In the post two weeks ago, I posed the question in the title “What Moves a Boat?” and went on to fill in the blanks around the answer which is the propeller because the engine “just” spins the propeller. In that post I went on to outline the relationship between the different types of Horse Power or HP measurements such as theoretical maximum power, power in the propeller shaft and most importantly power that is absorbed by the propeller. A lot of this has a very direct bearing on one of the key metrics most of us keep track of and that is the fuel burn rate or how much fuel does it take to go a set distance on a given amount of diesel fuel. However I spent so much time going through the power and engine side of the equation that I didn’t get to the propeller part itself. So consider this Part II of the “What Moves a Boat?” question.
Since that post and throughout the build actually, I received quite a few questions along the lines of “OK, but what about the propeller then?” and “Why did you chose to use a CPP Controllable Pitch Propeller on Möbius?” so I will do my best to answer these and other questions I’ve received in today’s posting.
Those of you who don’t find these technical discussions to be your cup of tea, probably most of you, please feel free to take a break from your devoted reading of these Möbius World blogs and I’ll try to have something different for next week’s update. And I will issue a warning right up front here that reading the article below may well bring back memories of your high school math and physics courses. I’ll let you decide if that is a good thing or bad?!
It may also help you to know that I named my previous boat sv Learnativity for a very good reason and she definitely lived up to her name in the 15 years I spent sailing her around the world. For Christine and I, two former teachers as well, Learning and Loving are the keys to living life well and that is what we aspire to do every day. Might help you understand what drives us to do things like designing and building Möbius and then writing articles like this one and others here on the Möbius.World blog so we can share all of our lessons learned with you.
Horses for Courses:
I promise to get to propellers as quickly as possible but it is neccessary to understand a wee bit more about the engine power that turns every propeller.
It is not widely understood that for most diesel engine models from most manufacturers, the same engine model can be configured for several different service ratings, each with their own different set of HP, torque and fuel burn ratings.
This is done so that the same engine can be set up to match the use case of the vehicle or boat that it is installed in. I am oversimplifying it but in marine applications these different configurations typically go from Continuous for engines needing to run at full load aka WOT or Wide Open Throttle 24/7 for days and weeks at a time, up through a series of other models as the use rating goes down to where the engine is only needing to produce peak output for a few hours a day. As you might guess as you go “up” this range, the Horse Power and the RPM goes up with each step as does the fuel consumption. It should also be noted that these same engines also start acquiring some “add on” equipment such as turbochargers, after and inter coolers, etc. but the base engine, block, crank, pistons, etc. are the same.
For example, below is a well done explanation by John Deere for their “M rating” system for their marine diesel engines. (click HERE for link to full PDV version) Keep in mind that these M ratings apply to the same overall engine model, let’s say their JD 4 cylinder 4045 4.5 liter engines or their JD 6 cylinder 6068 6.8 Liter model.
- M1 rating is for marine propulsion applications that may operate up to 24 hours per day at uninterrupted full power. These applications typically operate more than 3,000 hours per year and have load factors* over 65 percent. Possible applications: Line haul tugs and towboats, fish and shrimp trawlers/draggers, and displacement hull fishing boats over 18 m (60 ft).
- M2: The M2 rating is for marine propulsion applications that operate up to 3,000 hours per year and have load factors* up to 65 percent. This rating is for applications that are in continuous use, and use full power for no more than 16 hours out of each 24 hours of operation. The remaining time of operation must be at cruising † speeds. Possible applications: Short-range tugs and towboats, long-range ferryboats, large passenger vessels, and offshore displacement hull fishing boats under 18 m (60 ft). Marine auxiliary power engines for dedicated hydraulic pump drives, dredge pumps, or other constant-load marine applications should use the M2 rating.
- M3: The M3 rating is for marine propulsion applications that operate up to 2,000 hours per year and have load factors* up to 50 percent. This rating is for applications that use full power for no more than four hours out of each 12 hours of operation. The remaining time of operation must be at cruising† speeds. Possible applications: Coastal fishing boats, offshore crew boats, research boats, short-range ferryboats, and dinner cruise boats.
