I just picked up on this thread, and it reminded me of some comments from a pilot who flies full size and uses wooden fixed pitch airscrews.
He told me that the bolts and washers are there to hold the prop against the drive face, that the bolt holes should give good clearance to the full length of the bolts so that they are not subject to any shear forces, only tension. Obviously this is not easy to do on small engines but the principle probably still applies, and any breach of it could explain sheared bolts. The biggest snag is likely to be finding the ideal balance between having the bolts tight enough to hold the prop firmly on the drive face yet not so tight as to compress the wood. Good ole Zenoah, single bolt fully adequate, no problems.
This link from Sensenich may be illuminating. Scroll down to the last page.
http://www.sensenich.com/files/document ... 103243.pdf
Maybe the full-size people would like to use single bolt hubs, but the size of the nut and the spanner and the number of bodies needed to hang on the end of it would be prohibitive.
