Controllable Pitch Propellers (CPPs)

CPP fitted to an Artnautica LRC 58
CPP fitted to an Artnautica LRC 58

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).

Matt, Engineering Correspondent, is a Professional Engineer and true renaissance man, with a wide range of expertise including photography and all things boat design. He has a unique ability to make complex subjects easy to understand and he keeps an eye on the rest of us to make sure that we don’t make any technical mistakes. Working as M. B. Marsh Marine Design, Matt designs innovative powerboats of all shapes and sizes.

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Stein Varjord

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. 🙂

John Harries

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!

Marcus P

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

John Harries

Hi Marcus,

Dennis and I are working on a fun post about the LRC that we will publish in the next couple of months.

npf1

“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.

John Harries

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.

Carlos ?

John,
Article ahead if it’s time.
I’m very happy with our Hundested CPP installed on our 80′ Alum Cutter. Brilliant engineering.
Gc

John Harries

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.

Bill Wakefield

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…

John Harries

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.

Marc Dacey

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.

Eric Klem

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

Stedem Wood

A discussion about engine curves and controllable pitch props, interesting as they are, is nostalgic for me.

I grew up running an open 19-foot Fisher Boat built by Delquay in England. It was powered by a single-cylinder Sabb diesel through a CPP. It was great fun to operate, could be started by hand with compression release and would run at such slow speed that you could count the RPM by listening to it. Hull speed was probably four or five knots but it would climb over that without whimpering. It ran for a whole summer on 10 or 20 gallons of fuel and always brought a smile to those who came to look.

The dockside looks were all about the boat. It was reminiscent of the open launches used to bring navy personnel alongside ships at anchor during WWII. Opening the engine box, releasing the clutch and cranking the engine over by hand while people were looking was part of the show.

It would start with a couple of thumps while you were cranking, then take off on its own.

Once clutched in there were no gears, everything was done with the pitch control, throttle and tiller-controlled rudder. The combination was magic and gets to the point I want to make.

The boat is small, has a ballasted full keel, a fairly large prop and relatively small rudder, so it can be a handful and difficult to control in reverse. The magic part was the controllable prop, which lets you slide in and out of reverse pitch while using prop wash or forward progress for control. There is no banging in and out of gear or waiting for reverse thrust to build from idle after shifting. Forward or reverse, the thrust is seamless and infinitely controllable; that kind of control will let you put the boat just about anywhere you want it. You could spin it in one direction with the tiller over and just alternating the pitch control. Spinning the other direction took a little more work.

The boat is 46 years old now —–but sadly, after more than 20 years, the boat was re-powered with what was easily available; a small high-rev diesel with gear box (a couple of times, actually). It’s still an attractive, remarkably seaworthy little boat, but it’s nowhere near as much fun to run.

Stedem Wood
M/V Atlantis

Stein Varjord

Hi Stedem
Nice story about the old boat. The Sabb is made in Bergen, Norway and was the totally dominant engine maker for fishing boats. There were several hundred small factories for small boat engines along the Norwegian coast back in the day. I’ve used the same engine you describe. Really nice and ridiculously robust but a bit noisy and shaky. Runs on anything you feed it.

I’ve owned two Sabb model 2J, which is 2 the cylinder 30 HP version. That one is very smooth running. Like a sewing machine. But still as ridiculously robust. I once took over a boat in southern Turkey. It had been left on land unattended for more than 10 years, probably much more. That will make any engine seize up, which was certainly the case here.

I talked to an old guy at the factory, they still exist, Frydenbø Sabb motor, and asked him if there was any hope of getting it back to life. He just said yes of course. You can’t possibly kill this engine, unless you shoot it, with something big. I had a local Turkish old engine nerd help me with it. Only worked with old engines.

When he saw the engine he almost fell into tears of joy. “The best engine in history! The Rolex of boat motors.” 🙂 He later had to moderate his clam and include the Danish Lister engine as having the same level of quality. We poured oil in the cylinders, used a huge crowbar between the starter cogs in the swing wheel and the engine bed, trying to rotate both ways. I’m amazed that nothing broke from this powerful high power abuse. I was actually jumping on the end of the 1 metre crowbar that had 2 cm between the contact points. Many tonnes of force.

Gradually it started to move a bit, and then more and more freely until it swung smoothly. We put diesel in the system and it started easily. Crazy but true. This model also has the hand crank an decompression lever to start it, but you need to be a gorilla to start it that way. 🙂 I went on to run this engine on absolutely full throttle every day for two months, going from the Black Sea up the Danube river, heavy currents, and down the Rhine to Amsterdam and then to Norway.