- M4: The M4 rating is for marine propulsion applications that operate up to 800 hours per year and have load factors* below 40 percent. This rating is for applications that use full power for no more than one hour out of each 12 hours of operation. The remaining time of operation must be at cruising† speeds. Possible applications: Inshore crew boats, charter fishing boats, pilot boats, dive boats, and planing hull commercial fishing boats.
- M5: The M5 rating is for marine recreational propulsion applications that operate 300 hours or less per year and have load factors* below 35 percent. This rating is for applications that use full power for no more than 30 minutes out of each eight hours and cruising† speed the remainder of the eight hours, and do not operate for the remaining 16 hours of the day. Possible applications: Recreational boats in the U.S., tactical military vessels, and rescue boats outside the U.S.
Probably easiest to understand in table format like this.
Putting this all together, here is the table of the eight different models of the JD4045 four cylinder marine engines John Deere offers. As you can see the HP ratings range from 75HP @ 2400 RPM for the M1 version all the way up to 150HP @ 2600RPM for the M4 model.
Mr. Gee’s Power Curves:
Just before I finally jump into discussing our CPP propeller on Möbius, let me quickly summarize the power curves from Mr. Gee himself, our Gardner 6LXB six cylinder diesel engine that is fully NA or Naturally Aspirated with all mechanical fuel injection, no turbo, no inter/after cooler.
All 6LXB’s can be setup for several different configurations along the lines of the M ratings of the John Deere outlined above and in our case for Mr. Gee and our XPM hull and use cases, we have set it up for a 100% Continuous Duty able to produce 150HP @ 1650 RPM.
Thanks to Michael Harrison and the other great people at Gardner Marine Diesel in Canterbury England I was able to get this copy of an original graph of all the outputs of the Continuous 100% Duty Cycle version of the 6LXB taken directly while running on their dynamometer.
I took the best photo I could of this very old paper chart so you may want to click to enlarge to read it better.
As per this graph, there are 5 numbered curves mapped out:
- Max. power available from engine
- Max Shaft Power
- Power required by typical propeller
- Fuel consumption max power absorbed
- Fuel consumption prop power absorbed.
If it helps, I have done my best to extract the following data from these curves and put them into this brief chart:
As we now get into our discussion about CPP propellers (finally!), curve #3 is the most relative as this is the power that a “typical” fixed propeller can absorb at these different RPM so that’s the curve to keep in mind here.
Fixed vs Controllable Propellers
Fixed props FP are pretty straightforward and common so I don’t think I need to go into these in much detail. Their basic dimensions are outside diameter, pitch, # of blades, etc. Pitch refers to the angle of the blades and as this angle increases the propeller “bites” into the water more. You order a fixed propeller after carefully working with the manufacturer and providing them with the data about your boat such as hull type, displacement, engine power curves, cruising speed, etc. and they calculate the prop diameter and pitch and manufacture the propeller to match.
Tying this all together, a correctly pitched prop is one that allows the engine to achieve a few more RPM’s above its rated WOT or Wide Open Throttle. This is done to ensure that you can not overload the engine and damage it and most manufacturers will void the warranty if the boat has been “over propped”.
All very logical and reasonable until you start to look at it more closely or more likely you actually get out there and run a boat for awhile and see what the real world performance and fuel consumption numbers turn out to be.
What you end up discovering are two fundamental limitations of a fixed prop:
- A FP is pitched to be just right at one RPM, one HP output and one set of conditions or load. In all other conditions the pitch is less and less optimal. In this one scenario the fixed prop can be more efficient because it has been designed to be able to absorb all the power the engine can produce. However, at any other RPM or set of conditions and load a fixed prop is either over pitched or under pitched, running less efficiently and consuming more fuel.
- Most of the time in most conditions you need much less power and torque than the maximum power available and so you run the boat with much lower loads which usually reduces the lifespan of that engine. This is exacerbated by the trend for the past decade or more for boat manufacturers, under pressure from buyers, to put in more and more HP rated engines and so it is quite common for boats to spend most of their time running at 10 to 20% of their full load rating which sets them up for very nasty results such as glazing cylinder walls, running too cold, etc.
