Mitchell Gossman wrote:

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

Two items come to mind - weight distribution and propellor surface area.

Move weight forward to reduce porpoising. This will compromise top
speed, though, if you are proped right.

A 4-blade prop is one quick fix, but the right 3-blade will be fine too.
The porpoising you describe is likely from the inability of the prop
to hold the bow up while on plane - the power is fine from the engine
but it has to be applied to the water from the prop. You have to
increase the surface area of the prop by having larger blades or more
blades - there are options either way, the best would be to try several
different ones on a trial basis from a good prop shop.

Rob