First of all, sorry for that random, empty untitled forum post, the iPad wigged out and submitted my first attempt for no reason.
So, I'm trying to make a twin engine flying boat. I've been having problems, including a lack of side to side stabilization (at high speeds on water, it shakes back and forth, them flips.), staying upright (flipping right off the bat, even with a low CoM), and taking off at all. I've tried tinkering with things, and I managed to almost solve the staying upright problem but the others still plague me. Underwater wings won't keep it straight, and flat bottom airfoils won't give me enough lift.
I think I have solving of your troubles. Check my engine for boats. If it helps, write me in comments and I'll help you to modify it for your needs
@Jackie try to make double wings. Stick them one on top of another with no space in between. When throttling up, go easy, give it some time to pick up speed before adding more throttle. That way, the lift from the wings will probably prevent it from flipping forward. Test it with the air start, see if it flies. If not, fix that first. Buoyancy will come.
@Skua @MaleticAirplanes I have used side floats, and have a high to low buoyancy gradient from front to back, and small stabilizers underneath, yet it still refuses water transport. Maybe the CoT is too high, it's a high wing parasol design, with engine pods, rear facing, under the wings on pylons.
Maletic is right about the floats - you want more buoyancy at the front and less at the back, until it nearly tips over backwards when you start. If the design allows for it, small underwater vertical stabilisers can help keep you on course too, as well as improving low speed steering if you give them rudders.
To stay upright, I needs floats on the sides, front flip problem can be resolved with more buoyancy at the nose, and takeoff problem is probably because of high center of trust or too much buoyancy at the tail, or both.