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Controllable Pitch Propellers
Copyright 1998 - 2013 Michael Kasten
RATIONALE | COSTS | SOURCES | CONTACTS | TECHNIQUE | SUMMARY
I. CPP RATIONALE
What Is the Advantage of Controllable Pitch?
There are real advantages provided by Controllable Pitch Propellers (CPP). This article is intended to outline several of the reasons why CPP might be favored for a motor vessel, for a sailing vessel, or for a motor sailor.
For Power Boats
Long Range Cruisers (LRC's) will necessarily carry quite a sizable fuel load, typically some 12% to 15% of the vessel's displacement. With such a highly variable load on a long range trawler yacht or other long distance power vessel, if fitted with a CPP, the pitch can be increased when the vessel is light, and decreased when loaded without having to vary the engine rpm, and will provide maximum use of the engine's power.
When running free with wind and waves 'pushing' you along, the pitch can be made greater in order to properly load the engine. Conversely when headed into the weather, the pitch can be made more fine. Much like shifting into overdrive or second gear, depending on the requirement.
While it is true that a Fixed Pitch Propeller (FPP) can be more efficient than a Controllable Pitch Propeller, it is rarely given that opportunity... The FPP can only be maximally efficient at one rpm with one horsepower input amount and one load condition: i.e. those for which it was specifically designed. At that one rpm and load, the FPP is designed to absorb all the power that the engine can produce. At any other rpm, or any other vessel loading, the FPP cannot operate efficiently, being either over pitched or under pitched.
A correctly sized Controllable Pitch Propeller on the other hand is able to be efficient over a wide rpm range, and over a wide range of vessel loading. This is so because the propeller's pitch can be adjusted as needed in order to absorb all the power that the engine is capable of producing at nearly any rpm.
Even aboard a planing power vessel, one can use a CPP to provide fine pitch and higher rpm to get going, and increased pitch with reduced engine rpm once up to speed. If the vessel has two engines, and speed is to be low, one engine can be dropped out, and its propeller feathered to reduce the drag of the stopped propeller.
The Controllable Pitch arrangement will allow the engine rpm to be varied as needed for the most favorable reduction in engine vibration and noise, as well as to eliminate cavitation.
Finally, there will be no anxiety as with a Fixed Pitch Propeller over whether or not the correct pitch has been calculated for the vessel. As many boaters have discovered, predicting ideal propeller pitch is far from an exact science...! With the wrong fixed pitch propeller, efficiency will be very poor indeed.
With a Controllable Pitch Propeller, you can make pitch adjustments under-way to suit a wide variety of conditions. It is not a difficult matter to choose the appropriate pitch with a CP. One will quickly learn what works best, even without fancy instruments. Once you have sailed with CP, you will be very reluctant to go back to FP -- by comparison FP will suddenly seem rather primitive.
For Sailing or Motor Sailing
Should you be traveling aboard a sailing or motor sailing vessel it is unquestionably the best arrangement to have a CPP for the sake of accommodating the widely varying engine power requirements, whether under sail or not; whether running free or beating; or whether simply powering.
A further benefit is that some CP units will actually permit fully feathering the blades in order to effectively eliminate prop drag while under sail alone. See notes below about fully feathering installations...
If we run at speeds other than that for which a Fixed Pitch Prop has been pitched, the propeller is not being efficient, and we are wasting fuel. It is interesting to note that here in the US where fuel is relatively inexpensive, the Fixed Pitch Propeller is the main stay. In the US, if it costs us a little more per mile to drive our vessels, we have not really taken notice.
While that has been the case for quite some time here in the US, we are seeing big changes in that situation... Now (2008) with fuel prices closing in on USD $4.00 / gallon, any extra fuel used does begin to sting...!
In Europe where fuel is several times the cost in the US, the CPP is much more common. In Scandinavian countries the CPP configuration is nearly universal, and has been so since the beginning of mechanization of their commercial fleets in the early part of the 20th century. In other words, it is extremely well proven and well supported in northern Europe, especially in Scandinavia.
