webdr

Please forgive me if this is old news.

I was on the phone with Bill @ Top Secret and we were discussing an area that seemed relatively new in terms of rc boat theory.

If you leave a small space between the drivedog and the strut, about 1/4 inch or less, it allows the thrust created by the prop to be transferred through the flex cable and right to the collet, essentially pushing the boat at the engine, very near the center of gravity.
Conversly, if you butt the drivedog up against the strut shaft, all the thrust from the prop ends at the strut, and the boat is pushed from the stern, which can create adverse handling vs the aforementioned way.

Ive not tested it with the first method, but my avenger and rio 51 are set up this way. Ive yet to drive them so Ill see what happens. anybody want to shed any light on this subject?

Ron Olson

My Feedback: (1)

The gap is for cable wind-up where the cable shortens under load of the prop. This is for the round drive flex cable collet setups, the square drives are another story.
The next story is for the square drives which do push at the transom. Some people prefer this mothod and has been used on record setting boats. Any wind-up of the cable if any will shorten in the collet and not at the prop end.
Most are the preferennce of the boat's owner or the type of strut used.

webdr

Yes, thats what Bill was also saying, the space allows for wind up on round flex shafts.

Although it ends with a boaters preference, could any handling benefits be attained from pushing at the collett vs the back of the strut?
Matt

piper_chuck

My Feedback: (12)

It's not at all a boaters preference thing. If you don't leave the gap on the round shaft system, when the cable winds up it will exert pulling force on the bearings. In sort order, you can trash not just the bearings, but also the crankcase.

Ron Olson

My Feedback: (1)

..... not to mention a crankshaft!
I found out that little tidbit of information the hard way. I readjusted a strut but didn't check the gap afterwards and tossed the boat in the water. It did run a little better at that strut angle but the flex cable put too much stress on the whole driveline I watched several parts go "ploop, ploop, ploop" into the pond, the cable with the prop attached, the flywheel and part of the crankshaft.
It's amazing the amount of stress that can be placed on the driveline.

webdr

On a 1/4" flexshaft, how much gap should there be?

Ron Olson

My Feedback: (1)

I don't know for sure as there might or mightr not be any hard and fast rulles concerning cable diameter and gap.
While trying to dial in a carb with a boat in the water holding it still (a .21 mono then tried it later with another one) I was watching for cable wind-up but didn't see much even when revving up the engine. I came to the unprofessional conclusion that I didn't need as much gap as I thought that I did. One might think that you'd only need so much to where you had a slight gap when under power and with me holding the boat might be harder than it running at speed. My theories only.

Dan S

I have been using no less than 1/8 gap, could be a tad more at times but never less than 1/8.

Dan.

juicr

The longer the cable, the more gap you will need. I just set all mine at 1/4 in. with the round collet.

webdr

Good info, thanks guys.

Justaddwata

My Feedback: (3)

You can get a feel for how much a cable will torque up when you tighten a prop nut (or by pushing against the prop by hand) if you hold the collet nut with a wrench.

I believe 1/4" is good for long drive cables (sport hydros, riggers) and go down from there. Ultimately contact between the cable and strut is a good thing but only once the cable has been allowed to torque up (otherwise you will stress the cable and bearings). Having too much spacing between cable and strut will prematurely wear out your stuffing tube and possibly motor bearings as the load is transferred to everything but the strut.

Ron Olson

My Feedback: (1)

That's something that I was wondering about also if the length and diameter of the cable has an affect on how much gap is needed.

jetpack

My Feedback: (1)

As far as the CG aspect, there would be zero detectability. The prop is the X point of weight transfer and balance. You would need a HUGE amount of movement at the prop shaft to notice any type of CG change to the motor via the flex shaft - way beyond operating tolerance. When picturing this you have to take in account the thrust line itself, which is always pushing forward - the prop itself is the pivot point to the water that determines the CG, not where its post of strength originates.

The end play I was always told to carefully watch your thrust washer.

Starting with a new plastic thrust washer, slowly increment your flex shaft into the collet (using setting gauges - feeler gages at the drive dog gap) between each test run until the drive dog starts to kiss the washer with an even wear pattern. Using steel blue or marker on your drive dog face will help you determine this.