JAE Mini Sprint epic fail

[Eric Drust]

A few weeks back I bought everything to get a mini sprint going. I had it done for this weekend and brought it with me on vacation to try out.

What a blast! I am still blown away by how fast the boat is, and how sharp it can turn. All my neighbors got a kick out of seeing it, and a few were contemplating getting one themselves.

All until I put the third battery in it...

Boat went 100 feet and died. I rowed out to retrieve it and found the prop, prop shaft, nut, and drive dog all missing.

I have my guesses as to what is to blame. The coupler for the motor to flex shaft is out of round (or alignment depending on how you look at it), so the flex shaft literally "jump ropes" inside the stuffing tube, and the vibration is so bad on my boat, you can't hang onto the boat out of the water with the motor spinning. It will vibrate your hand to the point a reflex action causes you to drop it. I am guessing the intense vibration broke the weld joint on the end of flex shaft.

I am also pretty disappointed overall, as I went with a pre built one thinking I would be getting it in the water faster, and that the build job would be a lot better than I can do. Well, I was wrong on that. I had to spend many long nights sanding huge globs of resin off the boat just so the cowl would fit on it, and the stuffing tube is not even straight in my boat. It could be as much as 10 degrees misaligned, but there is so much resin around it, I don't think I can break it free and rework it. The misalignment in the tube forces the boat to turn left so I had to trim the rudder to the right by a ton to make up for it.

I am guessing with the combination of vibration, and misalignment, I am loosing a lot of performance potential, which makes me hopeful I can get a lot more out of a properly setup boat.

Does anyone know if a reliable flex shaft for this boat which won't break as easily?

 

[anthony_marquart]

Well don't get too wound up. It's not really unusual to lose a shaft and everything attached... Happened to me a month or so ago... You will need to get it aligned though.. If the cable is whipping like a jump rope the stuffing tube it too short. It's best to have no more than 1/4 -5 1/6 of un supported cable. You can add to it by adding a piece larger diameter tube over the existing one and putting a piece of the stuffing tube of same diameter as the original in side of that one...

The problem of needing rudder to go straight is probably not related. Even if the strut it offset to one side or another it should run straight. You probably just need to look at the sponson alignment for that.. no big deal especially on a JAE cause they are help in place by collet clamps.

 

[BrianTooms]

My flex broke inside the stub shaft my first time out with 3s but luckily I didn't lose the shaft. I soldered the stub back on and was good to go.

Offshore Electrics has some Octura cables assembled or un-assembled. http://www.offshoree...rod=oct-oc64pkg

One thing though you shouldn't rev your motor too much out of the water unloaded.

Here's a short vid from after the Gold Cup in Edmonton.

http://www.youtube.com/watch?v=f5h4RRqd2aM&hd=1

 

[Eric Drust]

I can't run my motor at any speed out of the water. Even the very lowest speed and the boat vibrates heavily, with the motor sounding like it will come apart.

My stuffing tube is not too short, the coupler for the motor and flex shaft is not concentric. As it rotates, the flex shaft spins off center by about 1/16 of an inch. I believe it is just a badly manufactured coupler and I need to get another.

My strut is not offset at all, it is not parallel with the rest of the boat.

 

[anthony_marquart]

If the strut is mounted to the transom tighly and it isn't square with the boat... the transom isn't square and you could have a problem... or you can just shim it to get it straight.

So if your tube is a little crooked,.. no big deal.. as long as the transom is square..

 

[Eric Drust]

This boat does not have a strut which is separate. The stuffing tube extends out the back of the boat, and is reinforced with the ski of the boat.

I think the transom is square, and I think the tube is aligned properly inside the boat. The issue is where it comes out of the boat, it was not straight when epoxied in place. It is bad enough one can easily see it by eye. I will try to post a photo to show what I am referring too.

 

[Martin Hamilton]

I can confirm that their is a problem with the coupling as well,I didnt use the stock coupling as i use a .048" wire drive in mine & use an Etti collet coupling. But the stock coupling i had had an oversize hole on the motor end, It should be .125" to suit motor shaft but is actually .130" ( 3.3mm ). Because it uses grub screws to tighten onto the motor shaft your pushing the coupling over to one side & off centering the coupling. I havnt checked to see if the hole is also off centre in the coupling as well which would make things even worse re every thing running true. Martin.

 

[Martin Hamilton]

By the way the shaft & motor will sound like a bag of nails if run out of the water, Its because you have no load on the flex & motor. The noise will stop when in the water with load on the prop. Dosnt help much though if you have some thing running out of true.

 

[Eric Drust]

I can confirm that their is a problem with the coupling as well,I didnt use the stock coupling as i use a .048" wire drive in mine & use an Etti collet coupling. But the stock coupling i had had an oversize hole on the motor end, It should be .125" to suit motor shaft but is actually .130" ( 3.3mm ). Because it uses grub screws to tighten onto the motor shaft your pushing the coupling over to one side & off centering the coupling. I havnt checked to see if the hole is also off centre in the coupling as well which would make things even worse re every thing running true. Martin.

You are spot on with this. Upon closer inspection the coupling fits the motor shaft very loose. At first I thought the motor had a bent shaft, but I removed the coupling, and ran the motor while holding on to it. I could feel very little vibration.

Can you give me any more direction with the wire drive idea? Did you have to relocate the motor at all? I am interested in anything which will make this boat run smoother.

You are also right about the boat sounding bad out of the water, but mine is so offset, it even sounds pretty bad while in the water. It is greatly reduced, but not where I would expect it too be.

 

[MarkTanski]

I had pretty spotty luck with chinese hardware, decided trying to save a couple of bucks isn't worth it, everything from bad collets to rudders that don't turn . Some of it has been very high quality like the TFL props and motor mounts, wish I knew who made what over there with all the-re branding!

 

[Martin Hamilton]

Re wire drive,Motor was lowered very slightly by rotating one of the mounting bolts down in one of the slots in the mount just to ease the angle between wire & motor shaft.Also made new stuffing tube with less bend more a gentler curve & used .098" teflon liner. I made the wire shaft which has a 3mm stub that runs in 2 ball bearings. Martin.