KIWIPROPS - SDC
Sail Drive Composite
Propellers
All
Composite
Folding - Feathering Hybrid Propellers for Sail Drives
Only .... February 2010
SDC ORDER SHEET
Units currently available are 2 Bladed for less than 40 hp
CONTACT US
 |
Kiwiprops has
developed a unique and
innovative hybrid
propeller
range manufactured solely from engineering plastics and glass reinforced
composites offers a targetted solution to the
unique design
challenges
facing
a propeller
to be fitted to a Saildrive
unit. Initially available only in 2 Bladed configuration for Saildrive units of < 50 hp ... typically the smaller Bukh, Volvo, Yanmar, Saildrives - Larger 3 and 4 Bladed units are currently under test targetting up to 85 hp - See prtotypes above. They offer users the choice an extension to the already well proven and accepted Kiwiprops range but target just Saildrive users. Blades sizes range from 14" to 17" initially - so offer Saildrive solutions to users down to 10 hp - lower than the smallest 16" Kiwiprop 3 bladed units |
PRODUCT OVERVIEW:
The
three over-riding functions required of a propeller are:
- Motoring
ahead at the highest possible speed at both
engine cruise rpm and full throttle as can be obtained from a fixed
bladed propeller
- Reversing
with the same utility as a fixed bladed
propeller
- Reducing drag to a minimum when sailing both on and off the wind
The extensive benefits that the SDC design solution offers a user can be summarised as follows:
- Eliminates propeller corrosion
- Optimal ogival foil shapes for motoring and reversing
- 100 % Reverse thrust
- Low drag when sailing
- Simple user pitch adjustment
- Fully lubricated internally
- Suited to any sailing speeds
- Allows
for 2 - 3 or 4 common blades
- Sheds any weed and ropes when
sailing
- Smooth no shock engagement in Ahead and Astern
- Very light weight
.. < 3Kg ~ 1Kg net of bouyancy
- Secure attachment to Saildrive spline
- Easy to fit - does not require blade removal
- Replaceable sacrifical blades
The initial series of production status units is now in Beta test which we intend to run for a mininum of two seasons normal usage.
The high time unit has accumulated nearly 200 hours. Addition units up to 35 hp have been installed and are under test.
These web pages are designed to provide potential Saildrive propellers users with a complete analysis of the unit to allow informed decision
making when contemplating the purchase of a new propeller to fit most popular Sailrives - initally under 40 hp.
Comments and queries are encouraged.
In designing a propeller that is optimal for the range of modern Saildrives now on the market there are a number of issues that must be considered and prioritised.
Propellers
like life
- are full of compromises and invariably involve design trade-offs in
developing an optimal solution.
For
Saildrives
virtually all of which are cast from Aluminium alloy - corrosion takes
on a
much bigger perspective than it has done in years past over traditional
shaft
installations.
For
shaft installations the
only metals involved were invariably Bronzes and Stainless
Steel both of which have
relatively good corrosion resistance from a marine environment.
While
one can debate
the corrosion potential of various metals and their associated alloys
AB2, Stainless
Steel 316, 2205,
Monel
etc in various applications, all
metals suitable for propellers will have a significant
electro-potential
relative to an Aluminium Saildrive leg.
In
addition modern
current interruption devices in
To
eliminate not just
reduce - the corrosion or conducting potential of a propeller on a
Saildrive
application,
composites offer a route which without debate eliminates
any
corrosion potential on the Saildrive
from the propeller itself. There are still restrictions. Carbon fibre
is a not
an appropriate reinforcing for composites as the carbon has a high
electro-potential relative to Aluminium.
Dry cell batteries have a carbon anode for good
reason. A full electro potential listing of metals in
salt water is at: Galvanic Series
Composites
and
engineering plastics are not as stiff or as strong as metals but the
issue is
whether they are adequate for the purpose intended. The larger boss of
a
Saildrive unit offers an opportunity to increase the size of the
propeller body
to lower the stress levels without adversely affecting the performance
or
appearance of the propeller unit.
All
propellers must
offer optimal motoring performance both in ahead and astern.
The
preferred way to
achieve that is to use optimally shaped ogival shaped foils with
progressive
pitch to accommodate the lower velocity at smaller diameters as used in
the well
proven traditional fixed bladed propeller.
Reverse
thrust in all
folding propellers is always less than optimal and particularly so at
low speed
as these types of units depend upon the mass in the tips of the blades
to hold
the blades open with centrifugal force. As the blades generate reverse
thrust
however this acts against the centrifugal forces tending to hold the
blades
open and in reality the blades are held only partially open leading to
very
poor reverse thrust particularly when manoeuvring at low engine speeds.
