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

[Raydee]

Since we are on the prop subject has anyone thinned down a prop too much and had a blade fold over on them? I am looking at some of the Andy Brown props I have and man are they thin. I was messing around thinning some of the props that I modded myself and on my X452 I didt realize how thin the prop was until I was just about done. I am worried that I thinned it to much now and it will either throw a blade or fold over on me....it may also be the best prop I have ever ran??

[Fluid]

Fuel prop guys don't understand the forces that an FE motor can put on a prop. I have received a number of props from top guys which are too thin....some have changed pitch and cup just from normal running. You can try hardening the prop to regain some strength, but that may make the blades too brittle. THe best solution is to run blades a bit thicker than the fuelers do. A thrown blade is potentially dangerous...


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[Raydee]

Yeah I had a prop guy do a prop for me for my Gas rigger and when I got the prop it looks awfully thin. I ended up throwing a blade on that boat that bent my strut and took out the rudder.

[Fluid]

I respectfully disagree that adding a mound of soft solder to the hub will anneal cast BeCu or NiBrAl props...

You can disagree all you want, but the fact remains that the average FE boater has enough problem soldering clean copper wire to a gold-plated connector. I can guarantee that many/most will overheat the blades with a torch and ruin the prop. Just because you (an expert I assume) can do it does not mean that most readers can too. Several well-known prop modders I know personally (old timers) didn't like adding solder since it can be thrown off while running even if it is added safely.

I try to refrain from recommending techniques that the average guy can't do. That or at least outline the risks so the readers can judge for themselves if they want to undertake it. The right solution is to return the prop for a better one, anyone can do that. I know you were trying to be helpful, but remember that not everyone here has your skillset.



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[bob_t]

Regarding thinned props ... On full size performance boats there were two common types of "labbing" a prop ... a "recreational labbing" and "full labbing" a prop. Recreational labbing matched up (or "blueprinted") the blades, sharpened them, balanced, and thinned them a little, but left enough material so that they were durable enough to not crack just due to small nicks (from debri in the water) or fail just from fatigue from normal use. Most, but not all folks, understood that fully labbed props had a finite life, and were done for specific purposes (racing, poker runs, etc), not just everyday recreational use, unless you had unlimited $$$$$. We often called those props "one hit wonders" because if they hit anything, just once, you either cracked a blade, or threw it. I've seen many cracked props and props with 1/2 a blade gone over the years of fooling with those boats. It was especially bad with props that were designed to be submersed, but jacked up high enough on the transom (high "X" dimension) that they started to surface. Those almost always had a very short life before they would throw blades. With thinned/sharpened props it is always a tradeoff of performance versus durability ... you rarely got both!

From my experience so far with FE rc boats, most of this still applies ....

[Doug Smock]

With thinned/sharpened props it is always a tradeoff of performance versus durability ... you rarely got both!

I agree.
Ray I have never thrown a blade but I have folded the tips back on SAW props because I made them a bit too thin. But hey, you don't know what too thin is till you go there right?!

Doug

[Raydee]

I agree.
Ray I have never thrown a blade but I have folded the tips back on SAW props because I made them a bit too thin. But hey, you don't know what too thin is till you go there right?!

Doug

Well my PT45 is only turning 27,000 or so rpm so I guess I will try it and see

[bob_t]

I got my new stainless steel PRB4019 prop and put it on the balancer .... way off balance! (even though their literature says there is no need to balance it unless you want optimum performance and top speed). I think this prop would destroy the driveline, especially the strut bushings, as is. Took a closer look at it (checked runout of blades and hub) and the hub is drilled/bored .015" off center, and on a slight angle, from front to rear, relative to the blades. Obviously, the "thick" side of the hub/prop always drops, in the balancer, and I'm not sure what to do next ... don't think there is enough metal to remove to get it static balanced. Based on the runout, I don't think it would ever dynamically balance, since the neutral axis doesn't align with the center of the bore.

Throw it in the trash?

[bob_t]

Well, after a lot of work, I got it static balanced ... you just touch it on the magnetic balancer and it spins forever, and stops randomly oriented A HUGE improvement from right out of the package - in the magnetic balancer, it would shake the balancer, it was that bad. I'm still a little concerned about what it is going to do at 26,000 rpm, though, since the blades are a little tilted (ie, not quite perpendicular) with the bore ... any thoughts/suggestions? Thanks!

[Mike Caruso]

I got my new stainless steel PRB4019 prop and put it on the balancer .... way off balance! (even though their literature says there is no need to balance it unless you want optimum performance and top speed). I think this prop would destroy the driveline, especially the strut bushings, as is. Took a closer look at it (checked runout of blades and hub) and the hub is drilled/bored .015" off center, and on a slight angle, from front to rear, relative to the blades. Obviously, the "thick" side of the hub/prop always drops, in the balancer, and I'm not sure what to do next ... don't think there is enough metal to remove to get it static balanced. Based on the runout, I don't think it would ever dynamically balance, since the neutral axis doesn't align with the center of the bore.