A CP arrangement does not require a reversing gear--only a reduction gear. Instead of changing the direction of rotation, the blades articulate clear around until they are chewing on the water in the other direction. There is no gnashing of gears, only a smooth transition from ahead to astern thrust, with any choice of the amount of thrust desired, from dead slow to full ahead or astern. It is also possible to use a higher engine rpm and shallow pitch, for example to hold station while applying full power to the hydraulics (anchor windlass, etc.).
What About Damage...?
In the Baltic and in North Sea harbors, ice is common. Still the CP prevails. CP units are designed with the blade being the weaker part. If you damage a blade, it's a simple matter to replace it, and both the shaft and pitch control mechanism are not affected. One can easily carry a spare blade. This protects the propeller hub, the shaft, the coupling, and the transmission from being damaged by a sudden stop or by striking a log or other debris.
In the event of parts being required, parts and spares can be ordered and delivered quickly worldwide. I have phoned an order for parts to Sabb of Norway for example. Parts are shipped right away by air freight and are received within a few days. That's as good as the service I get when ordering parts locally...!
In any case, this technology is nothing new, is robust and well proven... and therefore certainly nothing to be afraid of...!
Fully Feathering - or Not...?
For sailing vessels it is important to note that in order to achieve a fully feathered propeller it will always be necessary to use a standard reversing marine gear in combination with a separate servo pitch control unit.
Virtually ALL controllable pitch propellers make use of a hollow shaft that contains a concentric push rod which moves forward and aft over a certain range. Via an eccentric in the propeller hub attached to the blades and to the push rod, the pitch of the blades is accurately controlled. Most CP propellers have a limited range of pitch change, and most CP actuators also have a limited range of push rod travel. Thus, there are two types of systems for pitch control...
- CP MARINE GEARS: A marine gear that has an internal servo unit to control the shaft's internal push rod will always turn only in one direction. Thus, in order to achieve "reverse" all of the push rod travel must be dedicated to changing the blades all the way from forward pitch, through neutral pitch, to reverse pitch. In other words, the push rod travel is completely used up in order to achieve that range of pitch change, leaving no additional travel available to effect a fully feathered propeller.
While these internal servo CP gear boxes are quite suitable for motor yachts that do not require a fully feathering propeller, they are considerably less suited to sail boats due to the limited range of pitch variation available. To my knowledge, there are no marine CP gear boxes that are able to fullly feather a CP propeller.
- CP SERVO UNITS: In order to achieve a fully feathered propeller, it is necessary to use a typical reversing marine gear in combination with a separate servo pitch control unit, mounted in line with the shaft. This allows the pitch control unit to dedicate its entire range of push rod travel to provide a pitch variation from neutral pitch, to forward pitch, through to being fully feathered. Said differently, since the reversing marine gear can change the rotational direction of the shaft, the propeller does not have to achieve "reverse" pitch, and the range of the servo actuator can be re-dedicated to achieve a fully feathered state.
Among the manufacturers listed below, Sabb for example only makes CP Marine Gears, consequently they are unable to provide a fully feathering solution for use on a sail boat. By contrast, Hundested, Nogva and West Mekan manufacture their own servo pitch control units that mount in-line onto the shaft just aft of a reversing marine gear. These systems are able to achieve a fully feathered propeller.
II. CPP COSTS
Does a CPP Arrangement Cost More...?
To most boat owners, what matters most when considering a CPP installation, whether on a new boat or on a boat that is being re-powered, is usually purchase cost. It might at first glance seem that a Fixed Pitch installation will be less expensive to purchase. After all, isn't it simpler?
What is the real cost difference?
This question is brought up sufficiently often that I have made the following actual cost comparison. In so doing, it's possibly best to make this kind of comparison using equipment that is suited to the requirements of a specific engine. So, let's use the John Deere 6 cylinder turbo as an example: the JD 6068 TFM.
