I've got several class 15 based machines. My pride and joy is lovely old Reeds Sailmaker. That will happily sew through 10 layers of 5oz Dacron sailcloth, which is good enough for any sail I'm likely to make. My Janome/New Home 636 will happily go though through 7 layers of 5oz Dacron. I've not bothered pushing that machine to its limits though as I primarily use it make spinnakers and sail bags. If I need to go heavier I've got a Chinese Singer 20U clone.
I've got several class 15 based machines. My pride and joy is lovely old Reeds Sailmaker. That will happily sew through 10 layers of 5oz Dacron sailcloth, which is good enough for any sail I'm likely to make. My Janome/New Home 636 will happily go though through 7 layers of 5oz Dacron. I've not bothered pushing that machine to its limits though as I primarily use it make spinnakers and sail bags. If I need to go heavier I've got a Chinese Singer 20U clone.