An acquaintance living on a small island in the Oslo harbour used this engine. He would use it every day to and from work, etc. In the winter, he didn’t want the struggle with starting it, frozen motor, cold batteries, so he just left it running nonstop, from the beginning of November until end of April, it was running day and night. That engine still is in use on the tiny ferry to the island of my sailing club. Runs every 15 minutes in season, less in other times. More than 40 years of intensive use, every day almost nonstop, and almost no maintenance apart from oil and diesel. Try that with a modern setup…..!

RDE

Hi Stein,
Great story about the Sabb 30. There were several installed in Cape George 36’s in the late 70’s but all were eventually replaced with Yanmars and the like. I guess Americans just liked their engines to buzz along at 3600 rpm instead of going thump-thump-thump!

Marc Dacey

That was a great story and a testament to engine design I find lacking today.

Stein Varjord

This is quite off topic, but…. 🙂
The Sabb 2J 30HP isn’t actually that simple. For an old engine design, its quite complex, but everything is mechanical and overly strong. Instead of a timing belt, they have cogs, no adjustments ever needed. The diesel high pressure pump is one for each sylinder and run by a lever directly on the main shaft. The cooling water circulation pumps likewise. All is one integrated machine. Designed for open fishing boats for far offshore in arctic Norway in the winter. No daylight. Lots of bad weather. The customers knew that an engine failure meant they would probably die. As John has previously mentioned, when the fishermen have declared what’s the best, one should take notes. This engine is still considered the best ever on the small boats.

I think the main reason why new engines are not built with the same attitude is partly to compete on price in a market far less aware of quality, but more I think it’s “planned obsolence”. Or what I’d say is cheating customers. Here’s the trailer of a film about that….
https://m.youtube.com/watch?v=251qoGOqpdk

Eric Klem

Hi Matt,

There is definitely something to be said for the older style of design. Probably the most fun design that I ever did was for a 40 kW steam expander. In the early stages of the design, we were trying to figure out how to get the desired efficiency using a design architecture that had been used in previous designs. We were really struggling to come up with a geometry that our dynamic simulations predicted would have the desired efficiency so we went back to basics and ended up with a one-lunger which actually exceeded the performance target by a decent amount.

Of course, people are obsessed with power to weight ratios and metal is sold by the pound, both of which drive us towards high revving, power dense engines.

Eric

Ben Garvey

Hey guys – great discussion. I yearn for an affordable Hundestedt for my own boat. what a lovely piece of gear. Slightly off topic – but other variable geometry props have already been brought up. What are your thoughts on the Luke feathering props? they seem to be one of the older and more basic, robust and established feathering props out there. Any knowledge of issues/benefits/problems with them?

thanks all-
bg

Eric Klem

Hi Ben,

I worked on a 150T displacement vessel with a 32-22 luke feathering prop for several years. The drivetrain proved to be very efficient but had some issues which were in some ways related to the prop. The luke is one of the few feathering props to use curved blades which is good for powering but not as good for sailing. The prop was a bit harsh during shifting but most of the large feathering ones are.

Most of the problems were related to the enormous mass of the prop and the length of the hub. It was mounted on a 2″ shaft that was probably 8-10′ long which had significant shaft whip. Realistically, it should have been on a 2.5″ shaft given how much weight the prop put many inches behind the cutlass bearing to eliminate the whip problem. I am aware of 3 failures of the prop over a 25+ year and 6000+ hour life (it was replaced with a new luke a few years ago). In the first instance, the blades jammed but I can’t remember why. The second time, one of the rings that holds the hub together fell apart and the blades fell off but were reinstalled by a diver later that day. I was not aboard for the last incident but my understanding is that the blades jammed in the feathered position when shifting from forward to reverse entering the slip. This prop did not lead an easy life (it cut an awful lot of pot warp over many years) but it was rebuilt approximately every other year by Luke. Compared to other feathering props that I know of in similar service, it was not better or worse on reliability.

My experience was that they are good efficient props but pretty heavy and I would want to look into reliability more before buying one myself. Note, on our own boat I do not have a feathering wheel because of reliability concerns so I am conservative on this.

Eric

John Harries

Hi Ben and Eric,

As another data point. I have been using Maxprops for some 35 years and well over 10,000 hours of operation without a problem or failure.