Engine manufacturers recommend ways to try to reduce the consequences of running their engines in these low load conditions such as running them at WOT for a percentage of the time you have been running them at low loads, but you can see how this is far from desirable and a very poor fit for an eXtreme eXploration Passage Maker XPM type of boat and use case.
In summary then, with a FP boat, in many situations you end up running the engine inefficiently, using more fuel and reducing the lifespan of the engine. I don’t want to overstate this too much and there are of course thousands if not millions of boats running with fixed props so please don’t misconstrue my overview above to be saying that fixed props do not work. They absolutely do and can work quite well.
But as I repeat ad nauseum perhaps, we have our four fundamental SCEM principles for Möbius and all XPM type boats Safety, Comfort, Efficiency, Maintenance and so we are always looking to maximize all four of these and IF there is a better overall solution that helps us optimize one or more of these SCEM principles without compromising the others, then that is usually the Goldilocks choice we make. In the case of FP vs CPP, it became clear to us that CPP helped us make gains in all four of the SCEM categories, and especially so for Efficiency.
The CPP Efficiency Factor
I am going to resist the temptation, lucky you, of writing my own version of a deep dive into how and why a CPP prop enables you to achieve otherwise unavailable efficiency of both fuel consumption and engine maintenance and life span. Instead I will off load that explanation to the following two very well written articles on CPP propulsion.
** There is also a thread on the Trawler Forum with a discussion about the pros and cons of FP vs CPP HERE
The first is THIS one “Controllable Pitch Propellers” by the Naval Architect Michael Kasten’s at Kasten Marine Design. It was written back in 2001 but nothing has changed in this regard since and Michael does a very good job of walking your through the benefits of a CPP propelled boat. I will reference this article again a bit later as he also does an excellent comparison of the costs of building a new boat with FP vs CPP.
The other very worthwhile read is THIS eXcellent posting on CPP props by Matt Marsh. Matt published this very well written article back in April 2013 as part of a much larger “book” of which this is one chapter. All this is over on the eXcellent Attainable Adventures blog that John Harries has eXpertly curated over many years. If you are unfamiliar with this blog I can highly recommend that you spend a few minutes checking it out and I think many of you will want to subscribe.
The Wonder of Fuel Maps!
We need a way to talk about fuel efficiency of engines and boats and by far the best tool for that job is a Fuel map such as the one here from Wikipedia. These are also called Efficiency Maps or Consumption Maps where the horizontal X axis is RPM and vertical Y axis is Torque typically expressed in BMEP (Brake Mean Effective Pressure”).
This allows you to plot out colored lines of a given engine’s specific fuel consumption usually measured in units such as grams per kilo Watt hour g/kW/hr or grams per HP hour g/HP/hr and these colored lines are like a topographical map but instead of elevation of land, each curved line is a given amount of fuel consumption.
What is super helpful about Fuel Maps is that the specific fuel consumption lines are normalized so you can compare any two engines of any size because the lower the specific fuel consumption number, the more efficient the engine will be at that combination of RPM and torque. Does not matter if this is one of the world’s largest diesel engines such as the 14 cylinder Wärtsilä RT-flex96C that can produce over 100,000 HP @ 102 RPM (not a typo!) or a slightly smaller 2 cylinder Beta 10 engine that produces 10HP @ 3,000 RPM, you can directly compare their Brake Specific Fuel Consumption BSFC numbers.
Show Me the Money (numbers)!!
I know this has been a long and winding journey to get here and many of these acronyms and metrics can be overwhelming so let’s put this into more understandable every day units we can all understand.
While it may be counterintuitive to many the Wärtsilä RT consumes 171 g/kW/hr and the Beta 10 consumes about 330 g/kW/hr. Converting these numbers to efficiency, the Wärtsilä has a thermodynamic efficiency of 48.1% and the Beta 10 works out to about 24.8%. So as surprising as this may be, the Beta is about 50% less efficient and consumes twice the amount of fuel relative to its rated power output. Clearly I am choosing extreme examples as the Wärtsilä RT engine has held the record for the most efficient diesel engine in the world and it does weighs in with a dry weight of a svelte 2,300 tons so there is that, but you get the point of how handy it is to work wtih these BSFC.
