Prop Charts Prop
Cutting
|
Balancing
Author, Andy Kunz.
e-mail Andy at: andy@rc-hydros.com
Website: rc-hydros.com
The propeller fills a unique position in a boat in that it can be used to adjust
several
characteristics of the boat. To compare a boat to a car, the prop would be your
transmission, wheels, gear ratio, and tires. Quite a bit depends upon having the
right
prop, but finding the best prop for your boat is something of an art.
There are several different types of propellers for models, just as there are
for full-size
boats. Almost all racing props are surface-piercers, and run about half in the
water and
half out. Running in this manner solves several problems when compared to fully
submerged props, enough so that even some new merchant vessels are being
designed to
use surface props.
Octura makes several different classes of props to fit the different needs. The
all-digit
props (1435, 1932, etc.) are designed to provide lift and are therefore suitable
only in
hydros, while the X, Y, and P series props are multi-purpose props designed to
be more
efficient and provide minimal lift but maximum thrust.
On scale hydros, I have found that the X- and Y-series props give better
performance
than others. These props provide excellent thrust while angled relative to the
direction of
travel, and that makes them equally suitable for both solid and flexible
driveshafts.
Because of their higher efficiency compared to lifting props, they provide the
competitive
edge a racer needs.
Lifting props do not work well when there is much of an angle to the prop shaft,
so they
are far more limited in their applications. There are some boats which just won’t
run
without one (usually riggers), so you should have one or two in your prop box.
As we
will see later, there are certain reasons to choose one particular type over
another.
PROPELLER BALANCING
I’ve found one thing over the years that has improved my performance, extended
the life
of my equipment, cut down on noise, and earned me "attaboys" from my
peers. The
answer: prop work.
A good propeller, because it performs so many functions on a boat, is essential.
My
props are all balanced as described below, then polished and measured. They are
kept
stored in soft foam in a box, ready and waiting for just the right time. If you
ever meet
me at a race and are in need of a prop, just stop over at my table.
Proper Balancing
"Your prop has to be balanced," you often hear. But what is balancing,
really? What
most guys mean when they say their prop is balanced is that they’ve put it on
a prop
balancer and found that both blades weigh the same amount. This is static
balancing,
and is a good first step in the process of increasing performance. It has taken
quite a few
years and quite a few props, plus several good books, to find the things covered
in the
next few pages.
For balancing you need a good balancer. My personal favorite is the Top Flight
magnetic
balancer. This balancer can be so accurate that you can actually see the effect
of finger
oil when static balancing your props. It can also be used for a variety of model
balancing
jobs, not just boat props. It is the best for small boats, though, because it is
the only one
sensitive enough to really show when a tiny, low-mass propeller is balanced. It’s
worth
every penny of the $20 or so it’ll cost you.
Precision Equipment
Notice that I said above that the Top Flight balancer "can be" so
accurate that you can see
the effects of finger oil. To achieve that level, though, you need to make a
change. My
experience is that you will need to replace the horizontal shaft with a
precision piece of
steel. My recommendation is to obtain a "drill blank" of 1/8"
diameter from a tool maker
or tool supply shop. Have a machinist grind a fine point on it which is dead on
the center
of the rod. Even better would be to have him make you a shaft which goes from
slightly
undersize to slightly oversize for the propeller shaft size(s) you will be
using, but most
machinists do not have the tools to accomplish that. Be happy with a straight
shaft with a
single good point on it. (Note - it is only necessary to have a point on one
end. The
other end can remain square, or could have the sharp edges chamfered or rounded
slightly).
After you have obtained a good, perfectly straight shaft, the next thing you
need to do is
balance it. It can be done, but it takes patience. If you balance the shaft
properly, you
will be able to balance your propellers properly. If you cheat, you’ll lose
the benefit of
having such a good balancer. Don’t be afraid to take the time to do the job
right.
Use 320 or higher grit wet-or-dry sandpaper with light oil (I use haircutter oil
from my
barber for all oil sanding) to remove weight from the heavy side of the shaft.
Wipe the
shaft dry each time you check to see if the shaft is balanced. Remember, this is
a very
precise tool you are working on, and you want it to be perfect. The weight of
cutting oil
will fool you!
The next tool you need is a handle for holding the prop while you work it. I use
a 4"
piece of 1/8" driveshaft with a drive dog near one end, and a 1/8"
wheel collar (from the
airplane department at the hobby shop) to hold the prop on. It is best if you
file a small,
square notch into the side of the shaft so that the set screws do not allow
slippage.
If you are working with props for
other shaft diameters, use an appropriately sized tool.
You will also need a metal file. I bought a
12" metal file from the local Home Depot.