The
latest Yanmar and
Volvo engines have now reverted back to the 3000 max rpm of many years
previous
which has had the effect of lowering shaft rpm and thus exacerbating
the known
performance weakness of folding propellers in reverse.
The
traditional
approach is to add mass to the blade tips to increase the centrifugal
force but
this has the adverse effect of producing a less than optimal tip shape
and
impacting on motoring performance. It
also increases both the total mass of the propeller and what is more critical the
moments of rotational inertia or the resistance to the propeller being
rotated
at a different speed. The propeller acts like a flywheel and resists
any change
in rotational velocity.
Modern
Saildrives have
quite strict criteria for moments of rotational inertia and the shock
impact
generated after the Saildrive clutch is engaged. The very popular
Yanmar SD20
units have dog clutches rather than a clutch pack which generates very
abrupt
and noisy engagements and obviously requires low mass in the propeller.
Some
designs have
rubber bushes between the boss of the propeller and the body of the
propeller
that carries the blades to minimise this shock loading at start up. The
problem
this introduces is that the rubber bushes are very poor conductors so
in effect
isolating a very large proportion of the metal area of the propeller
from the
zinc anode.
This
can only have a
very adverse effect by creating an opportunity for corrosion on both
the
Saildrive leg and the propeller itself due to the dissimilar metals
that each
is made of.
Some
propeller
manufacturers will argue that their bushes are conducting but this is
generally achieved by the addition of carbon particles which in many
ways
exacerbates the electro-potential problems.
The
obvious way to
minimise the moments of inertia is to first reduce the mass of the
entire
propeller unit, and then to attempt to focus the bulk of the mass as
close to
the centre of rotation as is possible. Yanmar design
criteria for Moment of Inertia of Saildrive propellers are
avilable at: Yanmar MOI's
All
folding and
feathering metal propellers must also deal with the often harsh fore
and aft
shock loads imposed when the blades meet their stops upon opening.
Any
composite propeller
that mimics a traditional folding unit design will have insufficient
mass in
its blades to generate any reverse thrust from centrifugal force. Some
form of
mechanical motion will be required to ensure that any composite propeller
always opens fully and stays
open in reverse. At this stage another design issue emerges. In a
traditional
folder there is very little resistance to rotation of the propeller
unit once
the blades are fully folded. Any design for a composite unit then has
to have
the blades residing in a
folded or low drag position such that sufficient
resistance is
generated from the water to enable any mechanical action to take place when the propeller is engaged into
either ahead or reverse rotation.
This
functionality is
likely to conflict with the obvious requirement for a low drag
configuration of
the unit when sailing.
manufacturers criteria for start up forces at clutch engagement in both the ahead and reverse directions.
thus ensuring that a simple reliable and effective mechanical action always forces the blades out of the engaged position when sailing.
Remove any mechanical motion of the blades about their mountings when sailing thus eliminating wear
potential and any auto rotation caused by partial feathering or folding.
Variable
pitch is a
critical design feature for any propeller in that it allows for fine
tuning of
optimal settings for each and every vessel, and even for tuning
personal
preferences for engine cruise rpm on identical vessels to deliver
optimal
cruise speeds.
The
nature of all
Saildrive gear trains is such that they all have the same reduction
ratio in
both ahead and astern. This is very different from shaft drive
installations
where the reverse reduction ratio is often much higher than in ahead
and consequently
the pitch setting in reverse should be higher than in ahead.
Thus
variable pitch for
a Saildrive unit requires that when adjusted the pitch is modified
equally in
both ahead and astern to the same setting automatically.
Variable
pitch is also important from an
economic
perspective in that it reduces inventory in the supply chain
dramatically.
Traditional
geared
folders require inventory by Diameter
x
Pitch x Blade
#.
[
Being
geared each blade requires the teeth in a different position so is a
unique
part # -
It
also places
constraints on reassembly as each blade location is unique]
With
typically 4 diameters from 15 to 18, 5 pitch ranges from 10 to 14
and 3 blades for a 3 bladed folding unit -
this translates to 4 x 5 x 3 =
60 line items for
blades required in inventory, versus just 4 for an optimal
composite design with the blade root common
and only the tip shape varying to obtain the different diameters.
Thus
the injection dies
can be constructed with just
one common die and a set of inserts that go to make up
the die
set for each of the required diameters. This lowers the die costs
substantially
as the one main die block can then service the entire range of
propellers.
A
composite unit with
common rotation and variable pitch will thus only require inventory by
blade
diameter thus reducing both die costs and inventory dramatically and at
the
same time offering customers what is optimal as regards diameter and
pitch as
distinct from whats in stock.
