[Info-vax] SIMH disk geometry help required please

[Gremlin]

I have a Charon-VAX disk image of an RZ59 which I am trying to use on SimH v3.9.

I can configure the disk in the SimH ini file using "set rq0 rauser=9114" which should (?) indicate that it is a 8.9GB storage device.  When I start OpenVMS 7.3 I can mount the disk, but it is full of read errors... other Charon-VAX disks (all less than or equal to 2GB) are fine.

I think there is a problem with the disk geometry... SimH $show dev/fu shows the disk as an RA82 with:
Total blocks 17755614
Sectors per track 57
Total cylinders 20767 
Tracks per cylinder 15

While on the "real" Charon-VAX, it is:
Total blocks 17755614
Sectors per track 193
Total cylinders 5111
Tracks per cylinder 18

Is there some magic incantation in SimH to specify disk geometry?