Oh, and for those wondering, Mr. Gee, a Gardner 6LXB has a BSFC of 206 g/kW/hr which works out to be no less than 39.73%. If the engine is operated slightly below maximum torque, it does attain slightly more than 40% thermal efficiency. Now you can see why we chose to marry Mr. Gee to a CPP bride for truly outstanding efficiency, longevity and low maintenance,
If you’d like to know more about Fuel Maps, Matt also wrote up a very good explanation of these in his other post “Understanding an Engine Fuel Maps” HERE and reading that will help you understand what I’ve written below much better.
Fuel Maps for CPP driven boats
I will leave you to digest all these articles and charts above at our own choosing and speed but to get to the crux of it for our discussion of CPP props I will focus on the following 3 following three Fuel Maps from Matt’s great article above.
Here is an example of a “fuel map” that Matt created for his articles. This would equate to a typical 4L 100kW/135HP four stroke diesel engine. The thick Red line is peak Torque and the green circle is the sweet spot of fuel economy, power and torque we seek.
When we add in the Blue/Purple line of a Fixed propeller torque curve, the problem becomes very easy to see; the optimal green circle is a long ways away from the middle of that center Goldilocks Island we want and the prop torque curve never even gets close to Goldilocks Island at any RPM. With a FP this is just the way it is and there isn’t much you can do about it.
However, if we change to a Controllable Pitch Prop we can “pull” the green circle over here simply by changing the pitch and we now run right though that Goldilocks sweet spot! Being able to change the Pitch allows us to drag that torque curve pretty much anywhere we want it
I can imagine that some of you might feel that I am overstating the situation with FP boats to lead into the explanation of why we chose to go with a Controllable Pitch Prop or CPP, and perhaps I am. But all of these points above are based on the laws of physics to a large degree and just the way a FP and diesel engine works.
You might think about it this way; in a FP boat there is only one way that you can change the speed of the boat in a given set of conditions and that is by changing the RPM of the engine and prop. That works BUT these are often RPM’s that are much less efficient and you would otherwise not want to use if you had a choice. Turns out you do!
The ideal would be to be able to run your engine and prop ALL the time under ALL conditions, at their just right load conditions where they are most efficient fuel and power wise. As it turns out this isn’t all that difficult to achieve if we simply add the ability to change the pitch of the propeller at any time such that the engine is always running at its just right RPM and the boat is moving at whatever speed you want within its range. This is what a CPP does: just right load at any RPM and SOG (Speed Over Ground)
CPP props are not new or uncommon having been in daily use in boat airplanes and boats around the world for almost 100 years. For example almost all propeller driven airplanes have a CPP. Have you ever wondered how such a plane can sit there on the runway with its propeller/s whirring away and not be moving? Simple, the pilot adjusts a lever in the cockpit that changes the pitch to zero such that it is like a knife slicing through the air producing no thrust. When you’re ready to take off you just push that Pitch lever forward, the prop blades rotate more and more, producing more and more thrust and the plane zooms down the runway. Once the plane is in the air and finished climbing, the loads are much lower so you reduce the pitch accordingly.
Change the medium from air to water and the CPP in a boat works just the same way. Here is a short video that might help you see how a marine CPP works and looks as it is changing the pitch.
But Wait! There’s more!!!
Below is a short video of Uğur manually rotating our four bladed CPP on Möbius. This is from last year during the build but does a good job of showing you just what is going on under the water as we move our Pitch lever on Möbius.
There are several additional benefits that might not be immediately apparent until you get to know CPP a bit better and one of the biggest benefits worth pointing out is that if you can change the pitch from zero/neutral to full ahead, you can do the same in reverse by simply rotating the blades the opposite direction AND the shaft continues to rotate in the same direction.
Thus you eliminate the need for a fwd/rev transmission. You do still usually need a gear reduction box, ours is 3:1, to get the prop spinning much slower than the engine but no forward/reverse gears are involved. This has several positive consequences such as being much “kinder” to the engine and gearbox as there is no “clunking” in and out of gear and the other is that you can smoothly feather the prop from forward to reverse moving the boat just millimeters at a time if desired, which is eXtremely handy when maneuvering in close quarters, docking, etc.
