ianw:

I think your explanation of where energy gets wasted using round
collets and shaft tube liners is correct.

"Floating square drive" actually refers to two types of floating.
The flex shaft is floating within the liner (or the unlined shaft
tube) and the square drive tip is floating within the square drive
collet. All forward thrust is applied at the drive dog bushing
(washer).

JRCBD has more info on this subject.

Interesting Steve! Wonder where that up to 4mph gets scrubbed? Couldn't all come from driving thrust through the BEND in the stuffing tube, rather than keeping it
back at the STRUT, could it? I just don't know...
Welcome any thoughts Steve, as I'm a novice on my first build.
Thanks,
Gerry

I think that Steve should give some reasoning behind the suggested 4mph gain on speed.

For example playing devil’s advocate, even if true, if the boats are doing 20mph that is a significant advantage. If the boats are doing 40mph+ it is less of an advantage.

If as I suggested earlier you use a straight though collet connected drive (mine with no liner and well lubricated flexi) and take some time to get the gap end correct to suit the power plant, that system will deliver in terms of speed and good reliability with minimal wear on the flexi and the back end tube and thrust bearings etc.

It was mentioned about the importance of keeping the collet tight to avoid losing the shaft and prop should it come loose. Good point made. Personally with my collet even before its first use I slightly widened out the slots to be certain that when clamped up the inner section it did indeed clamp on the flexi shaft as opposed to seeming tight but risking those 4 horizontal slots coming together and not giving sufficient grip on the cable. And in that situation further tightening of the coupling would not be adequate to keep the cable tightly gripped.

Food for thought I hope.

Cheers
Craig

Just for further food for thought on two points with the straight through collet connection:

Consider the “shaft saver” clamp on device:

It should indeed prevent a lost flexi shaft and prop should the collet fail to grip the flexi. However that component will not stop the loss of the shaft and prop should the flexi break or snap within its housing tube. I would suggest that with careful application of the collet, there is no need for a “shaft saver”.

Consider the common question, “What gap should be left at the prop end to allow for flexi wind up etc? 6mm for most mono hulls is often quoted.

If a 6mm gap prevents the prop thrust washer from binding onto the strut end during wind up, then arguably one could have that gap set to say 10mm or even 15mm with no issues assuming decent support bearings at the strut end.

Logic suggests to me that if you start with a gap of say 6mm+ and reduce it via experimentation, you will end up with a flexible drive system that does share the forward thrust to both strut end washers and the engine bearings while at the same time minimising the negative effect of a flexi cable snaking within its housing (if too long) or contrary getting over stretched (if too short).

I will be interested to have your thoughts on this.

Cheers
Craig

It's a nice 'theory' on shsft saver ....... however I have broke a few at collet and spun afew out of collet and the saver kept my shaft in boat.

I will not run that type (Cable/shaft as 1) without a saver.

I run prop shafts on some too but for specific reasons - as it makes it a PITA to pull cable with shaft and I like to look at my cables.

I am building one now with propshafts but only because I have a gearbox with 2 out (L&R) and I needed square ended cables on gearbox so I went square on both ends of custom cables to prop shafts with square input.


Normally I would run welded cable/propshaft. Although I do see need on higher horsepower engines that seem to eat cable like some of twins ...... so what ever works for the application at hand I guess

Cheers

Just my 2 cents

Randy

I'm going to try a round collet on the new G4 Hydro that I am building. I'm keen to be able to remove the shaft easily, so will see how it goes. I will still feel fast to me.

If you use the round collet, the flex shaft is pushing on the engine. By holding the shaft rigid at the engine, you allow it to flex inside the stuffing tube. If it flexes inside the stuffing tube, it will rub on the sides of the tube in one or more places. If it rubs on the tube in one or more places, it will cause friction. Friction causes heat and loss of horsepower. The harder you push, the more friction you create. The more friction you create, the more horsepower you lose. Loss of horsepower means you can't turn that paddle wheel behind the strut as fast. What happens when you can't turn your prop as fast? Right "YOU SLOW DOWN"

If you use a "SQUARE DRIVE" you push on the strut, allowing the shaft to float free in the stuffing tube with less friction.

I'm not saying there is no friction or loss of horsepower with a square drive. But there is considerably less.

A local boater who ONLY runs square drive consistantly runs 18.5 to 19 second times in the 2 lap oval speed trials, and 1:05 to 1:10 6 lap heat times.

I appreciate your reply Steve.

When I see a claim made I always like some logical reasoning behind it as you provided.
Cheers
Craig

Hi Craig.
I too am wondering what gap should be left 'from back of strut to thrust washer' to allow for flex cable wind up. I measured that gap (about a 1/4" on my boat), and
put a wrench on the collet. Then, using two lock nuts, I put my torque wrench on the prop end and wound the flex cable up to just over 3 foot pounds, and re-mic'ed the gap.
And with a couple of repeats, it was coming up a .04 hundredths inch difference,
or in other words, a strong 1/32" or a shy 1/16".
Now that's on 1 foot of 1/4" flex cable with welded prop shaft I got from
Jeff at Insane.
So is 3 foot pounds enough to represent a hopped up Zenoah? I don't know...
And what to do with this info? Well if I try to share thrust between crank
bearings and strut, it just might help.
But, I'm still on the fence between collet and square drive...
and learning a lot here.
Craig, thanks for writing, and to everyone who's given advice in this forum!
Gerry

Hi Gerry
This is an interesting thread and I accept the reasoned points made by Steve.

In respect of your static test applying torque to the drive, I am not sure that it can provide a basis upon where to start with the gap measurement. When the flexi is rotating at speed with the boat underway many different forces come into play including the forward thrust applied to it from the prop.

With my Lynx mono and stock Zen260PUM and wrap to centre header, I started off with a 6mm gap and over a period of runs shortened the flexi end on my bench grinder a little at a time. I figured that once the thrust washer started to show the first sign of being in contact with the strut support, the gap was about right. I’ve left it that way since and after two years of regular running the original flexi shaft is still in good condition.

However of course every set up will be different. If for example I was using a tuned Zen the gap would need to be bigger. I think that much is down to testing, observation and slight adjustments.

One thing clear to me from this topic is that both alternatives if set up correctly do deliver good results. I’ve only used the collet system and because it has delivered with zero problems I am naturally inclined to stick with it.

Cheers
Craig