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

These motors, those "CNC Aluminum Outboards" I did build a spacer block to make them longer. Just required a longer than stock flex cable. Boat runs amazing but I break a flex cable after 2 or 3 runs and it's not the same one. They are counter rotating with the proper flex shaft for the reverse rotation. They break about 1/4" below the motor coupler. This time it really sucks because it broke and I lost the shaft assembly including the red anodized prop. Those props weren't cheap like $45 each and they are no longer made.

I am thinking maybe I am not leaving enough gap between the drive dog and lower unit? I have about 1.5mm gap. Think the flex cable is shrinking more than that under load and breaking the cable? And here I switch from the geared Aquacraft motors so I didn't have to deal with discontinued gears stripping out. Now I can't keep the flex shafts together and losing expensive props really sucks.

The usual cause of flex cable breakage near the coupler is insufficient cable support. There may be too much of a gap between the end of the stuffing tube and the coupler. By taking thrust at the motor you are putting the cable in compression which by itself is fine, but if there is an unsupported porting of the cable it can over-flex, stress and break. If you didn’t extend the stuffing tubes when you installed the spacers, that may be the problem.



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I did think about that as well. The stuffing tube is teflon and there is a about 12mm of exposed cable on the one that broke. In these pictures you can see the tubes and flex shafts. The one with the higher tube is the one that didn't break. The one where you see only the flex shaft is the one that broke, It broke about 1mm in the tube.

I will make new tubes and bring the tube to within one mm of the motor coupler. I have some brass and aluminum tubing and a bender, maybe I can make a brass or aluminum stuffing tube. Just have to get the bend just right. Might take a few tries. Yeah...I think you're right, to much space between the

Teflon tubing provides very little support. I’m not familiar with those lowers but to provide proper support you will need longer metal tubing surrounding the Teflon - if that is possible.



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The lower unit has 3 very short screws, then the lower splits in half exposing the groove for the shaft and stuffing tube. These are .130 flex cables. There won't be space for a metal tube over the teflon but I think I can make an aluminum stuffing tube and eliminate the teflon one. There is a lubrication port in them, that is the little green hose inside you see. You inject lube there and it forces it into the stuffing tube through a little hole in the stuffing tube. I think I can duplicate this with the aluminum tube.

But at the end of the day, I am confident the breaking is from the tube not getting close enough to the motor coupler. I have new flax shafts coming from OSE and they should be here Friday so I'll do the work and snap a few pics in the process so maybe it will be helpful for people in the future.

Another concern....

Thinking with a gap between the drive dog and lower unit when the motor is pushing the boat all the thrust is pushing on the flex shaft and thus the vertical mounted motor. I would guess that is adding to the breaking issue. Is there a way to ensure the thrust will be able to transfer from the drive dog to the lower unit? Is there a formula for cable shrinkage so we can minimize the gap? Maybe a formula taking into account the thickness of the flex shaft in relation to the length of the flax shaft?

How about in the gap a small ring of silicone tubing this way it can "squash" for the shrinkage but apply some of the thrust to the lower unit?

If you want to replace the Teflon with metal, use brass not aluminum. Most aluminum tubing is very soft and bends easily in use. Brass works great without Teflon.

Taking thrust at the lower unit is tough to do correctly, OBs which do this almost all use square-ended flex cable with no clamping collet. That lets the cable float and adjust itself for cable windup. There is no ?formula? for cable shrinkage, too many variables like cable construction, rpm, prop size, pounds of thrust, stuffing tube ID, etc. Using a short piece of silicon tubing will do nothing to help, it is too soft and will pop off. Best to take the thrust at the motor like 95% of all inboard models.

Yours is a great example of why the thrust location doesn?t matter much. The folks who say that taking thrust at the motor pushes the boat from that point and acts completely differently than taking thrust at the strut. With that kind of thinking, your OP boat which takes thrust vertically should have the transom leap out of the water when applying WOT. The fact that it does not clearly demonstrates the fallacy of that line of thinking.



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I agree brass is the material easier to work with. But I only have enough of the correct size tubing in aluminum so I'll try that first. Here is the lower unit split open. There is a very precise channel for the flex shaft and stuffing tube. There is no way the aluminum will alter it's angle in there so as long as I can keep it lubricated I think it will hold. I think I can bend the aluminum tube pretty easy using the cover plate side of the lower unit as a template.

In the picture of the full outboard you can see the spacer block and anti ventilation plate I made. I know in this picture it looks sloppy but I have some screws out so it's not buttoned up here but when it is fully assembled it is very tight and true. The spacer block was a MUST to make these F1 style motors work and only added 9.5mm to the flex shaft and stuffing tube. The anti ventilation plate really helped reduce the props cavitation while getting on plane. Night and day difference.

The idea of using metal tubing is to extend it closer to the coupler, giving support to the cable outside the housing. Simply lining the case with aluminum would be worse than using the Teflon since it will wear out very quickly.


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Yeah the metal tube won't work, at least not aluminum. No matter how hard I tried I could not get the bend without slightly flattening the the tube in the bend making it so the flex shaft can not fit much less spin freely. I will have to use the teflon. But I think I can put a short length of aluminum tube over the teflon tube between the exit of the mid section and the motor coupler. This should stabilize the flex shaft in that space just below the motor coupler.

Here is a dry fit of a new flex cable and teflon tube. You can see how tight it is and how tight the bend is. I may have to buy those scale Evinrude outboards with the gears. LOL

Ok so I tore down the motors and rebuilt them with all new bearings, new flex shafts and found a couple things that could have been causing the breaking. The brushless motor the way it mounts put a decent bend on the flex shaft between the motor coupler and where it enters the mid section. So I made some shims. I wanted to make it so I can lower the motor on the mount and have the flex shaft enter the motor coupler without even moving it. It is all tight now. I am going to test now.

If it gives me any crap I am starting over with two of these. https://www.offshoreelectrics.com/pr...prod=bel-99999

Broke a flex shaft in the first minute but upon disassembly I found this was my fault. I did not solder the motor end of the flex shaft very good so it unwound inside the coupler. I have extra flex shafts so I am going to repair the one that failed and tear down the other one just to be sure I didn't make the same mistake on that side.

If there is a problem on tomorrow mornings test I will be on OSE getting those new outboards. $300 + new brushless motors. Wife will be irritated but oh well. She spends money worse than me. LOL

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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I believe the solder job I did on the end with the flat spot was not good. I have redone it with new flex cables. I will admit it has been many years since I worked with flex cables so I may be messing it up. Especially with these small cables. I am used to the 1/4" shafts which are much easier to work with. This is what I am hoping will be good. This is much more stout than the one that just broke.

Ok then I think I got it dialed in....for now at least. I got it together last night, ran it for a good 20 mins this morning and it worked great. I redid the broken flex shaft and I pulled the other one out and beefed up the soldering on the motor end just to be sure.

Hauls ass and everything stayed cool. Water comes out in a nice stream on the back of the motors though those aluminum tubes which I did like that to make flushing with fresh water easy. The boat picks up water at high speed, water goes through the ESC's first then that water goes through the motors and out those little tubes.

Anyway, I'm going to check tightness on all the motor coupler and drive dog set screws and run it several times today and if the hold up I can hold off on $300+ for a new pair of motors..for now. :cool2:

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.

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Originally Posted by Fluid

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.

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

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.

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.

Some nice pics of the boat anyway...

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?

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....

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

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!

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.

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.