New 21' Larson I/O 5.0 L, porpoises at high speed with trim anything
other than full down trim.

Why?

What to do?

I saw the post on hydrofoil and porpoising, someone suggested that
removing the hydrofoil on his outdrive could make matters worse. How
about the reverse, install a hydrofoil to improve matters? What's the
downside? I am always skeptical about aftermarket devices like these
because you would think that a manufacturer would ofer them standard
or at least as an option if they meaningfully improved performance
with little downside.

According to the manufacturer, trim up improves speed and turning
performance when on-plane. I have not been able to play around with
this much because of the porpoising situation? Therefore, I pretty
much raise the prop only to avoid obstacles at this time. Am I getting
the point of trimming-up as far as boat performance?

Mitch Gossman