and my transducer is NOT exactly centerline. I've often tried flipping the boat over on different tacks and seeing / comparing what I get for SOG and Boat Speed, and yes, they are sometimes different regardless how many times I try and calibrate, and I attribute that to exactly what you say, transducer slightly off-center.

But, my hunch is NOT based on instrument readings, rather it is based my feeling / sense for how the boat goes, she just feels happier and faster on the stbd tack. Regardless what the instruments say.

Larry's suggestion could be good one ... never thought of that.

cheers, John