Taking a dive

[MJ13]

Can someone please help me understand what is happening, or offer any tuning suggestions on how to get my stock impulse to stop submarining or taking a hard nose down dive into the water? This has happen to me twice in two weeks, once on 5S, and once on 4s, costing me an unretrievable hatch cover and seal each time. On the most recent occuracne, it nearly cost me the entire boat. The first time, the boat ended up right side up, and I was able to drive it back to shore, but with the most recent occur acne, the boat ended up upside down.

Why is this happening, and what can be done to prevent it from happening again?