I never boil gammon/ham. I wrap it in foil and place it in the oven in a roasting tray with boiling water in (if its ham, I put a mixture of brown sugar and cinnamon on the top when it's cooked and brown it under the grill).
If you have different settings on slow cooker put it on high for a couple of hours then reduce. I put mine in yesterday at 1 30pm and was cooked by 5 pm.