Know the Load!
Just a very quick diversion to explain an eXtremely useful gauge on any boat and one that is of particular value on a CPP based boat and that is having a high temperature thermometer known as a Pyrometer or an Exhaust Gas Temperature EGT gauge. If you’ve been following along for the past few threads about engines, power and fuel consumption you will have noticed that the key metric that efficiency is based on is the % of load you are putting on an engine. To avoid confusion, keep in mind that Load is the power in either kW or HP that you are USING at any given time and NOT the total POTENTIAL power an engine can produce. Also keep in mind that load can not be measured by RPM, you can fully load or over/under load an engine at ANY RPM. Therein lies the challenge; If you can’t go by the RPM’s on the tachometer or the throttle position, how do you know what the load is at any given point?
It turns out to be rather simple to know the load when you understand that exhaust gas temperature or EGT is a direct proxy for load because as the load increases in a diesel engine, so too does the heat of the exhaust gas. Measuring the EGT is done very simply by having a thermometer that can measure high temperatures which is technically called a Pyrometer and what I will refer to here as an Exhaust Gas Temperature gauge. Very similar to what you might have to check the temperature of your oven or a meat thermometer, you insert a probe into the exhaust manifold, usually at the end or elbow where the exhaust is exiting the manifold.
My finger is pointing at the threaded fitting I have installed at the end of the exhaust manifold on Mr. Gee
I am using a Maretron EGT probe as this makes it easy to put all the EGT data onto our N2K/NMEA2000 network that allows us to display the EGT gauge on any screen, anywhere, anytime.
Installation is as simple as putting in the threaded adaptor that comes with the EGT probe, inserting the probe, tightening the nut and then connecting the wires into your N2K network.
There are also many gauge companies who make independent EGT gauges that just wires the probe to a dedicated display on your dashboard the same as you would do for things like oil pressure, oil/water temperature, RPM, etc..
Here is one example of a test setup Christine made to display EGT and Fuel Burn rate on any of our screens while we were doing our initial sea trials in July.
For those interested, this is an example of some of the various ways we can chose to display our engine and boat data on our Maretron N2KView screens. You can have as many of these screens as you have time to create and this one is an example courtesy of our friends James and Jennifer on mv Dirona.
OK, now that we know the exact EGT and therefore engine load at any given time, it is easy to adjust the Pitch lever to the Goldilocks load and efficiency we want at ANY speed and in ANY conditions. This is a significant advantage to any boat I would think but it turns out to be an eXtremely Big Deal on an XPM type of boat and use can in particular.
Why Does this all Matter?
If I have done a reasonable job of brining you this far, you now have a good answer to that question as the CPP enables us to operate near peak efficiency under almost any conditions and this adds up to significantly better fuel economy and lifespan for the whole propulsion system on any boat. Now put this in the context of an XPM style of boat that is intended to allow a couple to take their floating home across oceans to the far reaches of the seas which means that these boats will be underway on long passages running non stop for weeks or more and complete self sufficiency throughout their journeys. Therefore these boats have unusually large tank capacities for both fuel and water which adds up to a lot of weight that literally comes and goes over these passages and so the displacement (weight) of these boats changes a great deal from start to end of passage and over the course of a year. Thus the boat has a highly variable displacement and when you add into this equally as variable wind and sea conditions, an XPM’s propulsion system must bee able to handle ALL of these varied conditions and do so while continuing to be optimized for all of the Safety/Comfort/Efficiency/Maintenance priorities.
XPM liquid loads vary substantially during a passage and over the annual use of the boat and to put that into perspective, our total fuel tankage is 14,617L/3861USG = 12,410Kg and water is 7300L/1930USG = 7300 Kg/16100 Lbs for a total of 19,700 Kg/43,450 Lbs. That is a LOT of weight and amounts to the displacement of the boat changing by over 55% ! That is a huge range that the propulsion system needs to be able to deal with efficiently throughout and this is yet another way in which the CPP provides significant advantages. Being able to change the pitch in synch with the changes in overall displacement of the boat as the fuel and water volumes go up and down allows us to stay in that Goldilocks sweet spot on the Fuel and Efficiency Map ALL the time.