Please bear in mind that the following comparison was made at a given time, using the prevailing costs and exchange rates during early 2001.
Example CPP: Controllable Pitch Propeller Equipment
Sabb HVP 65-E as of 1999 was quoted at 60,000 Norwegian Kronor (NOK), ex works Bergen, Norway. At that time they were quoting a 20% discount for OEM direct orders. The exchange rate during April 2001 was USD $1.00 equals NOK 9.08. This translates to a cost of USD $5,286 assuming the 1999 NOK price did not change much until 2001, and assuming the same OEM direct order discount would still be available.
The completely assembled Helseth 3H-60, having a 60mm shaft, stuffing box, stern bearing, shaft tube, and 660mm diameter 3 blade CPP propeller, at that time was priced at NOK 25,000, with a 20% OEM direct order discount available. This assembly translates to a cost of USD $2,203, ex works, if making the same assumptions.
CPP Whole Shebang: USD $7,489 plus freight of around USD $350 and import brokerage of around USD $50, for a very approximate total of around USD $7,889 with shipping, but without taxes. Bear in mind that this is a 1999 price and is based on a 2001 exchange rate...!
Example FPP: Fixed Pitch Propeller Equipment
The Twin Disc MG-5050, also well suited to the output of the JD 6068 TFM, retails (in 2001) for USD $3,570. A "builder's" 10% OEM discount is sometimes available, so let's use approximately USD $3,213.
A comparable stern gear "assembly" is offered by Vetus and includes shaft, bearing, stuffing box, and shaft tube. Although that would be a fairly good "like for like" comparison, I don't have a price from Vetus for that equipment. However we can instead assemble the pieces one by one, again assuming a 60mm shaft and 3 blade 660mm prop (around 26 inches), and taking the retail prices (actual researched prices as of 2001):
- Coupling: $0.00 (usually supplied with gear)
- Shaft: Aquamet 22 approx: USD $1,500
- Machining on Shaft (Both Ends): approx: USD $500
- Prop: 26" 3 blade Ni Br Al prop (comparable to the Sabb / Helseth alloy): approx USD $1,900
- Bronze Sleeved Cutless Bearing: approx USD $200
- Stuffing Box: Varies, so assume approximately USD $400
- Bronze Stern Tube Materials: 5' x 3" OD x 0.25" wall: USD $180
- Machining on Stern Tube: Approx USD $400
- Flange Fabricated on Stern Tube: Approx USD $350
From the above, the tail shaft "assembly" amounts to a total of around USD $5,500, and the Twin Disc MG-5050 Gear comes to around USD $3,213. This gives the following total:
FPP Whole Shebang: USD $8,713 without any shipping or taxes. The FPP costs given here are using current (April 2001) quotes from suppliers and machinists in the Seattle area, and using materials of equal quality to the Sabb / Helseth system.
A Few Comments
The above cost figures have factored in the miscellaneous goodies, the machining and the fabricating required to create an "equivalent" system in terms quality, and in terms of the self enclosed type of shaft arrangement offered by Sabb / Helseth. Of course one could argue that a Manganese bronze propeller with an ordinary stainless shaft in a non-self-enclosed shaft tube will be less costly, but then we would not be comparing "like for like" in terms of quality and components.
The above cost comparison was made at a given moment in time, using prevailing exchange rates of that time (2001). Of course exchange rates have changed, but so have local costs in the US. I have been told that several of the above mentioned prices have increased over the years, however if we adjust the above figures for present day exchange rates and similarly adjust for the increased costs of a comparable FPP installation, the overall comparison will in all likelihood still show approximate parity.
Doesn't a CPP system require more labor and cost to install...?
A CPP system consists of a gear box and a self-enclosed shaft assembly, just like an FPP system. In North America I usually encounter no end of completely unwarranted whining from boat builders who are unfamiliar with CPP equipment. Consider the following:
- Mounting the gear box is in no way different than mounting a similar FPP gear and makes use of the same SAE flywheel housing sizes, etc. Zero difference.
