should the exhaust port in the drive be left open or blocked off so it cant pull air from the surface

I too would like to know this one. Should air be allowed into the prop. Mine are blocked off and the exhaust only goes out the transom. Should the propeller be different? No diffuser ring or a cone on the exit side?

I was told years ago by a good prop man to remove the rubber tube that connects the transom assy to the exhaust on the drive to allow air to travel through a through hub prop. Otherwise there was a suction created on the back of the prop. I always ran it that way and never tested it, but never had a problem either. I just took his word for it.

Hope that helps.
Dick