Symmetric blades ( ie the
same identical blade being
used for each blade position ) thus emerges as a design issue in
lowering the
overall cost of the unit and therefore its price and value proposition
to the
end use customer.
Corrosion
will always
be less in any marine environment where the internal parts of any
propeller
remain fully lubricated throughout the sailing season. In addition to
ensuring
smooth operation and minimising any potential wear from the operation
of the
unit the coating of marine grease will assist in minimising any
corrosion
potential that exists with the metal components that will still be
required for
the highly stressed attachment bolts.
Traditional
folding
propellers universally have exposed gears with the potential that
creates for
marine deposits to introduce
significant wear in the gear tooth mechanism of the unit.
The
ability to simply
grease the whole of the unit without removal from the Saildrive is thus
an
important design feature. Ensuring the unit is internally greased with
no exit
pathways will primarily ensure smooth operation without wear but also
ensure a
minimum of scale deposits from hard water which in certain locations,
typically
water with high lime content, can cause scale deposits which may
interfere with
the smooth operation of the feathering or folding function over time.
The
ongoing secure attachment of both the blades to
the propeller body as well as the
propeller unit itself to the Saildrive are a critical design
issue. Not
a season goes by where a number of users report the loss of propeller
blades or
the loss of the entire propeller from their Saildrive unit.
Being
mounted on a Spline as
distinct
from taper where the propeller can pull up tight on the shaft
Saildrive
propellers by their very nature will move slightly on the spline
whenever they
are engaged into either ahead or astern. This is also a common cause of
propeller loss as the locking mechanism fails and over time the
attachment nut
comes loose.
A
composite propeller
unit must incorporate fail safe mountings for both the blades and the
propeller
as a whole while at the same time ensuring the unit is simple to mount
with a
minimum of tools. The ability to mount and dismount the unit without
having to
remove the blades from the body of the unit offers a very useful
feature from a
customers perspective as Saildrives require the frequent changing of
the zinc
anodes. On many models this entails the removal of the whole propeller
from the
Saildrive unit. Newer units have a split
zinc which
can be replaced without removing the propeller.
Low
drag when sailing
is an obvious design objective for any propeller, but because of the
nature of
Saildrives where the leg and boss at the bottom of the unit present a
substantial frontal area and thus drag, the design of the propeller can
focus
on just the incremental drag from the design of the blades. The boss of
the
propeller will have no incremental drag effect as it is streamlined
behind the
larger lower gear case of the Saildrive itself.
While
two bladed
folders are often seen as having the lowest drag configuration that
is only
strictly true when off the wind. On the wind and reaching
conditions will generate
streamlines, due to the leeway made by the yacht, that
generate a significant projected area
relative to the blades if they are in a vertical position.
Projected
area
basically equates to drag so that more correctly the drag of any two
bladed folder is greater on the wind and
reaching when the blades
are aligned vertically than off the wind when leeway is virtually nil
or when
the blades are aligned horizontally.
Drag
from the blades
will be less when the blades are folded in a horizontal position.
An
optimal blade shape
will such that it will tend to shed any weed or flotsam encountered
when
sailing. In the case of a Saildrive this requirement is lessened due to
the
large gear case at the foot of the leg which tends to act as a first
point of
deflection for any impact.
In
addition the lower
sections of the blade will be of sufficient radius to prevent any
damage from a
rope being caught around the propeller under power.
Cumulatively
the above
criteria and objectives make for a very challenging design requirement
for any
new composite propeller that is targeting Saildrive units to primarily
eliminate not just reduce the corrosion potential from the bronze
propeller
components acting upon the Aluminium Saildrive legs in use today.
Couple
this to
compelling economics of the delivered unit which are taken as a given
makes for
an even greater challenge.
After
5 years of design
iterations and over 30 versions of the propeller constructed and tested
in all
operating modes onboard a sailing yacht we are confident that the
approach we
have taken on the SDC propeller design makes the optimal trade-offs.
In
addition to normal
motoring and reversing these included multiple engagements ahead while
sailing,
engaging reverse while sailing, engaging ahead whilst motoring astern,
engaging
astern whilst motoring ahead, feathering from a sailing mode, static
thrust and
full throttle tests.
The
next stage of the
test program involved a documented usage program on a Bukh 20 Saildrive
rated
20 hp @ 3000 rpm on a 2.25:1 reduction delivering 1333
shaft rpm @ max for over 100 hours over
a six month period. This was designed to simulate normal daily usage
with
multiple astern and ahead engagements followed by motoring periods.
Our
experience with the
Bukh engine range is that it is very conservatively rated and that this
can
reasonably be compared as very similar to
a Yanmar
3GM30
rated 24 hp continuous @ 3400 on 2.64:1 delivering
1287
shaft rpm @ max.
