SV27 Drive cable sheared off

[Zephrus]

Hi all,

I'm a relatively new boater. I have a SV27 I just overhauled/upgraded from stock. 5S-5000-150a pack, Feigao 540 7XL motor, 180 amp Hydra ESC, Graupner CF42 prop (prop is a bit big for this setup but I only do short runs).

So I just got it in the water for the first time today. At first I was using 14 cells NiMH for a test run. Everything went well. Motor and ESC running cool.

Then I pop in the 5S. This thing was HAULING. My lake, at the short end, is roughly 300 feet across. It was traversing that in about 3.5-4 seconds if that. WAY too fast for my relatively small lake. But it was going like a shot. Everything is running smooth for about 4 minutes, then it happens. My boat stops, when I give it throttle it's whirring but no forward motion. I'd figured I might have lost my prop. It seems to be sinking fast. So I get out the recovery boat, wind it up and bring it in.

Turns out my drive cable was literally sheared off where the drive cable enters the drive tube. Took the prop and most of the drive cable out the rear when it sheared, which is why it was filling up so fast. The motor shaft was bent (managed to get it 99.9% back to where it was). I've been trying to figure out the reason for it.

A few things. First, my drive coupler didn't seem to fit my motor shaft quite well, although they're both 5mm. It was just the slightest bit sloppy. Second, there was no inner liner protruding out of the front end of the drive tube. I'm thinking maybe the drive cable was chaffing up against the brass and, because of heat, friction and sharpness, finally cut it in half?

Anyone have any other ideas? I love the setup! The motor will need a bit more cooling for sustained running (the ESC still runs cool) but the the thing just flies! I'm figuring round about 55 mph or so and stable running pretty dry on near mirror water. Would like to GPS it when I get it going again.

So I don't know about the drive cable shear. It was a bit old so maybe that figured in . Any ideas/advice/suggestion much appreciated.

Rob

[scoota]

If the drive coupler was sloppy it would of not been running true, that would of caused vibrations witch would bend the motor shaft at the rpm your running , i would not run that motor again if the shaft is not a 100% true. as for the flex breaking, age/not running true cause of the coupler & and rubbing on the stuffing tube would of all contributed to it breaking , i always have a bit of the liner sticking out to stop the flex rubbing on the stuffing tube.. SVs have a habbit of the motor mount coming loose ,because they bolted on to soft ply wood, just check that its not loose...And when your running an XL motor i have a piece of foam under the motor to help support it , to take some of the extra weight off the mount .....

Jason

[Zephrus]

Thanks for the advice, Jason. I think you hit the nail on the head.

The motor mount was solid but something I didn't mention before. There was no room on either side of the radio box for the 5S pack, (and it would have been unbalanced if it did fit), so I had to put it up towards the bow of the boat. Not a common practice I'm sure, considering the CG issue, but just for a quick test run. The pack wasn't that secure and was right next to the motor. I only put a piece of stiff foam between the pack and motor to keep the pack from chaffing against it. During my run the boat did an endo .. landed right side up and I kept going. As I think back it wasn't long after that that the cable sheared. In short, although it was a dumb mistake I won't repeat, I'm sure the pack had to be banging up against the motor almost constantly while it was running. The endo probably tied it. Like you said, the motor mount in the 27 is only held in place by rather flimsy plywood, and the 540 is longer and a bit heavier than the stock motor. I'll definitely be taking your advice and giving the motor more support ... maybe even glass in a custom support to stabilize the motor better.

About the motor shaft ... I have it about 99.9 percent I believe. However that 99.9% is better than how it was when I originally ran it. Before the original run, I could literally see the tip of the drive cable coupler moving a bit from side to side as I turned the prop. It doesn't do that anymore. That is to say, it wasn't 100% the first time I ran it and is actually better now. I was so used to running the plain vanilla 30 mph SV27 that I didn't stop to consider the enormous forces that would be at work at more than twice those rpms and torque ... something I most certainly should have done. As they say haste makes waste ... and I was too anxious to get it in the water. As it stands now, there is no perceivable off-axis rotation. Whatever is there can be measured in microns.

Also I'll be going with an upgrade .150 cable as opposed to a .140 (I think) which is the stock size.
http://www.offshoreelectrics.com/pro...svcable&cat=52 if you're interested. I'll also be sure to have the teflon liner protruding from the stuffing tube a bit to eliminate any metal-to-metal chaffing issues. More support for the motor, and most definitely, keep that pack well away from it and secured.

Also it seems to me that the exposed part of the drive cable between the stuffing tube and coupler should have more support. I know it's not a long run but at the high rpms, like you said, even a slight discrepancy can become huge. Maybe some kind of mounted high speed bearing the drive cable runs through. Do you think that would be overkill? Also, Jason, about how much distance do you think I should have between the strut and drive dog? Up till now I've have about 1/8 inch. Do you think it should be more to allow for more flex?

[scoota]

the ose flex is a much better flex cable thats what i run ,i try and have 3-4 mm between the dog & the strut, sorry us Aussies work in mm , which is 1/8 to 5/32 for you guys.. which is plenty of gap .... you have that gap because when you load up the flex it shortens with the twisting motion , so if you hold the motor coupler & grab the prop & turn it you can see the dog move towards the strut , if you get what i mean, as well as the forward push from the prop thrust.. The bearing thing is a bit of over kill i think . i ran an 8XL for years with no problems ....

jason

[scoota]

Oh the standard SV flex is .150 .... i run this thrust bearing between the coulper & motor housing which will help take some load off the drive system http://www.offshoreelectrics.com/pro...?prod=dh-53136 i just seen in an another thread that your running a thrust bearing good move !!!!