- Mounting the self-enclosed shaft assembly is actually easier. Why...? Because the entire shaft, shaft log, stern bearing and stuffing gland are supplied as a unit, one which is already matched to the propeller and to the shaft coupling. In other words, there is less labor involved in the installation, less coordinating of parts, and overall considerably less fooling around with it all. It is simply a matter of arranging for the shaft log to be isolated from the structure (in the case of a metal boat), or mount it directly (in the case of a GRP boat), or bore a hole for it (in the case of a wooden boat). The installation could not be simpler. Here, the advantage goes to the CPP assembly.
In Europe, a CPP system is well understood, therefore welcomed. In Scandinavia, installation of a CPP system is assumed, since the advantages are quite evident.
III. CPP SOURCES
One should note that there are several CPP arrangements available from a variety of manufacturers.
Nogva Motorfabrikk is an excellent CP gear and propeller assembly maker from Norway whose offerings extend into somewhat larger sizes for both motor yachts and for sailing yachts. Nogva CP marine gears are available for installations of from 50 hp to 680 hp. Larger CP gear models also include dual hydraulic PTO, intended for heavy duty use in the North Sea fisheries. Nogva also manufactures their own line of 3-blade CP propellers, available with grease, oil or water lubricated shaft assemblies as we've described above. Yet another option available from Nogva is an in-line shaft hydraulic servo pitch control, much like the offering from Hundested. The in-line servo pitch control box allows the use of CP propeller and shaft assembly with a standard marine gear (such as Twin Disc or ZF).
West Mekan Produksjon is another Norwegian manufacturer specializing in propeller and shaft assemblies that are supplied with their own servo pitch control box, or alternately are compatible with the gear offerings from Sabb and Nogva. The West Mekan servo pitch control boxes allow their propeller / shaft assemblies to be installed with a standard marine gear, allowing sufficient blade travel for fully feathering the propeller under sail. As with Hundested, the West Mekan pitch control units are offered in mechanical, electric, or hydraulic models.
Servogear A/S of Norway also have a very complete offering, including propeller, shaft assembly, AND marine gear. Their offerings are unique in that Servogear also includes CP offerings for fast motor yachts. A distinct advantage of their propeller design for fast yachts is the presence of a propeller hub "fairing" which dramatically improves efficiency. See below for their contact information and web site.
Sabb Motor is now called Frydenbø Sabb Motor AS (name changed in 2008). Several years ago, the last remaining Sabb engines were discontinued, and replaced with a range of Lister Alpha engines having a power rating up to 50hp, and IVECO engines in larger sizes. For small boats, the Lister / Sabb / Helseth combination is an improvement over the prior Sabb engine offerings in many ways, primarily due to the Lister engines being half the weight, and also being less tall compared to the Sabb engines, making the Lister engines a much easier fit.
Sabb has re-focused their business on the manufacture of CPP marine gears, available for engines of 10 hp to around 300 hp.
Up to 30 hp or so, Sabb provides the HVP-25 which makes use of a solid shaft, with a Sabb manufactured propeller and stern tube assembly. Above 30 hp, the Sabb CPP gear offerings operate a concentric shaft, and Helseth is the shaft and propeller supplier. Helseth also supplies the complete shaft / log / propeller assembly. The Helseth range includes quite large vessels, as well as high speed CPP equipment.
Heimdal Propulsion of Norway is one of the oldest makers of CPP gears, shaft assemblies and CPP propellers. In 2011, Heimdal introduced a gear that can take power ratings up to 200kw. The Heimdal equipment range goes from there upward in size.
