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 Prop Cutting
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 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 wonít 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

Iíve 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 theyíve 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. Itís worth
every penny of the $20 or so itíll 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, youíll lose the benefit of
having such a good balancer. Donít 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 itís rather course. I like this myself, but you
should get what works best for you. A set of jewelerís files will be useful as well, for
removing flashing on the hub and blades.

Youíll also need a single, small can of Play-Dohģ 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 arenít
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 Ė 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-Dohģ 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
havenít already. You donít 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 donít
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 itís where I
sometimes cheat myself. If youíve taken the time to balance your prop as described
above, youíre already doing better than 95% of the rest of the guys out there. Hereís 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 itís done. Since there arenít 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 isnít moving. If it isnít running true, we remove weight from the heavy blade until
it is balanced. Hereís 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 isnít, 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 canít do that, just make sure itís perfect at operational speed.

The ideal spin balancer would spin the prop at the speed you would be turning it on the
water. I donít 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 doesnít, you
can find the heavy blade as you did on the balancer, but it might not be the prop thatís out
of balance! Try making sure your driveshaft is balanced as well Ė 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 Ė 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, donít 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 youíve been working your props for a while,
youíll 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, donít 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!