Flex shafts breaking. I must be doing something wrong. Help.

[ScarabChris]

Yes! I think the motors are dialed in. I guess I was just sloppy with making the flex shafts the first few times. I ran the hell out of the boat yesterday. I took it out on the real boat and ran it a local sand bar hang out, salt water and a little choppy water racing my sons little UDI Arrow. Now I can continue the cosmetic part of the boat.

I will get some running shots next time I have it out.

And on a side note if anyone is looking for a good entry level boat for their kids this is a really nice little boat. RTR out of the box and right around $200. It hits close to 30 MPH and has a self righting feature. The motor is a brushless out runner that is weighted so when the boat capsizes you just stab the throttle a few times in quick succession and the torque from the weighted motor will flip it over. This was a must for my 11 year old. LOL.

[ScarabChris]

I’m a bit confused, a new cable should not unwind inside the coupler unless you used reverse. The torque will tighten the cable, not unwind it. I haven’t soldered a cable in over a decade, but sometimes have used green LockTite. Haven’t had a cable unwind in years, even on 100 mph boats. Do you have the correct cable on the correct motor?




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Just to be clear on what I meant by "unwind". I am using the correct shafts for the rotation. What happened on that one that broke in quick was I did not do a good job on the soldering on the motor coupler end so the torque allowed the set screw in the motor coupler to shred the tip of the cable.

[JestDanny]

Are those the little mercury outboards from H.K. , if so no one has had very much luck with them to my knowledge, same old problem. I solved it by simply using the electric motor in another project and then tossed the rest in recycle bin.

Good luck Danny

[ScarabChris]

These little motors are pretty good. The only weak part is that flex shaft. You have to be super accurate on the length and angle of the motor mount. The only thing I hate is how short they are but then again they are made for F1 style boats. I made a 10'' spacer block and anti ventilation plate and once I got the flex shafts proper they work well. The anti ventilation plates were a must because the props would suck air in al most speeds making it over rev. The plates do the job just as they do on a real outboard.

[ScarabChris]

Welp...I had a weekend of good runs. I ran it 5-6 times in the canal behind my house. Ran excellent. But I broke a flex shaft on the last run of the day. Same motor and I think I know what is happening. When I run the canal I run the course so a large oval making right turns only. The shaft that breaks is on the outside of the turn.

Today was pretty choppy, I live on a canal that is open to the ocean with real boats coming and going so it gets choppy (for little RC boats) and today the boat was doing what we call in real boating "barking the props" where the boat gets a little air, motors rev high then upon re-entry the props hook up pretty hard. This is what happened. I remember being excited about barking the props then I took the sharp right turn which puts the port prop out of the water a bit, when the boat straightened out the prop hooked up and the shaft broke. So the initial damage I think was when the prop hooked up then the turn finally broke it. I will have to take the STBD shaft out and look at it.

Either way...this will continue to be an issue. That sucks because this boat runs awesome! But I think a geared lower unit might hold up better. At least this time I didn't lose the prop and drive dog. I have extra flex shafts so the fix is easy.

[ScarabChris]

Some nice pics of the boat anyway...

[ScarabChris]

Dammit...I think I know why this is happening. I have several right and left rotation flex shafts. These little lower units have a pretty tight bend. The cables made to rotate right (prop turning CW) can be bent much easier. The left rotation cables (prop turning CCW) do not want to bend. This is why the left rotation are breaking.

Seriously I have like 12 cables and every right (prop turning CW) rotation cable is super easy to make a sharp bend. The left rotation (prop turning CCW) do not bend so easy. Dammit! Where do I get high quality flex shafts? I always thought the counter rotating one was the weak one. But in RC boats the standard rotation is left (CCW) which are the ones breaking....am I right?

[ScarabChris]

Pics explaining the above post....The unbroken one is the right (CW) shaft. Every time I broke a shaft that was not due to my poor soldering they broke right at the middle of the bend in the lower unit. The right (CCW) shaft is the one that breaks. They do not bend easily!.

I need some good super flexible CCW .130 flex cables.....or just run both CW. Ugh....

[ScarabChris]

And just to be clear, there is a teflon tube in there and I keep them greased.

[Norwest]

I would say the cable is too thick and can't make the bend. Posted a pic explains breakage .I'm dumb but pic answers ? My answer would be to use smaller flex with liner and smaller high pitched props. I'm not trying to be a s/alec about this. Liner, just decease diameter by inserting smaller size till all is correct. No need to reply!

[ScarabChris]

Both flex shafts are sold as the same size, one is right and one is left. I would agree the left (CCW) does look and feel slightly slightly thicker and it is but not much. The easily bendable cable that is right (CW) measures at 3.18mm and the stiffer left (CCW) measures at 3.22mm. The only part that has me scratching my head is the left (CCW) is supposed to be the standard. The right is the "counter" rotation.

But I found an option. As mentioned in my above post my sons boat, the little white Arrow...uses the same shafts as my outboards, just slightly longer. But the solid prop shaft is the same and the flex is the same. But it very easily bends and it's a left rotation. So I took the prop shaft out of my sons boat and I made him a new one using the thicker one that doesn't bend. His boat is back together and works perfect. Now I will size the flex shaft that came out of that little Arrow boat so it works in my outboard. I will run it hard and see.... If it holds up hell I'll just order that shaft from the UDI R/C spare parts list.

[plinse]

Here we have some shops selling flex shafts as "hardened" telling the customer "hardened is cool, you can put more power on them".

In most boats you do a 10-20 degree bend on a much longer flex shaft. With your extreme bend the soft china-cables are propably the best for you .
When I saw the pictures of your opened outboard motor it actually caused a pain in my eyes as an engineer.
Something like that may be on the edge of feasibility but it will not be a high performance solution.

On top of the thread you also wrote that you have 1,5mm gap at the thrust bearing. I would expect this to be much too much.
Too little gap is bad for the known reasons, you get very high load on the thrust bearing causing drag and heat, in the end something fails.
On the other hand I also had boats filled with smoke coming from the flex shafts because the gap was too big.
As long as there is a gap at the thrust bearings the flexible shaft is pushed into the teflon which causes friction in the teflon tube. You can make the teflon melt this way ;-)
You can really produce a lot of heat this way and on my flex shafts of 18cm length 0,5mm work well and 1mm already made the teflon tube melt.

Of course this is a matter of how much load and speed you have on the shaft but when you hold the coupling to the motor and twist the propeller so the flex shaft compresses you can easily find out how much gap you will need. A little axial play in the motor shaft also helps to make the whole system less sensitive to the adjustment.