What is actually done when sharpening and cuping a model propeller?

Which faces of the raw propeller blades do you grind off and which ones do you leave alone when sharpening. Leading edge only or both leading and trailing edge of the blade sharpened? To a fine thin edge or to a clever shape

What exactly do you grind/bend? to "cup" a propeller?

Do you only sharpen the propeller edges or do you grind the whole blade to make it thinner as well? If so how thin?

Do you try for an aerofoil blade shape like a planes wing or unifom thickness across the blade?

What tools work best? Dremmel tool? Air grinder? What dremmel/grinder bits to use for what operation? All by filing? Wet dry sand paper? Stone?

How and with what do you polish the prop when done shaping?

Etc? Etc?

I just moved our prop thread from the electric forum and pinned it to the top of the general boating forum. Hopefully that will answer a lot of your questions.

Thanks for the heads up. I will go there with my prop questions.

A lot of this stuff is kind of subjective. I would love to do some testing with all this, or read about anyone that has done some testing.

Anyhow, about polishing...I brought this up before. It is said that a polished finish on the bottom of your boat will slow it down. So...why do folks polish their props, rudders, and turn fins to a high shine?? You will get answers on both sides of the fence on this...

As to cupping...I don't think most RC boaters cup their own blades. I think this is a more advanced method of working a prop. Problem being, you would need to cup both of the blades EXACTLY the same. There are some prop duplicators out there, but I have not seen one for props as small as most folks here seem to be using (for RTR type boats).

As to which side to work...I have heard of some folks not touching the front of the blade at all (the front actually is the part facing rearward when mounted in the boat). But, every picture I have seen of props seem to show that it has been worked. If you think about it, this is the side of the blade that makes the most contact with the water. It is actually pushing your boat. Personally, I use a dremel to remove any flashing from it...other than that, all I do is polish it when I polish the rest of the blade. I am not saying that what I do is right or wrong...just saying what I do....

As to what bits to use...I use a small diamond bit to remove any flashing off the barrel of the prop. I use the sides of a cutting wheel to remove flashing from the edges and flatten the rear of the blades. Then I use 2 or three different grit sanding disks to shape, sharpen, and balance the blades. Then I use dremel polishing compound if I actually polish the blade. One of my blades was just worked down to 600 grit sandpaper, and then left alone.

I think if you ask 10 different people how to do it, you will get 10 different answers.

As a disclaimer, I am pretty new to this. What I do either came from reading, from folks here (one in particular), or from figuring it out on my own.

Sean

Thanks Sean. The Dremel sanding disks to shape, are these used alone on the mandrel or is there a backer for the sanding disc I should get.

On the Prop thread they say that the Berrilium dust is poisonous. What precautions do you use when power grinding etc?

That's why I leave the fancy prop work to the pro's. I've got one buddy here in town that has the equipment to do all of the work so I let him do my props. It can get expensive buying the tools to do it correctly.
Polishing is subjective. Why put a lot of hard work into making a boat look good then having an ugly prop on the back? Those that are seeking every 1/10 MPH out of their boats leave them rough but I like the polished look. Besides that, it's about the only thing I can get my kid to do on the boats!

My precautions include:
1) Dust mask
2) Shop vac turned on and positioned to capture the filings
3) I don't use a power tool. I stick to files and sandpaper so I'm not slinging dust as far. Since I only do props for myself, the extra time isn't an issue.

I don't "polish" my props. When I'm finished sharpening and balancing them I finish with some really fine K&S flexi-grit and consider them done.

I think it is great that you asked this...because it is something I asked too. I am not only new to "boating", but I am new to "dremeling" too

Yes, sanding disk is on a mandrel with no backer. Watch your dremel speed. Them props get hot...and it is much better to work slower, than to take too much off.

As to your dust question...well...I would not want to recommend what I do...so I will leave it at that.

As an added note, I use one of those magnetic balancers, and apply pressure so that the balancing shaft is on one magnet, and very close to the other one (so it rotates very freely). I think this is probably more sensitive than what is actually needed, but it is what I use (have you ever thought about the balance of the drive dog??...with the set screw on one side...).

Sean