<... I can confirm that you can easily see past the impeller on this pump ...>
That tells me that two units plumbed in series will solve the problem of redundancy.
Ust both for TO, climb and alternate them in cruise.  The extra resistance will make
the operating pump work nearer capacity, but that is acceptable even if it results in
somewhat shortened pump life.  Let's remember, the goal is not unimpeded performance
with a pump failure, it's a "limp home" capability.  Series plumbed pumps or simple,
leaky check valves will do the trick here.
Just a theory ... Jim S.

Haywire wrote:

>         I can confirm that you can easily see past the impellor on this pump. This
> is actually an indication that the impellor is not optimized for the pump
> casing, however it is adequate to do it's job very well. This also allows it
> to allow flow to past even if it ever seizes, although I don't expect that
> to ever happen again since I've made a very small mod to correct this.