After
examination at
the end of this period no discernable wear or structural issues were
detected
in the unit which was in as installed condition.
The
next stage of the
program will extend testing to the very popular Volvo 2003 2030 and newer
D1-30 range plus the Yanmar 3GM30 and 3YM30 range all rated
28 29 hp with various shaft speeds between
1360 and 1460.
We
believe the unit addresses the
very complex
design issues facing such a product in a manner that by providing equal
emphasis to the functionality and life cycle economics will deliver on
an
ongoing basis a superior life cycle value package to all Saildrive
users within
the defined power range of the unit.
# 1: Why an all composite propeller ?
The
design
of this unit has been driven to address the
changing requirements from the ever increasing market share that
Saildrives now
occupy in the new build market and thus their overall market share of
drive
trains.
All
Saildrives from the major producers are cast in Aluminium
and while they make extensive efforts to address the potential for
corrosion
that this creates,
the presence of a large area of Bronze from a traditional propeller adjacent to the Saildrive leg generates an electro-potential between the surfaces driven by basic physics that can not be avoided. Zinc anodes can only mitigate this effect. A full electro potential listing of metals in salt water is at: Galvanic Series
In
addition
all Saildrives have the same reduction ratio in
reverse as they have in ahead quite different from the majority of
shaft
installations. This requires a different approach to pitch settings in
reverse
for optimal performance.
While an
all composite propeller will eliminate the corrosion
potential from a bronze propeller - a composite propeller will not necessarily eliminate
corrosion.
This is a
complex area and requires specialist input but can
be coming from a whole raft of areas. Onboard power, bad grounding,
marina
grounding, incorrect usage of copper based antifouling paint and even the boat(s) next door.
Incorporating
other design features
at the same time to eliminate poor reverse thrust, lack
of
lubrication, smooth engagement, positive mechanical opening and closing
and the
high drag from fixed propellers will deliver an optimal unit. The
advent of new
composites now makes this zero based design approach to the optimal
propeller
design specifically for Saildrives a reality and provides an economic
solution
to a number of long standing series of outstanding propeller design
problems
for sailing vessels. The approach
adopted will eliminate not just reduce the corrosion potential of a
bronze
propeller adjacent to an Aluminium Saildrive leg.
From
a design perspective it is best described as a hybrid unit. It is a cross between a feathering and a
folding propeller incorporating the best features of each.
Like all feathering propellers it has user
variable pitch and operates at 100 % thrust in reverse.
In
addition all three functions of motoring, reversing and sailing
are engaged and folded positively by a
mechanical motion the opposite of all
folding units which depend only upon centrifugal force to open the
blades and
water pressure to close them.
Like
all folding propellers the blades retract from the streamlines to
offer
minimal drag and the ability to shed anything that is likely to catch
on the
blades such as lines or kelp. The blades
also have progressively pitched ogival foil sections for optimal
motoring
whereas the foils on feathering propellers have slightly less efficient
symmetric foil sections with fixed pitch down the foil. An ogival
foil is flat to the rear and the section of a circle on the front face
and is used on all fixed bladed propellers.
Ogival
Sectioned Foil
Symetric Sectioned Foil
# 2: How does the unit operate in reverse ?
In a
nutshell the unit operates in reverse exactly as it
does in ahead.
The
blades
are mechanically opened and then locked so it will
operate exactly as a fixed two bladed propeller in both ahead and
astern.
When
reverse is engaged the water pressure acting against the
blades causes the blade assembly to rotate around the internal boss and
in
doing so initiates the internal
mechanism that positively rotates the
blades to an open position.
The
internal mechanism continues to rotate the blades around
the plane of their mounting bolts until the blades, while acting
against the blade
springs to ensure a smooth opening motion, come up against their
respective
pitch stops. At this stage the whole propeller begins to rotate in
whichever
direction has been selected and continues to operate as a fixed bladed
propeller. When sailing - the internal
blade springs rotate the blades back into the trailing position where
the
blades are aligned with the streamlines for minimal drag.
Thus the
trailing edge of each blade becomes the leading edge
during reversing like a fixed bladed propeller. This is no different
from
almost all other folding propellers.
What is
different is that the blades are held by the design
of their mounting bolt and the internal mechanism in their fully open
position
irrespective of reverse thrust at the same pitch setting that was
selected in
the ahead direction.
The unit
thus operates exactly like a fixed two bladed unit
in reverse to deliver similar thrust as such a unit would in ahead.
This will
always substantially exceed the reverse thrust
available from any competing folding propeller design which all use
centrifugal
force from the mass in the blade tips in a futile attempt to hold the
blades
fully open against the reverse thrust of the propeller.
