Flex shaft myth?

[scoota]

Maybe if we learnt to use the search engine , we could advoid all this tension , cause i reckon this question has been ASKED & ANSWERED a hundred times since i have been on this fourm !!!

[SweetZ28]

Like I said above, don't try to re-invent the wheel. Your suggestion will get someone into trouble. I tried your "test" on three of my boats, measuring the actual amount of cable shrinkage. One was 1/16", one was 3/32" and one was 1/8". Had I used your method I (or worse a newbie who took what you said as gospel) would have overheated a setup.

Bottom line is that each setup is different. It depends on the length of cable, the brand of cable, the stuffing tube diameter, number and severity of bends, etc. Far better to avoid a problem by having a bit too much gap than having too little. Remember there is no negative to a bit too much gap. If you are scared of having too much gap then follow the suggestion above and mark the strut with paint. Of course, that is like a "too late light" because the damage is already done by the time you find out you have a problem...

BTW, you need to wind the cable the opposite direction it rotates. It can make a big difference.


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By holding the motor from not moving at all and turning the prop puts way more stress than a boats prop in water ... you can hold your boat from moving farward with your hand in the water when flooring it but the prop still spins and slices through the water Unlike totally stopping the motor from rotating and twisting the and forcing the prop by hand. You must have a small cable in a large tube or something because all my boats none of them hardly even move when i do the test i posted above to them?

So i guess if you do my test and you don't see anything bind your ok run the gap you have set... but if you see binding when doing my test add a little more space and do the test until you don't see anything bind and you will be ok.

[dasboata]

If you twist them from the collect end they get shorter if you twist it from the prop end it gets longer,,, Props are turned by the collects check it out again

[keithbradley]

To anyone who was confused:

I wasn't implying that you use marking compound (or paint) on your strut and then run your boat untill it stops. That would completely completely defeat the purpose. I guess I didnt clarify that enough. Mark it, run it (apply full torque to the strut) and check it. Shouldnt take more run time than maybe 15 seconds. If there is damage done in that time, the strut gap wasn't your problem.

[keithbradley]

By holding the motor from not moving at all and turning the prop puts way more stress than a boats prop in water ... you can hold your boat from moving farward with your hand in the water when flooring it but the prop still spins and slices through the water Unlike totally stopping the motor from rotating and twisting the and forcing the prop by hand. You must have a small cable in a large tube or something because all my boats none of them hardly even move when i do the test i posted above to them?

So i guess if you do my test and you don't see anything bind your ok run the gap you have set... but if you see binding when doing my test add a little more space and do the test until you don't see anything bind and you will be ok.

How do you account for compression via thrust with this method? Do you assume there is none?

[scoota]

How do you account for compression via thrust with this method? Do you assume there is none?

i was thinking the same

[bwells]

It seems the best thing is a thrust washer between the drive dog and strut with say 3/16th inch gap thereby transfering the forward propulsion to the strut and not the motor. I do not know how you would check the flex "shrinkage" under load. All my flexes are 3/16th so I set the gap a tad bit less.

[sundog]

Maybe if we learnt to use the search engine , we could advoid all this tension , cause i reckon this question has been ASKED & ANSWERED a hundred times since i have been on this fourm !!!

Scoota. no tension here But this deserves to be asked again: So does the drive dog only serve to transfer (forward energy) to the shaft/cable?, meaning it doesn't (need to/want to) transfer torque to the strut/transom? Then (if so) would a square end cable (which slips into the square ended shaft) be more efficient? - it would allow for the shrinkage and transfer energy directly to the strut/transom, instead of through the cable and thrust bearing to the motor end cap bearing/ motor mount. - Are we trying to put a precise gap that will coincide with the (compression/shrinkage) to transfer the torque through the washer to the strut/etc? Or allow enough extra gap so the cable transfers the torque to the motor/etc.?

Should the torque be on the strut or the cable?

[SweetZ28]

How do you account for compression via thrust with this method? Do you assume there is none?

I guess i should have been more clear when you hold the motor from rotating and as you twist the prop in its rotating direction with your hand also apply pressure pushing the prop in tawards the rear of the boat... If you do this and still see a gap you should be fine.

[keithbradley]

I guess i should have been more clear when you hold the motor from rotating and as you twist the prop in its rotating direction with your hand also apply pressure pushing the prop in tawards the rear of the boat... If you do this and still see a gap you should be fine.

Very simple, but that may be an effective method... maybe, maybe not. I don't know. Sounds like it works for you. I dont usually like to twist my props by hand though. I sharpen them as much as possible, and they can cut through my girlish paws.

[SweetZ28]

Very simple, but that may be an effective method... maybe, maybe not. I don't know. Sounds like it works for you. I dont usually like to twist my props by hand though. I sharpen them as much as possible, and they can cut through my girlish paws.

Use a cloth or a something to protect your fingers.

[Punisher 67]

There is no doubt that sometimes the common belief needs to be questioned , I asked years ago if a square end ferrule on the strut would work with electrics and was met with some resistance , ya my friends uncles brothers sons grandfathers neighbor tried it and it didn't work on his electric but works on the uncles gas . it didn;t quiet jive in what I considered common sense and all-oh and behold another old wives tale turned on its ear.... it has worked on everyone of my boats and like a dam . But You cannot discount stuff that has been tried and tried and triple tried by a lot off seasoned boaters that have been around since before oars were invented so really think a lot of this through

Some of the advice given here will save you some money , agrivation and a lot of dispare

[scotthend21]

After breaking many flex shafts I started using an O-ring for my spacing. this will give the necessary compliance for the shrinkage of the shaft under load. I have not broken another shaft since.

[FRED]

myth

Scotthend21, you say you are using O ring for spacing. Now from what i just read, you tighten the coupler and the check spacing by laying the O ring across the space for the desired space, correct. Now alot of things have been pointed out here and common sense is the best answer. Now it all depends on lenght of cable, size of cable, size of prop, power of motor/s, racing or not racing. Why not go to solid shaft and be done with cable especially with very long cable setups. The Europeans use shafts from what i've read with great success. The only downfall is bearings at the strut area in the cleanliness dept.

[domwilson]

There are many posts here on the subject. Symptoms range from high amp draw and temps to loosing shafts. I once did an experiment to see how much the cable actually shrunk. Not very scientific. I took a teflon washer and installed it with very little gap and the washer nearly got shredded. Did again, same setup and cable and new washer with a little gap. Boat ran faster with lower temps. Washer was undamaged.
Many people on this forum have issues resolved with this one bit of advice of leaving a little gap. It may not be correct in leaving a little gap, but issues have been resolved in doing so. People offer advice here typically because they already went through the pains and expense so you don't have to.