Thanks Brian.
What set this off was that a client quizzed me on why the rigging screws on his boat had no locknuts or splitpins on them, and other boats around with open bodied turnbuckles did.
He had the old style Graham rigging screws, a kinda searig style of turnbuckle the early ones of which had no locknut or locking screw. Here in NZ there are many boats rigged back 'in the day' using these, and theres still a few about. Any way i have not yet heard of/seen evidence of on undoing it'self, despite the lack of locking devices, hence the twisting question.. They look like pic 10 in the link below.
http://www.trademe.co.nz/Trade-Me-Mo...-213264608.htm
Cheers