Ive got a 92 skeeter 18.5 ft with a 95 evinrude 150. Freshly rebuilt motor. Took it out today continuing the break in process and when i accelerate the motor starts to shake and it shakes the whole boat. A few times i guess i had the trim in just the right spot and it didnt vibrate at all, it shot out of the hole and scooted on. It has a 21 pitch prop. On a side note this is my first bigger boat with tilt/trim. Could it just be that i have to figure out the trim better? Or what could it be?

trim should not effect the motor shaking.
probably you have a plug fouling a little
You said you were breaking it in , is it double oiled ?

Yes it is double oiled. All plugs look fine. Theyre new along with wires. Spark has been tested and its perfect.

Is that prop in good shape? Any dings or anything that could make it un-balanced???

prop is bent or prop shaft problem.

put boat in N and key out, spin prop as fast as you can and watch the center to see if it spins off center.

send prop to steve and trade for at least a 23 or 24, 21 is too low for a 150 on a 18.5 boat imo

prop too small. need more cupping

The prop does have a few dings but I didn't think it would make a difference. My mistake. That's good to know. I had already planned going for a 24 pitch as soon as I get the funds, 1400 dollars on rebuilding the motor hurt me for the time being. I do appreciate the info guys!

Check the fly wheel !

If you didn't have this issue prior to rebuild I would have to suspect something related to the rebuild. Good luck! I would take it back to engine builder.

Does it do it if you rev it up in neutral? If so it isn't a prop issue.

I rebuilt it myself. Turned out the carbs were full of trash causing it to spit and sputter and shake the motor which in turn vibrated the boat. After a good carb cleaning shes running beautifully. I still need a 24p prop but the 21p will get me by for now. Thanks for all the input!