Why did early Hurricanes and Spitfires have two bladed props…

Rotol was formed from RR and Bristol as a new source for CSU propellers, and these were entering production and service over the winter of 1939/40 with priority being given to the Hurricane. The Merlin III was designed to take a CSU and you can see the little bulge for this low down on the port nose ring from very early on in Hurricane production (perhaps all but the first 50?). There was some discussion between the Ministry and DH regarding a CS version of the Hamilton design but these were eventually rushed into service in July 1940. I have not seen any reason for the delay but possibly the promise of the superior Hydromatic version was a cause for delay?

Simply shortage. The UK aviation industry was only belatedly getting up to speed with VP airscrews. There was also a shortage of CSU’s, and consequently, these were not fitted until some time in 1940 I think (When..?). Consequently, after the fitting of the VP airscrews, there was a somewhat unnecessary workload for the pilot, as well as reduced climb performance until the CSU’s were fitted.

Pure speculation on my part but I would think both the Battle and Blenheim would have struggled to get off the ground when heavily loaded without having fine pitch,the fighters would not have had quite the same problem so maybe just allocating a limited number of available variable pitch props until industry caught up with demand !