After some gales, I hauled it down for a check and discovered that the 3 wooden blades were all split along their length. Oh dear. So I sent off to CMS Magnetics of Plano, Texas, for a set of their 10ft 6in reinforced fibreglass blades plus hub.

Here are the blades, which all balance at the same point along their length. However, one weighs 3020 grams, another 3042 grams and the heaviest comes in at 3055 grams.

My friendly local engineering shop is making an adaptor to mate the very substantial 3/16 inch steel hub to my 1KW stepper motor. Once I've mounted the blades, I'll see if I have unacceptable vibration when they are spinning. If so, I'll try adding weight to the appropriate blade at the hub end, and if that doesn't work I'll sand down the heavier ones (I need to clean up and sharpen the blunt edges and reshape the tips anyway).