HiCarl
Ive got the same trailer and boat too I think and its a real pain. the bunks are adjusted more than once a year. not sure if its the SBS design but once they slip they keep slipping time and time again, A real pain in the ............
Ive had them set low and high, (excuse the technical language, or lack of it) too low and she rocks side to side on the centre roller but is easy to launch, to high and she sits flat on the bunks and is pushed way deep into the water to float her off.
Best I found is to have the keel just on the centre roll but with the weight on the bunks. The rubber centre roller will be crushed if its not already and al but useless as a roller but under weight it should just roll off as long as the resistance from the bunks isnt too great. I lowered the bunks completely then trolley jacked each support just enough to raise the keel and take the load. Jacking the front support slightly more to allow for the hull profile (helps her slide off).
Then you get into the whole arguement of greasing the bunks and whats best or lining them with low resistant materials plastic or silicon and/or replacing the centre roller with nylon rollers.
in summery - I dont like my SBS bunk trailer but I have heard bunks are better than rollers for support. hope that helps