1. What prop do you have. This is a heavy boat and it needs a good SS prop
w/ some bow lift in it to help hold the bow up when you start trimming. I
would suggest a good SS 4 blade.
2. Trim angle is all about your speed, weight distribution, and
conditions. As you increase speed - you should be able to trim out. If you
are running wide open and bump the trim and the boat starts porpoising -
there is a problem w/ either the prop or the hull or the weight is all
forward.
3. For trim to work - you need less weight in the bow (not more as
suggested). Move weight to the rear (especially a heavy cooler).
4. The hydrofoil will stop porpoise by pushing the bow down (same as
leaving the trim down). It will not help w/ performance in terms of getting
more boat out of the water which is the purpose of trim.
5. Trim tabs do the same thing bow allowing you to push the bow down more.
Will not solve this problem. The are great for helping to plane and stay on
plane at low speed. The also allow you to adjust side to side lean.

Try a 4 blade SS prop. I think it will solve your problem.

--
Tony
my boats at http://t.thomas.home.mchsi.com

"Mitchell Gossman" wrote in message
om...
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