They are electric or hydraulic stainless steel flaps on each side of the transom that help control porpoising and makes a better ride. And yes, they do make a big difference.
But to answer your question, you are right in that all the way down isn't the best economy and highest speed / lowest RPM work in your favor. As you trim up more hull is coming out of the water, steering gets easier, speed increases and so on. My trim indicator died, but I can tell by feel exactly how my engine is trimmed.
I don't have a fuel flow meter but I believe the best RPM range for fuel economy on a V20 w/ 150 hp o/b is somewhere between 3700 - 4000 RPMs. In your scenario of going around 20 mph or so I wouldn't worry so much about economy as adjusting the trim for the best ride, including pounding and spray from the hull. I've found there are times when speeding up and adjusting trim you can get on top of the chop and find a better, dryer ride than if you tried to go slower. Not always the case, but sometimes.
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1994 Wellcraft V21
Last edited by Blue_Runner; 06-05-2014 at 03:25 PM.
Reason: spelling error
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