Let's hear the reasons; applications and benefits of using flex cables and flex couplers. This is new to me so I don't know when and what's the advantages. What do you need for a 700/5mm motor and say a 3mm shaft. Do you get rid of the shaft and tube totally? Do Flex Coupler really helps stop drive line vibration in a 700 motor? IF so when should you use a flex cable and why? Is it worth the time and money? how about it for a 550 low turn motor?

First, Yes Flex couplers do reduce vibration greatly. I had the shockwave 26 that had a dog bone and it vibrated like crazy. A flex cable is even better, less drag and vibration. Time and money are no object
How ya doing Chuck?

Doing good my friend. - have you ever made your own and what's the difference between ujoints and flex coupler - I know what they both look like but - what does one do vs the other?

Well the dog bone is rigid and is connects the motor to the drive shaft because the never line up properly and where the dogbone connects to the out drive is where the vibration occurs because it is a loose connection. CVD's are a little better than dogbones, but they are not without their own problems. The flex coupler is fastened very tightly at each end and the flex in the cable makes the difference where the motor and drive shaft do not line up. So there is little or no vibration because there is no loose connection
. No I have not ever made my own.
I heard of one guy with a flex cable that hit something in the water and when he got the boat in the cable looked like a spool of fishing line that got all tangled. But I hear flex cables are the way to go and are supposed to be the best for speed and low vibration.