This has a fine cut for wood, but for metal it’s rather course. I like this
myself, but you
should get what works best for you. A set of jeweler’s files will be useful as
well, for
removing flashing on the hub and blades.
You’ll also need a single, small can of
Play-Doh® or similar putty. This will help us
make the prop the right shape. A stroboscope will also be needed to spin balance
the
props, and possibly an optical tachometer such as is used for model airplanes. A
stroboscope makes a bright, short flash which makes moving objects look like
they aren’t
moving, and the tach measures how fast something is rotating.
Finally, two small wood blocks are handy tools. One should be about 1" x
2" x 3", the
other 1" x 6" x 12". The larger one should be of a hard wood such
as oak or maple, the
smaller one should be of a softer wood such as pine. The hard block will be used
for
thinning the prop blades, the soft one for smoothing the lines. I have found
that the larger
block works better laying on top of my workbench, the smaller one in my hand.
Balancing Technique
Correctly balancing a propeller is accomplished by a tried-and-true engineering
principle
known as stepwise refinement. Begin by meeting easy parameters, then continue
meeting
harder and harder ones until you have achieved your goal.
I have balanced hundreds of props over the years (I think I keep Octura in
business
sometimes), and what is described below is the process I use for my own
equipment.
Everything depends on the tools you use, how well you know to use them, and how
much
patience you have to achieve the result. Save your pennies and buy good
equipment, take
some time to learn how to use them, and then be patient. It takes me about an
hour per
prop from start to finish, and I usually work on about 4 at a time. I also use
the process
described below after I damage a prop in order to make it usable again. Never
discard a
dinged prop – you can almost always cut it and rebalance to have a usable one
again.
Step 1: Dimensional Balancing
The first step to fully balancing your prop is to dimensionally balance it. The
goal of
dimensional balancing is to make sure that both blades are the same size. This
will keep
the load on the motor constant, which will help prevent power-robbing vibration.
Begin by mounting your prop in the handle. Remove the flashing from the hub
using the
jeweler files.
My next step with Octura props is to make sure that the prop is cut to the size
of the
mold. Many of these molds are apparently rather old, and sometimes I get props
which
are so different that you can see the differences between the blades without any
special
tools. Some blades are thick, some are thin, some have more flashing than
others, etc.
Next, place a clump of Play-Doh® on your work surface, then push the prop into
it to
leave an impression. Very carefully remove the prop, rotate it 180 degrees
(for a two-blade prop), and check to make sure that the impression perfectly
matches the other
blade. Use a metal file or emery board with oil to remove any oversize portions.
Repeat
until both blades are exactly the same size and shape, each time refreshing the
Play-Doh
to get a new impression.
Step 2: Sharpening
The next thing you need to do after getting the blades shaped the same is to
sharpen them.
Right now, your blades most likely have squared edges. You job is the make the
leading
edges as sharp as you can, but keep the trailing edge square.
First, make sure the trailing edge is square by lightly going across it with
your file if you
haven’t already. You don’t want to take your blade out of dimensional
balance, so go
easy. The trailing edge should be squared off relative to the face of the blade.
Next, use your file to remove metal from the front of the blades, moving the
blade from
the trailing edge toward the leading edge. This will give you the nicest cut.
You don’t
want to cut a edge, you want to thin the entire blade down so that the whole
thing is a
gentle taper.
After you are satisfied with the sharpness of the blade, use oiled 320 grit
sandpaper to smooth the blade and remove any imperfections that the file may
have
created. I prefer to use a 1x6x12 piece of wood as a work bench of sorts, with
the
sandpaper on the top side, moving the propeller to work it.
You can go to a finer sandpaper to make the prop smoother. I usually stop at 600
grit
myself.
Step 3: Static Balancing
Once you have dimensionally balanced your prop and sharpened it, you need to
make
sure it is statically balanced. The goal of static balancing is to make sure the
blades are
the same weight, which will definitely help with later steps. We also gain the
benefit that
both blades will become very closely matched in thickness, helping to maintain a
constant load on the motor when racing.
Use your prop balancer to make sure both blades weigh the same. This is
accomplished
when your prop will stay level on the balancer, without a tendency for either
blade to fall
vertically. You need to remove thickness from the heavy blade, never changing
the
outline. Use the hard block of wood and 240 or 320 grit paper.
Finished that? OK, now you are half done. Half? Yup, half. Your prop is balanced
tip-to-
tip, but probably not "top-to-bottom." You also need to be able to
make either blade
be on either side of the balancer for it to be balanced in this manner. Most
boaters are
happy when they can get the prop to stay level, but they forgot to make it stay
level in
both directions. When your blade is only balanced for one direction, you will
find that
the prop will run much smoother but not as well as it could.