Hundested equipment is made in Denmark and is of extraordinary quality. The Hundested propeller assemblies make use of a concentric shaft arrangement just like the Helseth equipment, and it is therefore 100% compatible with the larger Sabb gear offerings. A Hundested installation can alternately make use of a standard reduction gear combined with a Hundested CP 'control box' in the drive line (rather than a 'servo' type of marine reduction gear). Hundested offers a variety of sizes and types of pitch control boxes: manual, electric, or hydraulic. For installations above 300 horsepower Hundested offers a CP servo pitch controlling gear from Mekanord. In still larger sizes the gear will be manufactured by Hundested.
A variety of large vessel marine gear offerings are available from ZF, which offers a line of CP gears extending upwards into Mega-Yacht and even cargo vessel sizes... In fact, in the very large vessel sizes there are numerous other manufacturers of CP propellers.
Our specific challenge has been to assemble good information about CP equipment in the relatively smaller vessel sizes, accommodating up to approximately 700 horsepower.
IV. CPP CONTACT INFORMATION
Contact information for the above-mentioned manufacturers is as follows:
Nogva Motorfabrikk AS
+47 70 20 84 00
+47 70 20 84 10
Mr. Kjell Norvoll
West Mekan Produksjon AS
Konrad Skibenes: email@example.com Tel : +47 57 88 51 50. Fax : +47 57 88 51 51
Kjell Steinsåker: firstname.lastname@example.org Tel : +47 57 88 51 53
Hundested Propeller AS
Phone: +45 47 93 71 17
Fax: +45 47 93 99 02
Phone: + 47 53 42 39 50
Z.I. La Frayere
06150 Cannes La Bocca
Phone: +33 49 347 6938
Bei der Gasanstatt 6-8
+49 451 583 230
Korsør Propeller AS
Telephone : +47 71 20 29 00
Fax : +47 71 20 29 01
Heimdal Propulsion Norway AS
+47 97 57 90 38
+47 71 25 21 55
Bjørn Olsson email@example.com
Adriaan van Korlaar
Representing: SERVOGEAR A/S
Tel: +31 227 54 02 94
Fax.: +31 227 54 07 49
Mob: +31 653 63 11 93
Frydenbø Sabb Motor AS
Post Box 7170
Phone:+47 55 34 88 00
Fax: + 47 55 34 88 01
General Inquiries: firstname.lastname@example.org
Tor Isdahl, Sales: email@example.com
V. CPP TECHNIQUE
The "Basic Technique" Of Controllable Pitch Propellers...
Keep in mind that the technique I am about to describe is only possible to use in combination with a pyrometer in order to monitor the actual load on the engine. The reason for the pyro is that most marine engines can be overloaded and will therefore require that you monitor the exhaust temperature carefully using a pyrometer, which makes it possible to accurately gauge engine output.
At first a CP installation will seem to offer far too many choices... There are too many variables..! How can one choose the correct pitch or rpm..? After a bit of experience however, one will soon discover in a general way what works best in terms of the desired rpm and a range of pitch settings that work for the speed and weather conditions. The process is really fairly simple.
Once an operator becomes a little more familiar with their CP installation, the rpm and pitch that favors a given condition, vessel loading, or speed through the water will be virtually automatic. Prior to that, the pitch and engine loading can be roughly dialed in as follows:
- Select the pitch that seems appropriate to the requirements of the situation (by experimentation and recording or remembering the results of prior times).
- Increase the throttle to raise the engine rpm until there is no further gain in rpm.
- Allow the vessel to come to speed with the power being applied until equilibrium is reached.
- At that point, vary the throttle setting to be sure rpm is at its maximum for that pitch setting.
- Back off the throttle until you hear the engine lose a few rpm, then nudge the throttle back up to or just under maximum achievable rpm for that pitch.
- At this point, the engine is powering the propeller at its maximum ability, and no 'extra' fuel is being fed to the engine.
- Monitor the pyrometer to assure that the engine exhaust temperature does not exceed the manufacturer's recommendation.
- If the pyro reading is too high, just back off the throttle until the temperature is within tolerance.
