Hub balance issue - best to correct?

[bob_t]

I don't have a lot of experience (yet) at balancing rc boat props and trying to balance a Grim 42 X 55 on a magnetic balancer. The blades are well balanced, as they sit perfectly level in the balancer, but when you spin the prop, it always stops with the same side of the hub "down", again with the blades being perfectly level. From what I understand, the hub is the issue. I don't think I can remove enough metal from the heavy side of the hub, just fooling with the hub surface. When I add temporary weight to the light side of the hub, the prop will stop randomly like it should, but I don't think I can remove enough weight on the heavy side of the hub to correct it.

Is there a way to "add" weight to the light side of the hub on the copper/beryllium prop? I could fool around with the blades, removing some on the front of one blade near the root of the blade, close to the hub above axial centerline on the heavy side, and take some off the front side towards the trailing edge of the other blade, again, above axial centerline on the heavy side, but that just doesn't seem like the right way to do it, and just not a lot of metal there to remove to make a lot of impact. Any thoughts/suggestions?

Thanks,
Bob

[HydroMike]

When its done right the blade will stop on anyone of those blades, if it does not, then anything under ur hub is heavy just take a little at a time and keep checking. I'm not an expert but getting better. Good luck

[Raydee]

One of the prop experts I have asked this question to in the past said that he doesn't try to get the prop to balance snading on the hub since the hub is in the center and would need to really be worked to be able to get the prop to balance. Instead he removes material from the tip of the blade that is pointing down and the tongue on the other blade.

[bob_t]

Thanks! I appreciate the responses. The blades are balanced, but one side of the hub is heavy and its tough to do much metal removal from the hub, from the way it looks. So, my thoughts were on the right track - take some of the tip of the one blade and some off the tongue near the hub on the other. Based on how much "temporary" weight I added .... I hope there is enough metal to remove on the tip and tongue to get it to balance. From what I have seen so far, if a prop is out of balance it can be tough on the strut bushings, especially the one near to the prop. PB says you don't have to balance their stainless steel 1.6" X 2.5" prop unless you want better performance and higher top speed. I'll have one of those props over the weekend and will see just how close to being balanced it is, right out of the box ... I have my doubts, after seeing how worn a strut bushing was after only ~ 6 runs with one of those props, right out of the box, on another person's mystic.

[Mike Caruso]

Thanks! I appreciate the responses. The blades are balanced, but one side of the hub is heavy and its tough to do much metal removal from the hub, from the way it looks. So, my thoughts were on the right track - take some of the tip of the one blade and some off the tongue near the hub on the other. Based on how much "temporary" weight I added .... I hope there is enough metal to remove on the tip and tongue to get it to balance. From what I have seen so far, if a prop is out of balance it can be tough on the strut bushings, especially the one near to the prop. PB says you don't have to balance their stainless steel 1.6" X 2.5" prop unless you want better performance and higher top speed. I'll have one of those props over the weekend and will see just how close to being balanced it is, right out of the box ... I have my doubts, after seeing how worn a strut bushing was after only ~ 6 runs with one of those props, right out of the box, on another person's mystic.

Bob,
Look at my Gallery pictures for my Revolt you can see how I had to remove weight from the hub of one of my props. I use this old style balancer in the picture to get the prop close to in balance. Then I switch to the Magnetic type and if you're having trouble try using a little modeling clay on the light area. When it spins good remove weight 180 degree's from the clay. It takes time but well worth it. The propeller called the 'wheel' is just like a cars wheel and tire assembly, it can have both Static and Dynamic unbalance. Most of us can only Static balance but as they say there is more to the story.

BTW 45X68 on my boat one blade is .010" thin we'll see how long it last's.

MC

[egneg]

I would work the hub first and then move out if needed - some props are so bad that they aren't worth the effort. I would contact whoever you got it from to see if it can be exchanged for one that is hopefully better.

[icelert]

You could add weight to the light side of the hub using soft solder. Try adding a blob on the hub without using too much heat which would cause it to flatten out and spread around to the heavy side. Then whittle the solder down gradually until the prop comes back to a balance condition. Just remember that you can never balance anything perfectly, only to a tolerance that your driveline can work efficiently.

[Fluid]

Adding solder to a hub is not a good idea, you can easily change the temper of the prop with the potential to throw a blade if you get the metal too hot. Realize that a "perfect" balance is not required with surface drive props - the huge vibrations caused by the blades hitting the water many times a second make a little bit of blade imbalance insignificant. BTW I have checked the balance of props run on 100+ mph FE SAW boats, and none of them were in "perfect" balance; each one had a heavy blade. These heavy blades didn't drop like a rock on my magnetic boat prop balancer, but they did drop slowly every time. Bottom line - close is good enough. Spend that anal retentive time on shortening power wires or improving your soldering technique, it will gain you a lot more.




.

[icelert]

I respectfully disagree that adding a mound of soft solder to the hub will anneal cast BeCu or NiBrAl props. The heat required to attach the mound of solder is minimal if done with the proper wattage iron. I do agree that we tend try balancing at times to tolerances much higher than needed.

This advice is offered in case of a prop that has been bored off center with the prop casting and presents a condition like what the OP is describing.

[Doug Smock]

The blades are well balanced, as they sit perfectly level in the balancer, but when you spin the prop, it always stops with the same side of the hub "down", again with the blades being perfectly level.

Thanks,
Bob

So you have a leading and a trailing edge hanging below the axis of your prop balancer.
If you remove material from those two places my guess is that you'll find a happy place.
Do make sure that you have removed any flashing on the hub before you get started.

Good luck,
Doug

[T.C.]

I think Dougs on the rite track .
I also see nothing wrong with using a small carbide cutter in a Dremel to cut groves in the hub at the same angle as the blade.
You can remove a lot of material with out hurting anything. I read somewhere this was once used as a speed trick, only the whole hub was slotted.
Or as Icelert suggested add solder, but add it using an iron not a blow torch .
T.C.

[Raydee]

So you have a leading and a trailing edge hanging below the axis of your prop balancer.
If you remove material from those two places my guess is that you'll find a happy place.
Do make sure that you have removed any flashing on the hub before you get started.

Good luck,
Doug

I think this is what I said BUT a little more Pro sounding

[T.C.]

After all the options have been exhausted and drastic action needs to be taken.
Thats when I get the Dremel out.
T.C.

[bob_t]

I appreciate all of the input. The prop is a brand new Grim 42 X 55, right out of the box. It was pretty clean, no flashing ... just way out of balance on the hub. Not brave enough (yet) to try to detongue this prop and reshape the blade, still "getting my feet wet". Most of my experience is with real offshore performance boats/power packages, but the basics are still the same.
Thanks!

[Doug Smock]

Just remove material from the back of the blade. The blades need to be thinned a bit anyway. I use a 1" belt sander and a assortment of files. Wear a suitable mask.

Good luck,
Doug