A 15 horse is too much for a 14', that size motor is more compatible with an 18' raft, or could be ok on a 16'. I would not go more than 10 hp for that size boat, you would be fine with less.
What happens to the angle of the motor when you apply throttle? Is the foot of the motor kind of sucking down and forward? Since you are having problems with a smaller motor ( 4 hp is not to big at all) I am wondering if the angle is part of the problem, especially with a strapon mount, the the motor could be rotating down, instead of really pushing the boat forward like it should. That would explain some of the water being pulled into your stern.
You may be able to fix this by securing a block in between the motor and the hull, or better yet, a transom that bolts to the frame. Is the rocker of the stern flexing down significantly?
Short answer is probly better transom, and smaller motor.
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