New airboat build need advice
 

hey all,

I just dismantled a large Catalina seaplane (was a great aircraft if you wanted to practise crashing) decided I would make a basic flat bottom airboat using all the electronics from the plane.

Its going to be twin electric brushless engine, I'm trying to figure out what sort of prop's I should use and whether I should go for a push or pull, the tri-prop I have generate a lot of airflow but after flipping the prop over, it still pulled air rather than pushed it (what the?) and I'm thinking push prop would make more sense. Also being twin motor should I reverse one of the engines so they are opposite to each other (Will I get adverse yaw if both are going it the same direction?)

ha! no I haven't, other than stack of components on my table this is nothing more than some scribbles on a piece of paper at the moment.

I'm intending to make the boat roughly 25in long and 13in wide.

I have -
2x 30 Amp ESCs
2x DY-BM2815A-KV1100 Brushless Outrunner
1x 3 Cell 11.1V 3000 LiPo

Current props I have are 3 Bladed 8 x 6 x 3, everyone seems to be going for twin blade on their airboats, or would these do the trick?

thanks

thanks :) helpful stuff I think I will just get this baby built with what I have for now I can swap the props out easily later!

For a given motor, a 3 blade prop is generally a bit shorter than the 2 blade (1 inch being the basic rule of thumb with models). Which could allow the motor to be mounted a bit lower in the boat. Could help with the overall balance and performance of the thing as air boats can be quite tippy at speed.

Also, flipping a prop over does not reverse it. The only thing it will do is reduce the performance of the prop because you now have it on backwards. The prop blade is like a wing in that it has an airfoil with a distinct front and back (top and bottom for a wing). The front, top, curved side should always face forward.

There are 2 basic prop types, Tractor and pusher. So named for the configuration of prop/motor in the airframe. Basically, motor behind prop is a tractor. Motor in front of prop it's a pusher. If you compare a tractor and a pusher prop to each other, the only real difference you will find is the direction of the twist in the blades. They will be opposite.

With electrics there is no real need to have a dedicated pusher prop. Use a tractor and simply reverse the motor rotational direction. On a brushless (3 wires) motor simply swap any 2 motor wires. So instead of the 3 wires connecting to the ESC as:
1-1
2-2
3-3
Make it something like:
1-3
2-2
3-1
Motor power will remain the same.

photo:
ibb.co/g4xYY0t


Got most of it finished, just need to get some longer servo arms for more rotation of the motors and get the thing coated in something waterproof.

Your design is going to put a tremendous load on those servos. A couple of points:
1. The best mechanical advantage is achieved using the shortest servo arm and the longest control horn. But in your case, I'd shorten the control horns too and even try to go in a hole or two on the servo arms. As you shorten the control arms you'll see more effective throw from the servos. But you'll also increase the load on the servos. It's a balancing act in the end.

2. Those servos are probably not up to the task at hand. Any idea what their torque rating is? I really think you'll need something much more powerful. I'd also mount them with something more than glue on the bottom. They have mounting tabs, use them.

3. The pivot is all well and good as it is but the top of the assembly needs support too. When the motors are running the whole thing will try to tilt forward. This will attempt to jam the pivot even with ball bearings. That in turn will increase servo load among other things. A simple support (wood stick) from the front part of the hull up to the top of the motor assembly with a simple pivot on top will work. A simple pivot can be a brass tab on the support with a hole in it. Two brass washers, one above and one below the tab. And a long screw through it and sunk into the top of the assembly.

4. The control arms on your motor assembly appear to be brass wire or similar. This is too soft for this use, they will flex and reduce your control. They should be something like a servo arm that sticks maybe an inch or two from each side of the pivot.

5. Last but not least, when using two servos on the same control, you should use the same hole in the servo arm. You have one in the rear most hole and the other in the front most hole. Since you want equal travel in both directions, put them both in the center hole on the end of the arm.

Hope this helps.

thanks so much for your tips. yes that has been really helpful, I'll try some of your suggestions (thats unless I crash it before hand) :D

I have a fiberglass hull made 30” long n 18” wide looking for a good start on a brushless motor for it n what size prop to start at the hull weighs 4# needing opinions on motor size once it gets motor stand n cage built with rudders and battery all on it I’m thinking atleast 6# boat total