Copyright 1998 - 2007 Michael Kasten
Updated 7 May 2007
Page Contents:
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, often amounting to around 15% or more
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 made greater when the vessel is light,
and made more fine when loaded without having to vary the engine rpm,
and still provide maximum use of the engine's power.
When running free with wind and waves the pitch may be made greater. Conversely when headed into the weather, the pitch may 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
amount and one load condition: i.e. those for which it was
specifically designed. At that one rpm and load, the FPP is able to absorb all the power that the engine can produce. At
any other rpm, or any other
vessel loading, the FPP cannot, either being over pitched or under pitched.
A correctly sized Controllable Pitch Propeller on the
other hand is able to be efficient at a wide rpm range, and at a wide
variation in vessel loading. This is so because the propeller 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 a
heavier 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 fully to remove the drag of the stopped propeller.
The Controllable Pitch arrangement will permit a skipper to vary the engine rpm in service as needed to obtain the most favorable reduction in engine vibration and noise, as well as to vary the pitch (and therefore alter blade loading) to eliminate cavitation at any rpm.
Finally, there will be no anxiety as with a Fixed Pitch Propeller over whether or not the correct Fixed Pitch has been calculated for the combination of vessel and engine. As many boaters have discovered, predicting ideal propeller pitch is
far from being an exact science...! With the wrong fixed pitch
propeller, efficiency can be very poor indeed.
With a Controllable Pitch Propeller, you can make pitch adjustments 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 Sail Boats and Motor Sailors
Should you be traveling aboard a sail boat or motor sailor, 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.
Relative Efficiency
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 don't really
notice: it is not a huge bite.
While that has been the case for quite some time here in the US, we are
seeing big changes in that situation... Now (2007) diesel prices are over
USD $2.50 / gallon, and any extra fuel used does indeed begin to sting...!
In Europe where fuel is often three or four times our US cost, 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.
Maneuvering
A CP arrangement will 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 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, I have
ordered parts and spares from Sabb for my own Sabb diesel. I have usually phoned in the order, and the parts are shipped air freight the same day. I would ordinarily receive the parts within a few days. That's
better service than I often get when I order parts from Seattle, less
than 80 miles away...
In any case, this stuff is nothing new, is robust and well proven... and
therefore certainly nothing to be afraid of...!
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 Lugger 6 cylinder turbo as an example: the Lugger 668-T.
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, which is also very well
suited to the output of the Lugger 668-T, 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. I don't have a current price from Vetus for this, but we might be able to assemble the pieces one by one, again assuming a 60mm shaft and 3 blade 660mm prop (around 26 inches), and taking the retail prices:
- 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, as 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 inflation on the NOK,
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 cost more to install...?
No! It consists of a gear box and a self-enclosed
shaft assembly. Even though we encounter no end of completely
unwarranted whining from boat builders unfamiliar
with CPP equipment, we can point out the following:
- Mounting the gear box is in no way different than mounting a similar
FPP gear. 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. You just arrange 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). This could not be simpler.
- Again the advantage goes to the CPP assembly.
One should note that there are several CPP
arrangements available from a variety of manufacturers.
Sabb Motor has replaced their own engines in the Sabb lineup with Lister Alpha engines
up to 50hp, or IVECO engines in the 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 being about half the weight and also being less tall compared to the Sabb engines,
making the Lister engines a much easier fit. Sabb gears are available for engines
of 10 hp up to around 300 hp. Up to 30 hp or so, Sabb provides the
HVP-15 with a solid shaft and a Sabb manufactured propeller and stern tube
assembly.
Above 30 hp, Helseth is the shaft and propeller supplier for the various Sabb CPP gears available.
With these, a concentric shaft is used. Helseth also supplies
propeller assemblies for quite large vessels, as well as for high speed
applications.
Hundested equipment is made in Denmark and is of extraordinary quality.
The Hundested propeller assemblies make use of a concentric shaft
arrangement, and it is 100% compatible with the Sabb gear offerings. A Hundested installation
in the smaller sizes can alternately make use of 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
will
make use of a CP servo reduction gear from Mekanord. In
still larger
sizes the gear will be manufactured by Hundested.
Another excellent CP gear and propeller assembly maker from Norway whose
offerings extend into somewhat larger sizes is Nogva Motorfabrikk.
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).
West Mekan Produksjon is yet another Norwegian
manufacturer specializing in propeller and shaft assemblies that are
compatible with the gear offerings from Sabb and Nogva. West Mekan
also makes a variety of CP servo pitch control boxes, allowing their
propeller / shaft assemblies to be installed with a standard marine gear.
As with Hundested, the West Mekan pitch control units are offered in
mechanical, electric, or hyraulic models.
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 such as
Berg that supply equipment to very large vessels.
Our specific challenge has been to assemble good information about CP
equipment in the relatively smaller vessel sizes, accommodating up to
approximately 700 horsepower.
Contact information for the above-mentioned manufacturers
is as follows:
Nogva Motorfabrikk A.S.
N-6280 Søvik
Norway
www.nogva.no
+47 70 20 84 00
+47 70 20 84 10
firmapost@nogva.no
Mr. Kjell Norvoll
Helseth A.S.
Bakliveien 11-13
6450 Hjelset
Norway
www.helseth.no
Telephone : +47 71 20 29 00
Fax : +47 71 20 29 01
helseth@helseth.no
Hundested Propeller A.S.
Stadionvej 4
DK-3390 Hundested
DENMARK
www.hundestedpropeller.dk
Telephone +45 47 93 71 17
Fax: +45 47 93 99 02
Mogens Christensen (mogens@hundestedpropeller.dk)
is helpful and very knowledgeable about CPP installations for all kinds
of craft.
Sabb Motor
P.O. Box 7170
5020 Bergen, Norway
www.sabb.no
Phone:+47 55 34 88 00
Fax: + 47 55 34 88 01
firmapost@sabb.no
West Mekan Produksjon AS
Malakoff 3
N-6770 Nordfjordeid
NORWAY
www.west-mekan.no
office@west-mekan.no
Konrad Skibenes: konrad@west-mekan.no Tel : +47 57 88 51 50. Fax
: +47 57 88 51 51
Kjell Steinsåker: kjell@west-mekan.no
Tel : +47 57 88 51 53
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 pyrometer
carefully. A pyrometer makes it possible to accurately gauge engine output.
Even though at first a CP installation will seem to offer
far too many choices... It will at first seem impossible to choose a specific
pitch or rpm. There are too many variables..! 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 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.
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.
These thoughts are not meant to deprecate the
excellent FPP equipment 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...
Copyright 1998 to 2007 Michael Kasten
More Information on Controllable Pitch Propellers and Gears:
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