System Based Solution:
Another key benefit that helped convince me that CPP was the way to go for Möbius is that the CPP comes as an integrated solution. With a FP you typically need to spec, chose, install and buy each component; the FP itself, then a matching prop shaft, then cutlass bearings, prop tube, flanges, transmission, shaft seals, anti vibration mounts, Elecrical controls, and the list goes on.
In our case, we chose to go with Nogva a large Norwegian company that builds complete propulsion systems. They provided us with everything except the engine as we already had Mr. Gee, though Nogva does offer several major engine options from the likes of JD, Scania and Nanni.
We worked closely with the engineers at Nogva to provide them with all the details of the boat and how we wold be using it and came up with a propulsion system that consisted of their N4-215-65 CPP system that looks like this and includes literally every part you need from the prop at one end to the flange that bolts to the servo gearbox at the other.
Installation of the whole prop tube and shaft assembly Nogva shipped was eXtremely easy as we just inserted the Nogva prop tube into the aluminium shaft log pipe that had been welded in as part of the hull months prior. These two tubes slid into each other with about 10mm / 3/8” clearance between them so it was a simple matter of aligning these two shafts concentrically and then pumping the space full of ChockFast an epoxy filler made for this job.
The bright red flange you can see on the far Right here is that flange on the N4 CPP which I am not bolting together with the brown Nogva HC-168-C servo gear reduction box using the standard SAE1 flange on the back of Mr. Gee which is the Silver/Aluminium part on the far Left.
I can not overstate the benefits of getting the entire propulsion system as a complete system from the same manufacturer as it made both the installation and the maintenance of this critical system eXtremely easy and reliable.
Additional Benefits of CPP
This does not apply to us on Möbius as we went all electric, but for boats that have hydraulic systems for things like thrusters, stabilizers, windlasses and winches, CPP provides a significant advantage in that not only is the pitch always just right for actual load, it also provides the ability to have higher engine revs needed for the hydraulic pumps even when you are docking or stopping the boat. With a FP it is challenging to keep the engine revs up just as you need that bow thruster and winches the most while the boat is near standstill while docking.
Slow Speed Maneuverings
Speaking of docking, with a CPP you can move the boat with silky smooth precision 1mm forward/astern with nothing more than small movements of the Pitch lever forward/aft.
Repairing Broken Props
Given our intent to cruise in icy locations in high latitude locations, as well as the always present danger of an errant underwater log or coral head that can take a bite out of your prop blades, the CPP provides a much more manageable repair than a FP. With a once piece FP if you bend or break a prop blade the whole propeller needs to be removed, often the shaft along with it and have it repaired or replaced by an all new one. I have had to do this on previous boats and it is a big job that takes a lot of time.
With the blades on a CPP being separate parts and the center hub being much stronger and more robust, it is relatively easy to remove and replace just one or two prop blades and this can be done while the remains in place.
When I spoke with the Nogva engineers about this scenario they agreed to machine an extra set of four blades in the same run and were able to provide these at a very low cost. I carry these four new blades along with a set of O-ring seals and grease just in case this should ever be a repair I need to do in some far flung spot.
To fully validate all this and give me some advanced practice in such ideal conditions, I did a trial run by disassembling the prop and removing all four blades. It turned out to be a very quick operation with no special tools required.
I started by removing the eight SS Allen head SS bolts you can see here which let me easily remove the end side of the hub.
This now exposed the bases of the four blades which rotate around the square bronze block you see in the center. Each blade is machined to precisely slide into place on the boat side of the hub and then the end side hub fits over that to fully capture the props. A rubber O-ring around the grooves you see here, seals each blade to keep the water out and the grease in.
Some of you have asked “Isn’t this a very complex piece of equipment?” and while it can’t get more simpler than a single part fixed prop, these CPP props really are not complex at all.
And when you consider the whole propulsion system not having any gear changing transmission reduces the overall complexity considerably further.
When you slide the blades off the only thing that remains inside is the end of the SS Pitch Adjustment rod and the single bronze block that each blade pivots on. No gears, no bearings, just a lot of grease.