Throw it in the trash?

Next time take a pass if the shaft hole is tilted from 90 degrees to the blades. Mine where all 90 degrees to the blades but shaft hole size +.003". If they could control hub shaft hole size and put it in the center I would buy more of them. Tumbled to polish not so bad as there is plenty of blade to work with and shapes are nice. But for now back the the first props Ocutra. Tom has allowed the sport to grow in the 60's, 70's and 80's by supplying consistently good props as he does today and he listened to the racers just like his Daughter Trudy does today. Don't kid yourself into thinking what can a girl know, she grew up running a lathe and making parts ha. Dad still is drawing props so if there is a need they make it. Tom is a pistol and no age listed LOL.
Great people.
Mike

[cugino]

I got my new stainless steel PRB4019 prop and put it on the balancer .... way off balance! (even though their literature says there is no need to balance it unless you want optimum performance and top speed). I think this prop would destroy the driveline, especially the strut bushings, as is. Took a closer look at it (checked runout of blades and hub) and the hub is drilled/bored .015" off center, and on a slight angle, from front to rear, relative to the blades. Obviously, the "thick" side of the hub/prop always drops, in the balancer, and I'm not sure what to do next ... don't think there is enough metal to remove to get it static balanced. Based on the runout, I don't think it would ever dynamically balance, since the neutral axis doesn't align with the center of the bore.

Throw it in the trash?

Yes. Or i usualy put them on the shelf and used to show my friend how not to make props. Centered drilles hole si key parameter. If it this is wrong, there is nothing to be saved. Wasted money. All other defect you can fix, but not this. It is not good in high reving operation.

[mrwzrd59]

I use a dremel with a 1/8" ball carbide burr and just plunge a sphere in the heavy side of the hub...Sometimes the sphere gets pretty deep. Its a cinch that it is waay worse to be out of balance vs a hole in the hub. Grim Racers seem to suck the most with initial balance all over the map. I buy several and just try and work the best one...The others get gifted or tossed. Yea, they can be bad enough to toss! My other observations are that the hole size is a big problem if its not a decent slip fit on the balance shaft. I wrap the shaft with onion skin .0005" shim paper to get it closer to mean. The hubs centering and hole size are always the place to start if you're going to balance them correctly. The blades barely require anything if the hub is good. Back to back speed tests with an unbalanced prop vs one of my good ones (I figure I'm hitting 95% balance) is 2 to 2.5 MPH!!!! I did it three times to verify the result! So, balance those props guys OR buy better ones in the first place.

[bob_t]

Great idea about the dremel and spherical divot at the hub .... stresses are low at the hub, as long as it doesn't get into the blades. I got another PRB 4019 prop to try.
This one was only a little off balance in the hub, and the bore and the hub were rather concentric (ie, very little run out). The blades were really good too, and didn't take much work. Only took about 1 hr of fooling with it to get it balanced really well. Amazing how long they spin with almost no effort, when they are balanced well versus "just out of the package". I understand what you mean about the Grim Racer prop (based on my sample size of one). I'm not sure I can get that one to balance because the hub is so far out of balance - may need to try your dremel ball burr idea on that one. On my balancer, I have two conical hubs that "pinch" and center the bore/hub of the prop and they run very true to the theaded pin between the magnets. The pin and cones are within .002" of running true, so I know I am balancing square to the bore (and unfortunately, not square to the prop if the bore is not axially straight to the plane of the blades).

I'm not really thinning my blades that much, as I am more interested in balancing so I don't destroy the driveline (or end up with spider cracks in the transom at the strut mounting screws). I am not trying for top speed (at least not yet ), and will take the durability over the last .1 mph (for now, anyways). I think I actually have as much fun fooling with this stuff, to see just how good I can get it, as I do running the boats!

Thanks for the tips! I appreciate it. Sorry for the long-winded post, too.

[egneg]

This is where a good LHS can be a good thing. You can inspect the props and get the best one or not depending on how they look. I save receipts and packaging until I check initial balance and run-out. I expect more from a prop that costs more than others.

[bob_t]

You may find it hard to believe (I did), but none of the hobby shops in Cincinnati area stock any boat parts or accessories, literally, and I have been in most of the shops. One shop will order whatever I need from Pro Boat and they have it in a couple of days, but, it is all special order. They now know who I am, so they don't make me pay in advance, but it is still all special order. That particilar shop, at most, will have 2 or 3 boats in the entire store, literally. The local shops have tons of truck, buggy and truggy parts, and some plane parts, but never any boat parts. So, everything I buy is either special order from that one shop (they don't charge me shipping) or it's off the internet. To send something back, bought on internet, I would have to pay return shipping, which would be at least half the cost of most of the props. I envy those who have a good local shop ... it would be nice to be able to sort through the props to find one that looks good, or take it back if it isn't ... everything I get is pretty much a suprise when I open the package (have even got some things that were not the item ordered, and those were really big suprises when I opened the box/envelope !!!)