Dick Stevenson

Hi All,
Another data point: I have been using Maxprops on 2 vessels over 30+ years and nary an issue. Once, at cruising speed, I caught a lobster pick-up buoy between the blade and the hull. Stopped the engine cold and almost my heart with the thump and buried the blade inches deep into the float. Took a while to pry off, but there was no issue when we got going and the blade balance was still good.
Dick Stevenson, s/v Alchemy

Marc Dacey

Dick, line cutters such as the Shaft Shark are damned expensive, but they do provide some peace of mind. I have yet to snag a line or a pot in our current configuration, but I have in my other boat, which has a Gori folder, and it got ugly quickly. Hence my interest in line cutting.

Alan S

CP Props for yachts up to 20m are an interesting conundrum. Technically there appear to be advantages,(improved engine loading, coarse pitch motorsailing and fine pinch motoring into strong wind/sea) but when quantified they are quite small and struggle to justify the significant extra costs involved. My current installation is a 54hp Yanmar/14.5m 12t yacht/Varifold Prop which has averaged 2.5 litres per hour for all hours, so not alot to be saved. The previous drivetrain on the yacht was a NZ Trident CP Prop, manufactured here in the 70’s and 80’s. Quite a few installed in NZ yachts and pretty robust, but when it was time to re-engine I wanted a conventional drivetrain, and not to have to wind a handwheel to change direction, so out it went (and I subsequently sold it and it was installed in another new build..).
Steve Dashew had a Hundested VP2 prop on Beowulf VI and her later near sister had a VP4 (the smallest size now manufactured). He spoke highly of these in his Cruising Encyclopedia, but has not used them in the FPB series. Nordhavn use a VP4 in their 56 Motorsailor (noting in a review that the prop assembly was twice the price of the engine), however once again not used in any of their power trawlers. The smalest Hundested installation I have heard of in the de Ridders Magic Dragon (built in the 60’s) fitted with a VP 1.
Probably the most prominent application for Hundesteds and similar these days is in 40m+ yachts (of the sailing type). These typically have a 500 – 700hp engine and actually motor alot on passages, so good drivetrain efficiency and the ability to feather a large prop when sailing are both important.
To my knowledge West Mekan (per Denis’ installation) is the only available CP unit that could comfortably fit in a 15m yacht, even so it requires almost 1m of space for the actuator, which has a significant impact on engine location.
For the ultimate in propellor control how about a CV Transmission installed on an ECU common rail engine with an engine load feedback loop to the transmission to set the gearing. Not perhaps for remote área cruising.

Ben Garvey

Good feedback, and a very interesting thread. Thanks folks.

bg

Nikolas

Hi Matt,
Something I found out from a very credible source regarding CPP systems:
A very small number of marine engines are rated for controllable pitch propeller use or can receive emission certifications per that use. Those that can be rated and certified are usually very heavily constructed, rated for low power continuous duty and have fuel systems calibrated similar to tractor ratings. Not rated engines If it occurs to have a load related failure, the manufacturer could void warranty (although they usually don’t).

Apparently at low RPM’s there is only a fraction of the oil volume, oil cooling and coolant flow required to keep everything as cool and well lubricated as it would be the case at higher RPM’s where higher EGT would be expected. Certain temperatures can be OK at relatively low RPM but rapid engine wear can occur as well as failure at even lower RPM’s and relatively high temperatures.

Nikolas

John Harries

Hi Nilolas,

I think you are certainly right that matching a CPP to a recreational rated engine (M3-M5) would be a very poor idea since these engines are only designed to provide their full power at any RPM for very short periods of time (typically 30 minutes max, followed by at least four hours of lower load).

However, any engine that is rated commercial (M1-M2) should be perfectly happy with a CPP as long as the operator knows how to adjust said CPP properly (as detailed above). After all, all a CPP does is match the prop to the engines published max power curve at each RPM. Therefor if an engine manufacturer bans CPPs from their M1-M2 offerings what they are in effect saying is, we published this power curve, but we are fibbing about it.

And there are plenty of commercially rated engines around (we own one). We simply don’t see them a lot in yachts because the builders like to fit M4 and M5 engines so they can claim more power, even though said power is not usable in a displacement boat, or at least not without screwing up the optimal prop setting for cruise.

More here: https://www.morganscloud.com/2016/04/17/how-to-select-the-best-power-and-propeller-settings-for-your-engine/

And we will publishing a post from Matt soon on understanding engine ratings.

