Bike_Mech the usual belts for sewing machines are round leather ones. However I run a car v belt on mine at the moment as I couldn't get one in time to finish a job I was doing.
No the belt is not supposed to slip at all. The adjustment is on the lever that goes from the footplate up to the...
Sailmaker here. Also make boat covers, mend horse rugs and do leather work.
I have a Durcopp and a Singer industrial machines and a Jones and Singer domestic machines.
I think you will find that seam tape will not stick to Ultrasil or any coated material like that.