radio controlled model : Need help in setting up the motors

So i'm trying to setup 2 electric motors for my boat (it's a small RC boat). I just wonder which way should i put the motors: 2 motors under the water or 1 under and 1 on air. I've attached the 2 pictures with the idea. Can anyone give me some advice how to deal with this matter?

One more thing, how do you make a propeller?

 

I'm assuming your RC unit can control each motor separately. If so, I'd want them both in the water, side by side and fairly far apart- keeping the boat symmetrical. This would allow you to steer and even spin the boat with throttle control only.
Functional propellers are tricky to build. You might try tracking down a book on propeller theory, but most modellers I know just buy the props as they're kind of tricky to make one-off.

 

thanks.
About the propeller, which one do you think give more thrust power: 3-blades or 2-blades propeller?

 

Prop selection is a complex black art, guided by a bit of dimensionally-inconsistent math that drives engineers insane. The selection of prop shape, diameter, pitch and number of blades is guided primarily by three things: how heavy is the boat, how fast is it intended to go, and where is the optimum torque band of the motor. The formulae used for large boats don't scale very well to models- I'd suggest getting in touch with a good hobby shop or club, there's a lot of really helpful and knowledgeable people in both.

 

Well, i guess i have to test with 2 and 3 blade propeller then.

One more question, when i have 2 propellers under the water, should i use 2 propellers with opposite side (i think they called left hand and right hand), and spin different directions? or should i use 2 propellers same side and spin in one direction?

 

look at the duoprop this RC guy made! http://www.boatdesign.net/gallery/showphoto.php/photo/539/cat/500/ppuser/690
have ya also seen the RC flying wingboat having its airprop fwd? its all on this foreum somewhere including making a wooden prop