Slide each blade back in place, bolt the end cap back in place and you end up with a fully operational CPP.
Before we splashed the boat back in February we of course put on the black International InterSleek silicone based Foul Release paint an all the underwater aluminium surfaces and coated the Nogva CPP with similar silicone PellerClean. Now seven months later with very little movement unfortunately, the good news is that there is almost no growth on either the CPP or the bottom surfaces and what little we’ve found comes off easily with a simple wipe with a cloth.
In my discussions with the Nogva engineers and other research before making my decision to go with a Nogva CPP, I was impressed by the attention Nogva had paid to the problem of prop blades transferring noise and vibration into the hull. As I understand it, Nogva provides propulsion systems for work boats used in aquaculture and the use case of these boats in particular need to have robust, efficient and reliable propulsion in their very demanding situations.
Like XPM’s these work boat hulls are usually made of aluminum, which can be prone to noise and vibration problems. Nogva’s solutions counteract these problems by minimizing the propeller’s impulses toward the hull.and they have gained a lot of experience though their R&D into this.
I will need to get more nautical miles on our Nogva CPP to more fully understand how well this all works but based on our sea trials to date, the whole propulsion system is very smooth and working very well so far.
OK Wayne, but What about Cost?
This is perhaps the most asked question or concern when others are considering FP vs CPP for their boat. For those considering changing their current boat from fixed to CPP it would be a move costly conversion in terms of both time and money and I don’t think the payoff would be there. However the opposite is the case for those of us building a new boat where everything has to be purchased and installed either way. In this case the CPP turns out to be no more and some have suggested less total coast than a fixed prop.
Perhaps the best explanation of this is a very thorough comparison that Michael Kasten’s did and wrote about in that article I mentioned up near the beginning. HERE is that link again to save you from scrolling up to find it. Michael did this research back in early 2001 so the actual amounts he quotes have of course changed, but based on my more recent research and purchasing all the costs have scaled up equally and so I think his examples still hold up. In the beginning of this article Michael does a good job of providing an overview of how CPP props work and why he too sees them as a better and more efficient type of prop for the boats that he designs, but if you scroll down to “Part II Costs” you will find his comparison of pricing out a like for like Fixed Prop and a CPP.
Near the end of this comparison he goes on to cover some of the same points I mentioned above as to the cost and labour required to install a CPP vs a FP. He arrives at the same conclusion as I have with is that installing a CPP system is actually less time and effort than a FP. Installation wise there is little to no difference between installing a transmission for a FP vs installing a servo reduction gearbox for a CPP so that is a wash cost and difficulty wise. However installing a CPP shaft system is much easier than the more “distributed” FP components.
Michael ends with names and links to all the CPP manufacturers he was aware of at the time and these will provide those interested with a good starting point for doing their own research.
I had read Michaels article several years ago before we Möbius was even a twinkle in my eyes and so I referred back to it and used it to help me do my own research and comparison of the pros, cons and costs of FP vs CPP and I came to the exact same conclusion that a CPP is no more expensive or difficult to buy and install than a fixed prop and could be less. Given the significant advantages and benefits I’ve gone over up above you can hopefully understand why this became a “no brainer” decision for me to make. Nothing since then in our experience with buying, installing and now staring to use a Controllable Pitch Propeller has changed and it has already exceeded our hopes that this would be the Goldilocks propulsion system for Möbius. I fully expect that opinion will continue to improve over the entire time we are running Möbius and enjoying all these advantages of the increased Comfort and Efficiency our Nogva CPP provides as well as the significant reductions in fuel costs.
Isn’t a CPP Difficult to Operate?
