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Porpoising is when your underway, the bow comes up, then down, then up, then down, and no matter what you do the bow keeps coming up and down.

My Outrage 17 II had a tendency to porpoise if I trimmed the engine out a little too far. I could correct it with the engine trim.
If you can't correct it with engine trim, then you need to try adjusting the height of your engine or try a different prop.
Many people and some mechanics assume they MUST put on some sort of fin on the engine or trim tabs.
NOT always true.
That is the easy way out and requires little thought process. Many times the fins will reduce overall efficiency and performance. I see far too many boats with the fin on the engine... Why?

As you can probably tell, I hate fins on an engine. I'm sure there may be a need for them for some people and some Whalers.... But not very many....