Markus

Hello Matt
A propeller usually has a higher pitch angle closer to the hub and a lower pitch angle closer to the tip of the blade to advance trough the water the same distance per revolution. For example pitch angle 25 degrees near the hub and 15 degrees near the tip of the blade. By using a CPP, the pitch angle of the entire blade is reduced let’s say by 20 degrees. That would leave the pitch near the hub at 5 degrees but the pitch near the tip at MINUS 5 degrees. To me that looks like an inherent deficiency in the CPP design. While bringing the engine to its optimal operating point, it severely degrades the hydrodynamics of the propeller. Am I missing something – what are your thoughts?

John Harries

Hi Markus,

This is an older post so that I’m not sure Matt will be monitoring it.

I’m not really sure of the details of an answer, but I do know that CPP’s work and provide a large efficiency benefit for boats that operate over a range of engine loads. After all they have been used by fishing boats in Europe for years, so there is a benefit.

That said, I also know that for a boat that will operate at a given engine load most of the time, a properly matched fixed prop that is optimized for that load is more efficient than a CPP.

Like so many things around boats, it depends on the application.

Eric Klem

Hi Markus,

You are absolutely correct that there are inefficiencies in a CPP due to the fact that pitch and angle are not the same thing as they depend on radius. A good CPP setup will be optimized for perfect blade angle at the normal operating condition. Some people confuse CPP’s and feathering propellers and forget that CPP’s can actually have a perfect blade shape for a given slip ratio and speed (feathering props can too but very few do, Luke is one example). As you move further from this operating point, the efficiency will fall off. It is important to keep in mind that the pitch changes we are talking about are relatively small in the grand scheme of things so the effect is fairly small. Also, CPP’s usually have large hubs due to their mechanical design which results in less variation in blade radius/angle again minimizing the effect. Given the benefits of a CPP, they outweigh the efficiency losses due to this specific effect.

One of the things that has always surprised me is how small the losses are due to this effect. For example, look at a Max prop or any other flat bladed prop which suffers from this exact issue. They do have a loss in efficiency but it is remarkably small.

Eric

John Harries

Hi Erik,

Thanks again for taking this one. I know the rough facts, but the details are way above my propeller pay grade.

David Eberhard

Our boat is a 25 ton steel cruiser, with a Yanmar 4JH57, 2800 RPM continues max rating. 2.71:1 reduction, spinning a 23” Autoprop. Our usual cruising speed is 1900 RPM, burning 0.9 GPH, moving at 6.5 knots. In smooth water at idle she will easily do 3+ knots. The thrust at very low speeds is amazing. Very little prop walk, easy to maneuver in close quarters. We have had the Autoprop on our boat since launch in 2002..all of the reading I have been doing here the last few days has me thinking about adding a EGT, calling Yanmar to get exhaust gas temp info, Callings Autoprop for torque curves for our prop at different shaft RPMs. All of this to see if we are near maximum torque at different RPM. In neutral while sailing it will spin right along quite nicely. By leaving the engine in gear the prop stops spinning and will pick up 0.5-0.75 knots of speed. The blades are asymmetric in so much they are rather shaped like an airplane wing. Thick on the leading edge, tapering almost to a knife blade trailing edge. This makes them less likely to be damaged from hitting something along with the ability to spin around it. Because the blades spin around the leading edge is always the leading edge regardless of direction. Equal thrust in forward and reverse as long as the reduction is the same. Great stopping power.

With regards to reliably, I personally know of one boat that had a blade fly off, hitting the bottom in the process. Thankfully it did penetrate the hull. It is possible to order a replacement blade. A couple of days later I towed them into a marina. Picture this, our 25 ton boat was towing their 20 ton boat with a loss of about 1.5 knot from our usual cruising speed. The prop was doing what it was supposed to do. Talking to him about it, he had never serviced the prop. At every haul out I check the bearings for play and adjust as necessary. Vitaly important is to make sure that the adjustment bolts are secured so they cannot back out. Currently we have 2500 hours on ours and will be replacing the bearings and races on ours next haul out.

We are very happy with our Autoprop and highly recommend them. I know I will need to reread the first few chapters of this series a few more times and look at the curves for our engine in more detail than I have on the past before I fully understand everything presented here.there is always something to learn.

John Harries

Hi David,

Thanks for the real world report, always the best kind, on the Autoprop. I agree, there is a lot to like about the product. In fact I have considered fitting one myself. What deterred me was the reliability reports, particularly shedding a blade, and that we have a hydraulically activated transmission (as most boats have today) so fitting an Autoprop would also require fitting a shaft lock.

Anyway, definitely more efficient than our Maxprop, but the tradeoffs did not work for us, particularly since we have spend so much time in remote places where shedding a blade would be a disaster.