Flex Wind-Up

[FloatDaBoat]

I’ve read several posts about how much a flex cable shortens under load, but the specific cable length was never addressed in any of them.
So, here’s my question: Utilizing .130” flex, is 3/32” (2.38 mm) enough clearance between the drive dog & strut, to allow for the torsional compression in an eight-and-a-half inch length of cable (measured from the motor coupler to the front of the prop shaft)? I’ve provided a photo for visual reference. The prop is an Octura X440. Trying to avoid a Tug-O-War situation here. My wife’s tired of hearing that I broke it & need more funds to fix it again.

[domwilson]

Looks about right to me. Maybe a little more like 1/8" or 5/32". But try it out and see.

[wolf IV]

i’ve read several posts about how much a flex cable shortens under load, but the specific cable length was never addressed in any of them.
So, here’s my question: Utilizing .130” flex, is 3/32” (2.38 mm) enough clearance between the drive dog & strut, to allow for the torsional compression in an eight-and-a-half inch length of cable (measured from the motor coupler to the front of the prop shaft)? I’ve provided a photo for visual reference. The prop is an octura x440. Trying to avoid a tug-o-war situation here. My wife’s tired of hearing that i broke it & need more funds to fix it again.

I RUN ABOUT 3/16'' ON MINE .. BUT THEIR .187'' DRIVES...NOT SURE THE DIF. IN SHRINKAGE? great job on the hardware!!

[Gerwin Brommer]

I also think 3/16th is far better..............

[paulwilliams]

Without consulting the manufacturers' documentation, I would *assume* that there is a directly proportional relationship between flex length at rest, and length under a given load.

In plain English, a long cable will shorten more under load than a short cable.

I have no doubt that the operating conditions we subject our flexi shafts to exceed by some margin the limits of maximum rpm and torque as defined by the cable manufacture(s). If you're regularly breaking flexis, then you either need a bigger flexi or a wiredrive.

Paul

[bustitup]

I’ve read several posts about how much a flex cable shortens under load, but the specific cable length was never addressed in any of them.
So, here’s my question: Utilizing .130” flex, is 3/32” (2.38 mm) enough clearance between the drive dog & strut, to allow for the torsional compression in an eight-and-a-half inch length of cable (measured from the motor coupler to the front of the prop shaft)? I’ve provided a photo for visual reference. The prop is an Octura X440. Trying to avoid a Tug-O-War situation here. My wife’s tired of hearing that I broke it & need more funds to fix it again.

In my opinion go about 1/16" more than the pic you posted and take a piece of fuel line about 1/8" long and feed it onto the flex shaft before you feed it into the strut so that it ends up in between the drive dog and the strut....works as a seal and cushion

jmho......

[Grimracer]

rule of thumb

Distance back = one cable dia.

Grim

[FloatDaBoat]

Thank you all. Your answers are very much appreciated!!!

Guess I’ll go with the median suggestion - - 5/32”.
If someone had sufficient data on all the variables involved & adequate testing facilities, I’m sure some sort of answer could be arrived at. Even then, static test results often differ from actual dynamic realities.
I like the idea of having a sealed strut to keep water out. I don’t think a piece of silicone tubing is the answer, though. Something along the lines of an O-ring seal might be better; O-rings probably wouldn’t be able to withstand the high shaft RPM’s. Oh. Well . . . .

[Fluid]

The clearance necessary depends. Depends on the length and diameter of the cable, the prop diameter and power level, the inside diameter of the stuffing tube, the weight of the boat and how much cable is exposed between the stuffing tube and the coupler. Two separate forces are operating together to shorten the cable: torque and compression. The twisting action coils up the cable - then releases it twice per revolution. The push from the prop is transfering all the thrust to the motor (and then through the motor mount to the hull) which shortens the cable proportionate to the acceleration/speed of the model. There is no perfect answer, except that using Grim's suggestion above has worked well for generations of R/C boaters.


/

[TRUCKPULL]

I was told by Ed. Hughey at the 05 Nat's in MI.

Distance back = one cable dia. for each 10" of length.

Larry

[Drag Boat Bob]

The clearance necessary depends. Depends on the length and diameter of the cable, the prop diameter and power level, the inside diameter of the stuffing tube, the weight of the boat and how much cable is exposed between the stuffing tube and the coupler. Two separate forces are operating together to shorten the cable: torque and compression. The twisting action coils up the cable - then releases it twice per revolution. The push from the prop is transfering all the thrust to the motor (and then through the motor mount to the hull) which shortens the cable proportionate to the acceleration/speed of the model. There is no perfect answer, except that using Grim's suggestion above has worked well for generations of R/C boaters.


/

Twice per revolution?...

Wouldn't that depend on the number of poles a motor has?

Of course I am new to this sport and have little experience, but it would seem logical given the how electrical motors work.

I am not disagreeing with your analogy, just trying to understand the physics involved.

[TRUCKPULL]

Twice per revolution?...

I beleive he is refering to a 2 blade prop NOT a 2 pole motor.

Larry

[Drag Boat Bob]

Twice per revolution?...

I beleive he is refering to a 2 blade prop NOT a 2 pole motor.

Larry

Now why would a propeller cause any change in twisting compression unless it was in and out of the water? Even then, why twice per revolution?

[TRUCKPULL]

Two blades X(Times) one revolution = twice
2X1 =2

We do run surface props so it is only one blade in the water at a time.

Larry