Impulse Fan Loss of Momentum
 

I'm modelling an impulse fan, as I have successfully previously, and am getting unexpected dramatic loss of momentum, much earlier than it should. See Image attached.

The exit velocity near the fan should be around 17m/s, not 6.3m/s and should still be above 1m/s for about 40m odd.

The meshing, while certainly not perfect, has been fine enough in the past for early results, however this is far too skewed.

Any ideas?