Despite
manufactures various claims - All folding propellers
in reverse operate
with the blades in a semi-open position to deliver mediocre reverse
thrust.
# 3: Is the pitch user adjustable
Can it be
done underwater ?
The
pitch can be very easily and quickly adjusted by the user with just an
Allen
Key.
At
the rear of the propeller body
facing aft for easy access is a
stainless steel set screw for each blade.
Turning
this screw in by one whole turn will increase the pitch by 2. Likewise
a
change of pitch of plus or minus 1 can be obtained from ½ a turn of
the pitch
screw.
This
can be easily done underwater with dive gear.
The
pitch screws are self locking into the body of the propeller and remain locked after adjustment.
They have been machined to provide tolerances for an interference fit
that will
ensure the screws stay locked.
When
the face of the
blade that acts upon the pitch screw coincides with the joint line on
the
mounting surface this equates to a
reference point of 12 inches or 300 mm of pitch. This is the most
popular
setting.
Pitch
adjustments can then be made about this reference point.
While
the design of the unit accommodates differing pitch settings on each blade it is
important
to ensure equal pitch settings on the blades.
Unequal
pitch settings will lead to vibration, noise and reduced propeller
performance.
Mounting
the Allen Key in a short length of dowel will ensure it floats if
making pitch
adjustments underwater.
# 4: Why
dont the blades fold up fully ?
With
a composite propeller there is very little mass in the blades and any
folding
or hybrid design can not utilise this mass to open the blades in
reverse by
just using centrifugal force as all conventional folding designs do.
For
the internal mechanism to operate there needs to be a torque operating
against
the rotation of the body to provide sufficient force to allow the
internal
mechanical mechanism to open the blades in both ahead and more so in
reverse
where the reverse thrust generated from the blades will tend always to
fold the
blades back into the closed position.
Allowing
the blade tips to partially project in the closed position provides
sufficient
force when the unit is engaged to engage the opening mechanism yet
provides
very low incremental drag. Remember on Saildrive applications drag from
the
frontal area of the leg and lower gear case will always far exceed the
incremental drag from the small thin and highly streamlined blade tips.
Frontal
area basically dominates any drag equation.
We
believe that typically in operation one blade will always align in the
disturbed streamline off the leg and make no incremental drag
contribution.
The
design of the unit has made a conscious trade-off of the very small
extra tip
drag vs the benefits
of 100 % reverse thrust and no corrosion potential.
Drag will be less than a typical
three bladed
feathering propeller for two reasons. There are only two blades not
three and
the area projected into the streamlines when the blades are retracted
will be
substantially less than a feathering unit where the blades are always
open.
Thus
the hybrid nature of the units design
~ 80 % folding with
20 % feathering from a drag perspective
yet 100 % feathering from a reversing
perspective.
# 5: Do
I have to set the pitch when the unit is received ?
No. Unless requested
otherwise - the unit will be delivered with the pitch targeted to allow
the
engine to achieve its rated max rpm under
full load
which is required for warranty purposes on all new engines.
This will
be based on the information collected from our database of
engines in use and comparative data.
Higher
pitch settings will increase cruise speed for a given
rpm at the expense of achieving maximum engine rpm and higher power
output.
Every
installation is however different. Exhaust
back pressure from design or
corrosion, fresh or salt water,
altitude, cleanliness of the prop,
engine age and compression, auxiliary take-offs such as compressors
&
alternators will all impact on engine rpm achievable.
Potential
boat speed which can be affected by loading and
weather will in itself alter engine rpm as the effective pitch that the
propeller sees
is a function of speed through the water.
Lower boat
speeds than normal will translate to slightly
lower engine rpm.
Typically we would
expect over 90% of users would not need to adjust the
pitch from the original setting that was shipped with the unit.
# 6:
Can I adjust the reverse pitch independently?
The
simple answer is no. The reverse pitch
always equals the ahead pitch.
Because
all Saildrives have the same reduction ratio in reverse as they do in
ahead due
to the nature of the bevel gear drive train Saildrives require the
same pitch
in reverse as they do in ahead.
The SDC
approach of mechanically locking the blades open in
reverse provides exceptional reversing performance
in that the unit
simply
operates as a fixed bladed propeller would in reverse.
Thus there
is no requirement to adjust the reverse pitch
independently from the pitch setting in ahead.
# 7: Do I need to take the unit
apart before
mounting it on the shaft ?
The unit is mounted by first
unscrewing the front face cover
after removing the 3 x M8 316 Grade Stainless Steel locking screws
around the
forward perimeter.
The
boss is then exposed when the body holding the blades is removed from
the boss.
