Flex Drive Minimum bend radius question

[BESM]

Newbie to FE here.
I am designing a flex drive system for my 1/12 scale Bluebird. I plan to use a OSE 0.130 cable assembly with a 3/16" prop shaft (ose-cable-130L).

I would like to know the minimum acceptable bend radius of the stuffing tube. I do not intend to use teflon lining, just going with grease and a 3/16 OD K&S stuffing tube.

Followup questions:-
Using a Speedmaster 21 strut (ros-spdss-002), the catalog page says use 1/4" stuffing tube. I am not clear how this is all supposed to look since I have seen no engineering section drawings.
For example does the 1/4" tube go through the strut fully and then how does it mechanically connect to the 3/16 stuffing tube. Presumably a piece of 7/32 tube would do the job.
Is the stuffing tube normally held in place by soldering or Loctite or just pushed into place. In the latter case I wonder about unnecessary water intrusion into the drive system.

What is/are the reason(s) for a flat bottomed strut versus a round one?

Thanks

Bruce M

[tbonemcniel]

How much power are you putting through this boat? .130" cable may not be enough.

There are different thoughts on the bend radius, some like to let the stuffing tube come down below the boat to the correct height, and "get the bend over with" with a fairly sharp bend, and have the stuffing tube running straight back along the length of the boat. Personally, I do not like this. I make my bends as gradual as possible, starting as soon as they exit the bottom of the hull. I heat the tube up with a propane torch until it's cherry red and then air cool in the areas I'll be bending. This makes it much more malleable. Once I complete the bend, I heat it up again until it's cherry red, then dunk it in water. This makes it much less malleable.

So the strut has a hair over 1/4" hole in it, allowing it to accept 1/4" OD stuffing tube. It won't be loose in the strut, but it's not tight enough to be a press fit, but a slip fit. So that the stuffing tube is able to slide in and out, but not wiggle. If it were me, and I was using a stuffing tube less than 1/4", the ID of the 1/4" tube is 7/32", so I would buy a piece of 7/32", whose ID is 3/16", and telescope the three together. The three can be locked together by using Loctite 640, which is made for cylindrical fit mating, and is particularly effective on active metals, like brass. You could also solder them together.

As far as securing the stuffing tube through the hull, I use epoxy over and around the stuffing tube, with a build up inside the boat. You can also lay a piece of fiberglass or carbon fiber cloth over the stuffing tube inside the hull where it comes through.

A flat bottomed strut will provide more transom lift at the rear of the boat, which is often (but not always) helpful in hydroplanes. A round bottom strut will provide less transom lift at the rear of the boat. I would start with the flat bottom as being a scale boat, you're likely to have more weight on the rear than a sport hydro (wings etc) that the extra lift from a flat bottom strut can help with. If you find later that you have too much lift, it's easy to swap it out to a round bottom strut, since your 1/4" piece of stuffing tube slides in and out of the strut itself.

[BESM]

Hello tbonemcniel

Thank you very much for your detailed reply. You have answered my concerns nicely and provided excellent guidance. I have been advised to go to a 0.150 flex shaft by Offshore so obviously I will do this.
I may have other questions down the line.
Best regards
Bruce