I have a 15' Arima Seahunter with a 70hp Johnson(want a Yamaha!). I played around with the exact problem you are describing. Tried everything. New prop, tuneup, another tuneup, adjusted the trim, etc... Finally discovered that I could really help things out by moving some weight forward. I suspect you have the same problem with that 4-stroke back there. Let me guess... motor -400lbs, plus a 9hp kicker, 15gals gas for 70hp, another 5 gals for kicker, probably two batteries, downriggers with two 15lb balls.... Sound about right?

I ended up moving my two batteries up front. Doesn't sound like much but the difference was unbelievable. Then discovered that I could plane at a much lower speed which made rough water much more bearable. When I repower my Arima, I plan on going with a two-stroke 75hp Yamaha(220lbs!!) and a four stroke 6hp Yamaha kicker. My little Arima will fly!