry that is taking budget precedence, so we will have to be content to continue absorbing information and knowledge about this topic for now.
Thank you again for this opportunity to do so…
I don’t have any personal experience with the Autoprop.
Anecdotally, I’ve heard mixed reviews. The efficiency and performance gains seem to be real, but the device’s reliability and manufacturing quality may still be open for debate.
Hi Matt,
I heard the same concerns about Autoprop reliability directly from a very reliable source just a couple of weeks ago.
PYI also told me that Maxprop were working on a automatic pitching prop, but that was a year ago and I have heard no more.
Just to add to the discussion, I have a four-bladed VariProp on our steel motorsailer, along with an Aquadrive (which looks to be in the boat pictured above) and a hydraulic transmission. I chose the VariProp because its pitch can be adjusted in the water and because I can set separate pitches for forward and reverse. The prop also autofeathers in neutral or when the motor’s off. Admittedly, I can only adjust for an “optimized” pitch in forward, and it will require experimentation and recording to determine where the sweet spot is in terms of fuel consumption and ideal RPM load on our engine, presuming the typical flat water or light seas under which we’d most likely motor at all, but having a really torquey pitch in reverse, where consumption isn’t really an issue, has already paid off in docking and maneuvering in tight spaces situations. So I can see the appeal of this technology even on a sailboat, but I’d have to be doing a great deal more motoring than we intend to do to merit the switch. That said, it does solve a lot of problems with engine wear and fuel burn and is very interesting to read about.
Hi Matt,
I have been only an occasional user of CPP’s (mostly with the old Sabb systems but also with one other) but I will say they are really fun to use. I don’t have good comparative data so I can’t speak to their efficiency although I remember the different applications as all getting good fuel economy. Some systems still incorporate a gearbox while others eliminate it completely. Eliminating the gearbox can give slightly higher efficiency if the correct gear reduction is included in the engine or a slow speed engine is used provided you don’t mind always having the prop spinning. The gearbox buys more options including in terms of prop wash and maneuverability but is probably more complicated than most users would like. I like your thought on incorporating electronic control (of course with manual backups) of the engine speed and pitch so that you simply command an amount of power and the system provides the optimization.
In all of this, I think that it is important to keep the overall efficiency in mind. Using the fuel island plot you included and using the numbers off your last post on the subject, I got an approximate increase in engine operating efficiency of 15% by using the CPP. While 15% is very significant, it can quickly be wiped out if other losses in the drivetrain are increased. This is where many of the arguments for diesel electric propulsion get into trouble as it does not take many energy conversions or additional losses to quickly erase a 15% gain in engine efficiency. A CPP with an enormous inefficient propeller hub or other significant mechanical losses could easily eat up most of the efficiency gain.
For my own use, I believe that a transmission with 2 forward speeds would be perfect. I could use the taller gearing for normal cruising operation and the other gear for the rare max power events. The addition of another forward gear should have very limited efficiency hit. While I like the CPP, this strikes me as a less complicated way to get the same efficiency boost.
Eric
Your point about the overall efficiency of the system is well taken, Eric. I see this fallacy quite often; someone will spend a lot of money and effort improving one part of the system, while completely ignoring some other part that ends up counteracting the entire improvement.
Multi-speed transmissions are the next chapter of this series, and they’re one of my favourite solutions too.