"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.

Mitchell - first and easiest fix could be to put some weight forward. I
would also recommend that you consider trim tabs - either fully adjustable
tabs or "Smart Tabs" type. You can find a lot of discussions on both types
on www.iboats.com forums - search on those two subjects for a lot of (what I
think is) good info. Good luck and have fun with your new boat.

Cam