The internally splined
boss of the propeller is then ready to
slide
onto the externally splined shaft of the Saildrive unit with the
front
face piece already fitted.
After
the appropriate checks which are covered in the user manual, the unit
can be
mounted on the spline and the nut tightened up using a standard 25 mm
A/F socket drive.
The
nut is then locked in via a vernier adjustment with a stainless steel
split pin
to the boss. After greasing the body of the unit can then slide back
over the
boss externally locking in the split pin whereupon the front face piece
is
screwed back on, tightened up and the locking screws re-inserted around
the
perimeter with Loctite.
The
unit is then ready for operation.
At
no stage is there any requirement to remove blades from the propeller body nor are
the pitch settings disturbed during this process.
Typically
this whole operation would take
~ 5 minutes for
The
operating manual supplied with the unit and available on the web covers
this in
detail with appropriate pictures and diagrams.
# 8: How strong are the composite
blades ?
Clearly
the
blades are not as strong or as stiff as bronze,
but the issue is - are
they strong enough for the purpose for which they have been designed.
Virtually
all modern aircraft have composite propellers and now structural
components.
The
DuPont Zytel HTN53G50 blade material contains 50 % glass by
weight and is thus both very
strong and stiff.
DuPont have extensive technical information on
their web
page regarding the physical characteristics of the many different
grades of
Zytel they have available.
Another
design issue is that composites and the economics
they enjoy allow a blade to be sacrificed in a catastrophic situation.
With a
substantial impact on the blade tips
( always
the
first part to hit ) they should sacrifice the blade -
hopefully
leaving the propeller boss and Saildrive undamaged.
We
believe
it is better to loose an easily replaceable blade
costing ~ $ 100 than a whole propeller
or drive train when hitting the ground or a floating log or mooring
chain.
This
can be very expensive in a Saildrive installation where the whole leg
is at
risk.
Ropes
caught around the unit will simply stall the engine and
in each of a number of cases where this has happened to date the unit
has
emerged undamaged.
The blade design with well rounded leading and
trailing
edges at the root is designed for these inevitable events.
We have not
yet had
a blade fail in service and are confident that they are stressed
correctly for the application they serve.
Remember composite propellers
are now
freely available for outboards up to 300 hp.
So in
simple terms the answer is quite strong enough !
# 9: Should
I buy spare blades to carry on board ?
Our
advice is that if you have a catamaran with no keels
that is
going to allow the props to hit first if ever grounded carry a spare
set of
blades.
If
you are heading off on a world cruise with the potential to be in
problems such as dead
trees or
coral heads then take a spare blade or set.
In many
ways the cheapest and best insurance is to carry a
cheap fixed 2 or 3 blade unit to get-you-home.
For
general use we see no more or less need to carry spare blades on board
than you
would with any other prop.
We would expect very little demand for spare
blades
other than in the above situations.
# 10: How
is the unit lubricated how often should it be greased ?
This
topic
is covered in the user manual but in summary the
unit as delivered contains lubricants sufficient until your next
maintenance
haul out. Each side of the unit will need to be lubricated after
removing the
small Pozidrive screws located just forward of the blade using a needle
nose
grease gun. You will need to remove the outer guard off the needle
portion of
the grease gun.
One hole leads to the internal
mechanism in the front of the unit. The other leads to a lubrication
groove
that carries grease to the surface between the body of the propeller
and the
internal boss that is connected to the drive shaft.
This then
is repeated for the other blade on the other side of
the body.
Each of
these four grease points should then be filled with
a high quality marine grease eg Shell Nautilus Marine Grease - NLGI No 2 or
equivalent.
Lubrication
is covered in detail in the manual supplied with
the unit.
# 11: Does the unit run smoothly
?
Yes this
is a factor often commented upon in the feedback
we have received from SDC testing coupled with the inherent design
attributes.
For any
rotating mass the design criteria to minimizing
vibration is to first lower the total mass of the rotating assembly and
then
ensure that the mass is concentrated as close as possible to the axis
of
rotation.
The SDC
Propeller is much lighter than comparable bronze
units at under 3.0 kg but more
importantly - because of the composite blades being much lighter than
the
typical bronze blades of say a 2 bladed folder, the mass is in effect
much more
focused towards the center of the unit thus reducing the Moments of
Inertia.
The
ogival
sectioned foils on the blades with progressive
pitch also assist with smooth running.
Combining
these factors produces a very smooth running unit
that offers minimal vibration and noise.
Blade
clearance to the hull is less critical than with
traditional folding propellers as the blade tips are a finer section.
They do
not require the thicker sections needed to contain the mass
required to improve the reversing thrust from the additional
centrifugal
force in an attempt to hold the blades
open in reverse.