Ideally, this will be within the preferred engine rpm range (the sweet spot) for the speed or conditions. If the pitch is too heavy for the weather conditions or the vessel load condition, there will probably be audible cavitation. Pitch should be set well clear of the cavitation range, i.e. more shallow. If the pitch is too shallow, the engine will not be loaded by the propeller and will run right up to its maximum rpm. In either case, make the requisite pitch adjustments until all is well. If conditions change (wind, weather, motor-sailing, etc.) then another series of nudges to the throttle are made to be sure you are not overloading the engine.
If the pitch is too heavy for the throttle setting (i.e. more throttle than the achievable rpm warrants) there will possibly be a darker exhaust smoke. With a pyrometer the exhaust temperature will begin to exceed the engine maker's published limits, and the throttle must be backed off.
In my own 34' schooner Emerald I installed a Sabb engine, which had an integral 2:1 gear with built-in CP control, etc. With the Sabb heavy duty slow turning motors, it is not necessary to use a pyrometer as a guide. It is sufficient to listen to both propeller and engine, and nudge the throttle until it is right. Per the Sabb manual's instructions, black exhaust smoke is an indication of excessive engine overloading.
It is far better to use the "nudge" technique described above for the throttle / rpm setting, rather than the exhaust color. This technique will quickly get you a workable preliminary throttle setting. Keep in mind though that for modern-day engines, even the "nudge" technique is not a sufficiently refined indicator for fine tuning the pitch or the throttle setting, and the pyrometer reading must be consulted.
The above-described technique is primarily used for familiarization with the CP equipment. With the pyro readings, one can gauge the load on the engine precisely. Using these techniques, and by listening to the engine and the propeller, eventually you'll know what's 'right' among the large variety of settings possible, i.e. in an intuitive way that does not rely on anything more than your senses and your common sense.
Of course all of the above is not to say that you must run the engine at its full capacity all the time...! For example, if the engine is idling you can vary the pitch as needed for the perfect salmon trolling speed... If the engine is charging a battery or running a refrigeration compressor or a water maker, you can back off the pitch some in order to allow the engine to propel the boat at maximum efficiency while still using full engine power.
VI. FINAL THOUGHTS
We have reviewed a simple cost-comparison case to illustrate the economics as well as the overall rationale that drive our recommendations in favor of a CPP installation, whether for a motor sailor or for a passagemaking power vessel. An added benefit is that the CPP system will be more efficient in use, and will be much more adaptable to variable vessel loadings, weather, maneuvering, or sail-assist situations. With these advantages the economics in favor of a CPP system become yet more compelling.
With a Fixed Pitch Propeller (FPP) system, once it is installed you will inevitably find yourself in an unknown situation in terms of whether the pitch has actually been calculated correctly for your vessel. In an alarmingly large number of cases it will not be. Therefore, in order to make a thoroughly fair cost comparison, one really should factor in the probable added costs of a second or third haulout and re-pitching ceremony...!
This would never be necessary with CPP equipment.
As Bjørn Olsson of Heimdal Propulsion has noted:
"...all vessels operate in varying conditions (varying load, varying wind/wave conditions). Therefore the theoretically determined "design point" [for a fixed pitch propeller] has no correspondence in reality.
For this reason, it is impossible to design a fixed pitch propeller with high accuracy. In reality it will always operate "off design point." In addition, when designing a fixed pitch propeller correctly, reverse rotation must be considered. Reasonable FP reversing properties will always be at the expense of FP forward properties. A CP propeller does not have these weaknesses, as the propeller blade may be optimized for one rotation direction, and the pitch may be easily fine tuned to match the operating conditions. Consequently, the CPP will be more efficient than the FPP in all conditions..."
These thoughts are not meant to deprecate the excellent FPP equipment that is available. They are rather an attempt to encourage an open mind when making this choice during the course of a new vessel design, as well as to encourage the same on the part of builders who oddly are often too rigidly stuck in their ways to consider new (actually old) or unfamiliar technology...
More Information on Controllable Pitch Propellers and Gears:
Please see our AVAILABLE BOAT PLANS web page.
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