Airboat Woes....
 

So last week while on holidays, I had the pleasure of playing around with my airboat. It had also had an engine upgrade from the finniky MDS 40 to an OS FSR 40!. Firstly this engine is a very nice starting and running engine. Secondly, I had also increased the size of my air rudders from the original to twice the size. Same height but twice as long. Also since I did not have a rescue boat handy, I had to resort to grass runs. Wow, once it gets going, it is darn fast. But as I am new to this still, it seems to be very twitchy/unresponsive/very sensitive to control input. So I am now not sure of the rudder size or is it just because it is on dry ground. Only time will tell. Two weeks minimum before I get back to the cottage. Anyways, on the first run, it must have hit a little bump at full throttle as it proceeded to do a 360 flip! To my horror, I was cringing while watching, but to my amazement, it landed right side up without any damage, engine running! Yay me. Next couple runs were good. Then the last run.... Similar to the first, but this time it tried a double flip in a different location. It is now back here at home for repairs, although minor. Bent up my rudders and snapped my rudder control horn only. I was very impressed with the design and as well that I was able to build it that rugged. Anyways, just wanted to share my experiences I recently had.

Try using some expo on rudder to desensitize rudder around neutral. Hi and low rates could also be adjusted to make rudder less twitchy. The servo and rudder horns have multiple holes for this reason as well.

I guess I think a little better explanation of the twitchy/unresponsive comment might be in order. I've never needed to adjust any settings on my radio to get the type of control I want other than adjusting travel/endpoints. If you're turning full right and it takes its sweet time to turn or doesn't turn very sharp, that tells me either there isn't enough throw or not enough surface area on the rudder(s) to get adequate control. If it turns too quickly or too sharply and spins out easily, perhaps the rudder has too much throw.

If I may ask, do you happen to have a 'suggested' amount of travel for the rudders? I am sure they will all be different based on design, but is there a starting point. The reason for enlarging them was last year in the water, it favored a turn in one direction. Not that it did not turn th eother direction, but rather one way was more pronounced in my opinion. I should also mention that the control horn is low and up front on teh inside of one rudder in line with the hinge-line as on an airplane. At the back end of the rudders (center of overall height), I have 2 control horns facing each other and joined squarely with a conrtrol rod. Thank you.

Sounds like the way I've set up my twin rudder boats. Hard to say on the deflection, 45* to start. But it depends on rudder size, boat weight and bottom friction. It is normal for the boat to turn sharper in one direction due to prop torque. Realize that turning relies on air volume and air velocity across the rudders, keep the throttle up for best turning. Alternatively, blip the throttle hard to assist in low speed turns.


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Thank you. That sounds about what I have for movement then. I will just have to fix her up and get it wet to find out for sure though. Two weeks.....

Yeah - I shoot for about 45 degrees of throw also. Make sure both rudder blades are parallel to one another throughout their travel too.

Here is one of my boats with air rudders fitted. This boat doesn't need more than about 30 degrees of rudder throw to get good steering and control.
This is another of my boats with too small of rudders on it. It goes straight nicely but spins out easily. Putting bigger rudders on it made a remarkable difference; however I don't have a video of the boat with bigger rudders on it. This boat has 45 degrees of rudder throw.

You need some H2O, and those things would fly! I used to live down in Louisiana and sometimes when I'd be out fishing, hunters would come through in airboats like those. They would basically take a flat bottom john boat and laminate the bottom with that white plastic stuff like cutting boards are made of. Then they'd put a big V8 on it, always open headers, gawd awful loud. They would drive right up the boat ramp and park them in the car lot. Those videos brought back some fun memories. I've got one airboat, but it's just a cheap plastic thing with a single rudder and a 40 OS. It moves a lot of air though, I just bought a 3 blade prop for it today. Not sure how it will work out, but it was cheap enough for a try. Good luck with your boats, they are really nice.