Should I offset my rudder?

[pcressey]

I am experiencing the stern of my boat shimmying, or bouncing left-right-left-right while under acceleration. I have a video of the boat running for a few minutes, and you can see the instability of it while achieving maximum speed. Would offsetting the rudder help the stability? What could be causing this effect?

The setup is a 2650KV / 180A / 4s 5200mah / 42mm 1.4

Thank you.



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[Doby]

Your strut angle is wrong. Its going to try and lift the back of the boat constantly and give you "prop walk".

Start with a neutral setting (prop shaft parallel to the bottom of the hull) and go from there.

Also, you are pushing almost 40,000 rpm on 4s, that's really more of a 3S motor.

[Fluid]

You also need to have a 3/16" gap between the drive dog and the strut. Right now you are pulling the strut back and down when running, making the problem worse. It is hard to tell from the video but you may want to raise the strut a bit too.


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[lenny]

What are your temps at ?

They may come down some if you get the strut shaft leveled out with the bottom of the sponsons,
And the boat may pick up more speed to.

So beware of blow overs when you first test it like that,
You may need to move the cells forward a little if that start to happen a lot.

Also ditch the plastic washer to and just keep a good gap like Jay said.

[pcressey]

Doby, I had the strut more leveled out, but it was lifting the front of the boat quite a bit, and it was on the verge of blowing over at high speeds. I don't have a wide shot of the boat from the side, but the angle of the transom hardware is actually a few degrees, and is not level with the hull, thus that angle is exaggerated in the photo... if that makes sense. The motor is peaked with the setup, but it runs quite efficiently, so I don't plan on dropping the KV's for now.

Fluid, it is a HK boat, so it has a fixed unit within the strut that connects to a 5mm flex cable. I will try ditching the washer and create a 3/16" gap. I actually modified the strut this past weekend so I could raise it a bit. The prop is just 1mm-2mm above the horizon of the sponsons.

Lenny, after a strong run the ESC is around 105F, the motor 95-100F, the batteries 100-105F, and the deans connectors 110-115F. I have a bunch of new Castle 6.5mm bullets to redo the connections, so that should help the temps with the battery/ESC connections.

Here is an album of the boat: http://forums.offshoreelectrics.com/...p?albumid=1191

[tlandauer]

How rigid is your rudder linkage? It seems the thin plastic servo arm is also susceptible to wobbles?
My MG was the same way, changed to off set and the problem was gone but with one big catch---the boat has more porpoise ( wah wah wahs). This is opposite of what people say and was also very surprising to me as well. I mean according to the consensus that the off set rudder should eliminate the wah wah wahs.
I never did learn the reason ( why the in line rudder would induce the left, right & left jitterbug ) and I suspect some kind of prop wash disturbance and the fact the Proboat stock servo/linkage set up is not beefy enough. I prefer the ride with the in-line rudder over the off set for that reason. Alas, too many boats, too little time, it will take another 6 months before I will revisit my MG.

[tlandauer]

Beautiful lake, where in California? If I may ask.

[martin]

If you have the 2 piece flex & stub shaft its not designed to have a large gap at the dog, its designed for the dog to run against the Teflon washer. As theirs no flex wind up shortening the flex & stub assembly like with the 1 piece shafts.

[pcressey]

tlaunder, that is a good point about the linkage to the servo. It isn't the best, but I am not sure it is so bad it is causing the problems. I will give it a closer look. That is Lake Balboa in the San Fernando Valley in Los Angeles.

martin, thanks for the info. I will leave it the way it is.

[Jeff Wohlt]

Make sure you have NO slop in linkage and yes, pretty common on rudder when in prop wash. I also like a two sided servo horn and wires and two on the rudder...if possible. Rudders take a beating and some just do not set it up right from the factory...one small linkage expected to run 40 mph with a tiny wire to hold it all in place. I make sure and use a 078 wire or larger and metal horns but I also like the adjustable linkage screw/pin at the rudder.