Hub balance issue - best to correct?

[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!

[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 !!!)

[mrwzrd59]

Same situation here in southern MI. The boat stuff just dosent fly out the door like the four wheeled junk. They have a tupperware tub full of "their boat stuff" and they treat it like a grab-bag and have bargin prices. I picked up four props for 5 bucks each ranging from 2 grim 42 x 55's and several stainless proboat OE props. One of the Grim's nearly hit the circular resepticle but I hung in there until it hit maybe 90% balance. We (son-in-law and myself) have three boats and are really trying to record our results with GPS, radar, battery/esc heat and rpm data logging...We dont race, just way curious minds (engineer/geeks)!
We are hitting the channel behind their house (now that the weeds are down) this afternoon with plans of shooting some piers with the GoPro. Should be a hoot!

[longballlumber]

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!

I have an address you can send the your bad Grim props too... I will pay shipping.

Later,
Mike