Just a guess based on 8" props that the motors are too small to get model up on the step and able to lift off. With large higher wattage motors, large enough props, props may get spray from the bow and lose thrust. I don't like to say this, but maybe your design has motors too close to the water. You might be able to install wheeled landing gear and take off from smooth surfaces, no water. Take this advice with a grain of salt, it's worth what you paid for it. Good Luck!