Problems with Stayin on plane

I have a 1986 Silverton 340 Convertable and have trouble staying up on a plane once i get there when going with the seas. I will get up to 16 knots at 2800 rpm then it feels like the boat sinks back into the water and reduces my speed by 5 knots. Could this be a trim tab problem or something else??

 

I could be completely wrong, but, it sounds to me like the following seas are "Helping" the Boat up on plane, but due to inadequate power, boat cannot sustain planing speeds.

 

When you get up on plane is it safe to assume that your prop is not breaking the surface?

 

How big are the "seas"? Are you on the face of the wave, or the back of the waves? If on the face of the wave you get a boost, if on the back of the wave it tends to slow you down. If traveling a little facer than the wave, you plane down the face and into the back of the next wave which then slows you down. Kind of a yo-yo effect.

 

Peter is asking the right questions, and we need a more detailed description. Describe the seas, where the boat is in relation to the front or back face of the swells, how does the boat perform at 2800 rpm in sheltered waters, what is your trim tab setting for the situation you are describing? Has the boat been repowered since new? what engines do you have, etc. Can't have too much info.