I'm just thinking here. Everything I've read and everybody here seems to agree that the blades of the fan should be 1/2 in 1/2 out of the shroud. I have a piece of 8" round duct that has a booster fan in it. It blows pretty hard for an 8" fan. It seems to me to be the same as a shroud. Why wouldn't the fan pull if it was completely inside this sleeve we're talking about?