Another of the most common questions I receive and so I will close out (bet you thought that would never happen!) by doing my best to answer this final question. A couple of quick caveats for context here. First there is no question that Christine and I are much more familiar with operating boats with a Fixed Prop and their typical Throttle + Fwd/Reverse levers or combined single lever versions. Switching over to CPP therefore presented us with some initial learning curve and at first it all felt very strange as everything was SO different. No “clunk” as we were used to when you put a FP into gear and you knew that the boat was going to move forward right away and increase speed as you increased the throttle and engine RPM. With the CPP there is no noise at all and the boat does not immediately jump forward, or reverse, and so at first you are a bit uncertain what is going to happen. You know the prop is turning at all times as you can see some of the turbulence coming out the sides even when you are in the Zero Pitch/Neutral position and moving the throttle forward increases the engine RPM but the boat just sits there. However, as you push the Pitch lever slowly forward in absolute silence and lack of any other indication, you notice that the boat is indeed moving forward and the more you push the Pitch lever forward, the faster you go. Pull the Pitch lever back and you very quickly slow down but again no other indication other than the visual confirmation of gauges and surroundings that you are slowing down and stopping.
We both spent some time out in some calm open waters to try out this all new propulsion control system and the strangeness soon faded away and began to feel more and more intuitive. Set the RPM where you want them and then increase the Pitch to move forward or reverse with extremely smooth and strong control.
Our first few experiences with docking this all new boat would have been challenging enough so with the added newness of a CPP it was all the more so. However all the surprises were very good ones as you were able to so smoothly and completely control the movement of the boat. With a very big four bladed of just over 1m diameter and an equally large rudder controlling the stern of the boat while docking is like having a stern thruster. We also have a very powerful electric bow thruster and as we have practiced using the combination of these fore and aft controls we have already gained a lot of confidence in our ability to control Möbius while doing such close quarter manoeuvring and even more so when we get underway. All still VERY early in our learning process but it has been a great start so far.
So the best answer I can provide at this early stage is that there is no question that learning to operate a CPP does take some time but it is time well spent and I’m not sure that this is very different than any system on a boat. Like all our systems, It takes a bit of time to learn where the sweet spots or Goldilocks settings are and become familiar with them so they become routine.
Operation of the CPP for cruising can be done in two different ways; set the Pitch and adjust the throttle to reach optimal loading of the engine or do the opposite, set the Pitch to where you have learned you think it will be best and then use the throttle to move you up to whatever speed you have found to be optimal for a given set of conditions. As I’ve covered in the sections up above about EGT we have learned that we basically run the boat based on the EGT reading once we are at the speed we want. We are learning to watch the EGT numbers to be sure we stay well below the maximum EGT/load which in the case of our Gardner 6LXB is about 450C/840F. If the EGT number gets too high, just back off the Pitch a bit. At this setting, the engine is powering the prop at its maximum ability, and runs well loaded at max. efficiency.
You don’t want to set the pitch too shallow as the engine will not be loaded by the prop and will run straight up to its maximum rpm. Nor do you want to set the pitch too steep either for the given rpm as that will overload the engine. Dark smoke and a increasing EGT are a signal for overload.
In situations where you want to be moving much slower, you set the RPM lower and the Pitch higher to load the engine sufficiently at low power range and low fuel consumption. In opposite situations when going uphill in adverse weather we will set the RPM higher and the Pitch lower or more shallow to allow the engine to come up to speeds with higher power output.
The recommended practice for docking with a single prop vessel is to set the RPMs higher rpm (about 60 to 80%) and then use the Pitch lever to do the needed and often hard over manoeuvres. We are learning to trust that we can push or pull the Pitch lever in either direction at these higher revs and it does not harm the system. This is quickly feels very comfortable as you experience the eXtremely fine control you have over moving the boat incrementally or quickly with just the Pitch lever.
Clearly I am in NO position to be offering advise here about running a CPP well and how to best handle a CPP powered boat, but these are my early lessons learned and I look forward to bringing you more and more as we get out there and log more hours and nautical smiles on Möbius.
Whew!! If you have made it this far, you are probably almost as tired from reading all this as I am from writing it. But even if it takes you, and me, more times to re read this and learn more, I do hope this has at least been interesting and informative for you no matter where you are at in the comparison between Fixed and Controllable Pitch propellers.
VERY happy with how well Mr. Gee and his Nogva CPP bride get along and how they propel us with such eXcellent Safety, Comfort, Efficiency and Maintainability.
Thanks for coming along for this long and winding ride and please join the discussion by adding your comments and questions in the box below.
Hope to see you back here again next week.