Spin Balancing
After you have finished static balancing, you need to make sure your prop will
stay
balanced when running at top speed. This is a lot more difficult to do, and it’s
where I
sometimes cheat myself. If you’ve taken the time to balance your prop as
described
above, you’re already doing better than 95% of the rest of the guys out there.
Here’s how
to get that extra 5%.
Spin balancing is what they are doing to your car tires when you have them
balanced at
a garage. The tire is spun, and a computer inside the tire balancer tells the
operator where
to place a weight and how big it should be. He will spin the tire multiple
times, until the
computer tells him it’s done. Since there aren’t any such tools available
for us, we need
to make do with what we can. This is another one of the other reasons I like my
Top
Flight balancer.
Basically, we will be spinning the propeller while using the stroboscope to make
it appear
as if it isn’t moving. If it isn’t running true, we remove weight from the
heavy blade until
it is balanced. Here’s how to do it:
First, make a small mark with a permanent marker on the tip of one blade. Next,
you
spin the prop on the balancer shaft. While it spins, shine the strobe on the
prop, watching
carefully to see if the shaft is rotating perfectly. By watching carefully, you
will be able
to see if the shaft is running true or moving in a circle. The ink mark will
allow you to
identify which blade is heavier (the heavy blade will appear to be on the
outside of the
wobble). Lightly oil sand the heavy blade to adjust it. Repeat this until the
prop is
moving in a perfect circle, with no movement of the shaft. If you have done a
perfect job
of this, your prop will still be perfectly static balanced. If it isn’t, you
will have some
vibration at certain speeds but not at others. The goal is to make it perfect at
all speeds,
but if you can’t do that, just make sure it’s perfect at operational speed.
The ideal spin balancer would spin the prop at the speed you would be turning it
on the
water. I don’t have a tool which would allow me to do that, so I just spin the
shaft with
my fingertips. If I ever figure out how to get 30K RPM on the balancer, it will
be a good
day at the races!
Mount the prop on the driveshaft in your boat, and run the motor up to the
expected
operational speed, using the optical tach to find that speed. It will probably
be between
50% and 75% of the maximum speed your motor turns, so you can work it from that
angle as well. (This is where stick radios with the spring return taken out can
be very
handy.) Check to make sure the prop runs true at operational speed. If it doesn’t,
you
can find the heavy blade as you did on the balancer, but it might not be the
prop that’s out
of balance! Try making sure your driveshaft is balanced as well – it should
run true at
even full RPM. This is a very fine detail, but it will help keep you from
wasting power.
One little hint – always mount the prop the same way on the driveshaft. I like
to make a
very tiny notch which identifies how the prop mounts to the drive dog. This will
help
you keep your drive system balanced, not just the prop.
Step 5: Polishing the Prop
All my props, when I finish working them, are polished to a high shine. The
shine, I
believe, gives the propeller a smoother surface for slicing into the water.
Since most
polishes use a wax binder, you are also waxing the surfaces, making them
slippery.
To polish your prop, mount it firmly in the holder. Put a polishing wheel in
your Dremel,
and put on your safety glasses. For polishing compound I use a four-grit set of
sticks that
I bought at Sears. I find the variety of grits much more useful than what Dremel
has to
offer. Use standard polishing techniques to put a bright finish on your prop. I
recommend that you do common sense things like always run the wheel so the prop
does
not dig into it, and try to keep from holding the prop in a way that it could
fly into your
face or body if it comes loose.
Whatever you do, don’t use a full-size benchtop grinder for polishing. These
props are
just too tiny for that! You do not need to concern yourself with the possibility
of taking
a prop out of balance by polishing it. The amount of metal removed and of wax
added
is not of sufficient consequence.
Storing Props
After you have your prop balanced and polished, the next step is to accurately
measure it
and store it. I purchased a small plastic box with a dozen or more compartments
to hold
my props. Each compartment has a small piece of soft foam which fits snugly in
the
bottom, and another which covers the prop from the top. All the props are
arranged in
order by diameter as measured by a vernier caliper, with higher pitch props of
the same
diameter closer to the next size larger in diameter. A piece of card stock fits
in the lid of
the prop box to be used as a key to the prop sizes. On race day, I can pull
exactly the
prop I need for any boat. I also mark certain props which I have found
particularly good
on a hull, and never loan those out.
If you have followed these steps, your prop should be about as balanced as is
humanly
possible. It may take a couple hours for each one at the start, but you will
immediately
hear and see the difference it makes. After you’ve been working your props for
a while,
you’ll probably get down to the point where it only takes about an hour each.
If one of your propellers is damaged (minor!) in some way, don’t throw it out!
Remove
the damaged area with a file, then follow the steps to rebalance it, starting
Step 1. That
damaged prop might become the new star performer if it is reshaped and balanced!