The cleanflametrap website is truly fantastic -- a wealth of information.

However, at the cleanflametrap site's page for water pumps cited in the previous message, the last statement reads "...And my favorite part - the stamped impeller just looks more efficient to me, though the bronze is pretty...."

I'm not sure I agree with that particular (illustrated) stamped impeller really being more efficient.

What I'm thinking is that the rotating vanes have to push the water around the periphery to the output passage. On the cast (Hepu) impeller, the water is trapped (albeit only one side) by the "wall" of the cast impeller. But on the stamped impeller, water can escape on both sides.

I'm not satisfied by my statement, though, as far as making my point. But if by analogy you imagine putting your hand in a tub of water and making a paddle stroke, you can tell that while you're pushing some water ahead of your hand, a lot of water escapes to the right and left of your hand, so you're not too efficient -- this would be that stamped impeller with the gaps between each "paddle" (for lack of a better term?). However, if you were stroking your hand alongside a wall, this might prevent some of the water from escaping on at least one side of your hand, making the stroke a little more efficient -- this would be the cast impeller.

Any comments or thoughts?