Which is the best type of flex drive. I bough a boat with a square drive, which is really easy to disassemble and grease etc. The shaft has broken so I need a replacement. I am also building a new Gen4 insane hydro, and want to use the same system on it. Seems the square drive is not popular and not many parts available?
Which is best?

They are pretty popular on the west coast. It really all boils down to personal preference in most cases. I prefer collet/ welded cable myself.


www.modelgasboats.com

Sent from my iPhone using Tapatalk.

I also like the welded cable collet set up. Just rememeber to keep your collet tight! If it comes loose your cable and prop will end up at the bottom of your pond. You can also get shaft saver collars to prevent this. The argument by guys in our club is that square drives put prop thrust on the strut and not transfered to the crank of your engine.

Thanks guys, I did wonder about the force issue, do they believe that the thrust against the crank causes problems? Logic would say that the push on the crank may be enough to cause problems as the prop bounces around? I was also wondering what happens as the rubber mounts move, (if the flex is rigidly attached to the crank).

Hi Ian
I’ve only ever used the collet type with no problems.

The one thing I like about tapered collet type joints is that they give good concentricity provided that the parts are machined accurately and fit properly.

As to the force issue, I sometimes worry about what is the correct gap at the strut end between the strut support and the propeller thrust washer. I’ve got that gap now adjusted (reduced from originally 5mm) such that after a flat out blast the thrust washer shows just a smattering of wear. That suggests to me that the thrust is now shared between the prop end and the engine crank bearings.

Even if the gap was big enough to transfer all forward thrust to the crank bearings, I would not imagine it to be an issue as long as the solid drive end of the flexi shaft is supported in bearings that don’t have too much play from wear (and well lubricated).
Just on the issue of concentricity:

Consider a solid circular shaft mounted into a housing secured with a locking grub screw. The set up will likely be off centre leading to potential vibrations. I found that evident when using various lathe mandrels for turning wooden pens. The best mandrels for me are those secured between tapered concentric joints.

Cheers
Craig

Craig, thanks for the info. I had pretty much decided to use a collet, (not square drive). Except for the possibility of loosing a prop with a snapped shaft, everything else seems to make more sense with the setup you describe. No-one seems to have horror stories about destroying crank bearings, so I suspect that the crank is fine handling the thrust load, (cause must people don't seem to worry about sharing the load like you have).

Heard that flex cable shrinks in length under load.
If so, how much gap should I give (between strut
and thrust washer), per foot of cable?
I'm using a collet and welded cable for this my first build.
I'll try to attach a photo with arrow for you...
Right click on photo, then click open to see, if I did it right.
Thanks,
Gerry

.250 TH = 1/4 INCH


RICK R

I have been told by someone who is very well versed in going fast that a collet drive will actually slow your boat down by up to 4 MPH.

I'd like other peoples thoughts on this as well, cause I have wondered. From other discussions I've had, it appears that if the cable thrusts against the motor rather than the rear of the drive, the majority of thrust acts through the bend in the stuffing tube. I am guessing that is why Teflon stuffing tubes commonly fail when using a collet drive. Logic would say that the flex cable pushing like that will chew up more energy in friction. I can see why pushing on a washer on the end of the drive would be more efficient. Have other people found this to be true?