# 12: How
much tip clearance is required ?
While the design
of Saildrives generally provides for more than adequate clearance
of any propeller,
som
This can also occur
in narrow catamaran hulls where
again the engine
is raised in the hull
To obtain
clean water away from the water that is dragged
forward along with the motion of the hull and impacts on propeller
performance
- all propellers require clearance from the tips to the hull.
Having
low
tip clearances can also cause vibration to be
transmitted through the hull from the radial disturbance caused by the
propeller
blades displacing the incompressible water as they sweep past the hull.
Some
rules
of thumb use 10 % of the diameter but higher
clearances generally have little additional impact on improved
propeller
performance.
Due to
the
low boat speeds - typically 6 ~ 7.5 knots involved
with low powered displacement yachts
clearance is not as critical as on many applications.
With thin
tips, unlike folding propellers which use the mass
in the tips to provide reverse thrust from the centrifugal force
generated, our
empirical experience is that clearance can be lower with virtually no
impact on
performance or vibration.
We would
suggest no less than ½ or
12 mm in a tight situation
obviously more is better.
This is not an issue in the great majority of
Saildrive installations.
# 13: How
robust is the internal Vectran drive mechanism?
Vectran
has
only recently become available and has truly
exceptional properties.
Only the
advent of such materials has enabled a design such
as the SDC unit to become viable.
While
very
expensive the small quantities used in the unit
make for an economical solution.
It has
virtually no stretch and
Using an
engineering plastic addresses the corrosion issue
targeted in the original design brief.
The tensions
involved in the Vectran are
very easily calculated and have been kept to less than
~ 25 % of rated breaking strain at full
power.
In addition the
torsion springs on the blades
eliminate any shock loads at initial engagement.
Fully lubricating
the unit will assist in removing any wear between the Vectran and
urethane body which
is self lubricating to an extent anyway.
While chemically
inert to salt water and oil based products Vectran will however
deteriorate
when exposed to UV
radiation.
Having the Vectran
totally enclosed
within the body
of the
propeller and then installed underwater
eliminates this as an issue for this application.
We currently
believe that it will have an indefinite life in this application.
# 14: Will
an SDC Propeller improve my motoring performance ?
All
Saildrives with one exception have been generally well
designed to convert engine rpm
to appropriate shaft speeds for optimal
propeller performance.
The
quantum
of any
improvement available will then
depend upon the extent to which your exiting propeller is optimal.
We would
expect an SDC unit to achieve very close to optimal
motoring performance that will match that of a well sized fixed two or
three
bladed unit.
Fixed units will always outperform traditional folding
units due
to their thinner
blade tips.
Folding units require mass in the blade tips to increase
reverse
thrust which translates to thicker tips and less efficient foil shapes.
Our
experience is however that in many situations the
existing propeller is not well matched and in these installations we
can often
deliver increased motoring performance.
The design of the SDC unit with
raked
blades places the blades further away from the disturbed streamlines
around the
Saildrive leg which is a desirable design attribute for improved
motoring
performance.
Typically
improvement will come from replacing a fixed or
folding two bladed unit with insufficient area
or a three bladed unit
that has
not been sized correctly in the first instance.
Variable
pitch allows the unit to be set economically to what
is optimal - as distinct from what was in stock.
The same Saildrive
installation in a heavy displacement vessel requires a very different
pitch
setting
from the same unit installed in a catamaran with its
much higher cruise speeds.
Higher boat
speeds lower the effective pitch that each blade
sees in operation so require higher pitch for the same
engine drive train.
Personal
choice of cruise rpm can also be addressed with a
simple pitch adjustment which allows
for matched optimal pitch settings
thus
delivering optimal motoring performance for each individual
installation.
# 15:
Will an SDC propeller improve
sailing performance ?
Versus a
fixed bladed propeller this is always very difficult
to comment on due to the very large number of variables involved. What
is known
from the published work from MIT in their lab is that a fixed three
blade 16
propeller mounted horizontally as a Saildrive unit is will produce drag
of over 70 lbs at 8 knots with the unit
locked
against rotation. Allowing it to rotate will significantly increase
drag over
the above figure.
This was
reported in Practical
Sailor October 1993 and January 1995
issues.
Drag at this level
will have a very significant effect on sailing performance.
Our
empirical experience is that a typical 30
to 40 foot vessel hard on the wind in
about 15 knots of breeze will achieve
maybe an extra 0.75 ~ 1 knot of boat
speed and point about 10 degrees higher into the wind with very
significant
increase in VMG over a locked fixed three bladed propeller.
Reaching can deliver up to an extra ~1.5 knots
again very dependent upon the individual vessel and situation.
