You probably won't get much usable advise here because most of us don't have much experience with boats that use the little spinny thingy out back. 

As for the trim tabs, you're going to have to try them in several different positions with similar water conditions (and probably a GPS) to determine if you are making gains with the changes.  Use a long straight edge along the bottom of the boat and out next to the trim tab (if down) and measure how much down it is (or get the hull leveled and use an angle finder on the trim tab) to get a base line.  Then adjust it so it is level with the bottom, then up about 4 degrees.  After each change go give it a good test day and compare your top speeds vs ride quality.  You might find that you prefer the ride over the added mph. 

Good luck.

Hey there fellas, I have a 83, 21 ft open bow Carrera, with a 330 hp 350, and Volvo 290 outdrive. I was curious how to make my manual trim tabs work best for top speed. Does this boat even need the trim tabs? Right now, they are slighty down. What is the best position to have them in? What kind of top speed should this boat get?

Thanks

I assume you have what they call "manual tabs" sort of like a set it and forget it type deal?

3* up from square with the bottom... The 290 Volvo is somewhat of a pig for speed, see if you can find yourself a Duo Prop lower... I have the same boat I am building with a Mercruiser.

GT

Thanks for the info guys

GT, how big a project is changing over to a mercruiser drive? How about cost?

trying not to be a killjoy but this is a gigantic job.

The largest problem that you have is the transom cutout is completely different and will need to be completely replaced including gel coat.

The best recommendation I can make for the best bang for the buck with me to find a duo prop lower.

I can expand more on this later.

Sent from my SGH-T989 using SoCal Jet Boats mobile app

Not a  killjoy at all. That's what I needed to hear. I ain't looking for a project of that size. Now, putting a duoprop lower, that may be doable. I'm gonna try my manual tabs in a 3* up position and try to trim the motor up as far as possible. I'll let you guys know how it goes.

Thanks

Thanks! Might be a little hefty on the pocket book right now