In short
the sailing performance improvement will be
dramatic and it is for this reason one would switch from a fixed blade
propeller.
Versus an existing folding
propeller there will be no
improvement in sailing performance
but a dramatic improvement in
reversing
function with corrosion potential
eliminated.
# 16:
What does the unit weigh ?
A unit
fitted with 17 blades including the attachment nut
weighs less than 3.0 Kg
Smaller
or
larger blades will alter this by
~
+/- 0.2 Kg / inch.
This
simply
equates to the material content of the marginal
blade tips as the rest of the unit is identical in all sizes.
In
addition
because the body of the unit displaces a
significant volume of water
the weight
under water reduces to
under 1 Kg
We
believe
this is by far the lightest folding or feathering
propeller available.
Typically
a
17 bronze 2 bladed geared folding propeller will
weigh between 8
and 10 Kg depending upon design.
Three
bladed units are typically heavier than two.
Self
pitching designs and feathering bronze or stainless
units are much heavier again.
Catamarans
therefore can achieve installed weight savings of
as much as 12
18 Kg simply by adopting all composite SDC type hybrid propellers.
Some
modern Saildrives have the option of selecting either direction of
rotation.
Most
older
units can only run Left Handed in ahead.
The
only logical reason for running Right Handed may be the perception that
in catamarans,
there is
In
reality, because the units are so far apart physically ( unlike the
twin shafts
of a launch ) manoeuvring is simplified and the reverse wash of the
propeller
which can cause prop walk as it blows over the hull has little
effect.
Saildrives
all have the shafts horizontal which in itself reduces prop walk as
the
wash of the propeller in reverse is not directed up over the hull
to
the same
degree as in a typical inclined shaft installation.
SDC
have made a carefully considered decision to only manufacture Left
Handed
propellers for Saildrives for the simple reason there are huge economic
savings
by not duplicating dies and inventory.
There
is no user advantages to offering an
alternative
rotation.
The
direction of rotation in ahead can be easily altered by simply swapping
the
direction of the control cables at the Saildrive for those units
currently
running
Right Handed that wish to install SDC units.
A
key design objective for any Saildrive propeller is to have minimum
drag from
the propeller when sailing.
Drag
for any object at low speed under water is primarily determined by the
frontal
projected area.
In the case of a Saildrive unit with a feathering or
folding
type propeller installed this will be dominated from the drag of the
Saildrive
unit itself.
The
drag from the propeller will then for practical purposes simply be the
incremental drag from the extent that the blades of the unit
project
out past
the projected area of the Saildrive leg itself and the projected area
of these
blades relative to the actual streamlines around the leg.
Measurements
we have done, which are quite difficult to obtain realistic results,
would
indicate that the incremental drag of a two bladed
SDC type hybrid unit
over a
Saildrive leg with just a propeller boss with no blades is of the order
of 1 ~ 1.5 lbs or less than 0.5 ~ 0.7 Kg
at 7
knots.
These
measurements are done when sailing flat off with streamlines symmetric
about
the Saildrive unit.
Sailing
hard on the wind where leeway will typically be of the order of 10 deg
will
increase the projected area of all propeller types ( except the self
feathering
Kiwiprop and Autoprop designs ). This is further complicated in the
case of a
two bladed folding unit as to whether the blades are lying horizontal
in a
minimum
projected area to the actual streamlines or vertically where a
folded
propeller will have a significant projected area to the actual
streamlines and
thus increased drag.
Three
and four bladed folding propellers sailing on the wind will have a
projected
area irrespective of the position of their blades.
With
an immersed weight of less than 1 kg vs 7 ~ 10 kg of a typical Bronze
folding
or feathering unit you need to also reduce the drag for the SDC
hybrid
by the
incremental drag that the additional weight
the bronze units will apply to the hull.
It
will not be large but it will not be zero.
Taken
over all sailing conditions we believe the net drag of an SDC x 2 blade
hybrid
unit mounted on a Saildrive leg will be very similar to the drag
from a 2 bladed folding unit.
It will
always be less than feathering type unit where the blades are always
fully
extend into the streamlines.
It will also likely to be less than the
drag from
a three bladed folding unit again depending upon the angle of leeway
at the
time.
When the blades are retracted into their " folded " position - just like all typical folding units, the blades will extend aft further than a traditional fixed or feathering unit.
While typically this is no more of an issue than for any comparable folding type propeller, users that are contemplating a switch from a fixed or feathering type propellers
should check carefully clearance to the skeg or rudder if this is placed closer to the rear of the propeller than is normal.
The overall length of the unit at it's "
longest " point from the rear face of the collar on the Saildrive
that the unit pulls up onto is less than 16"
or 400 mm