Chicken bald back

[sjp]

I've had 3 chicks out of 7 eggs hatch 2 eggs were clear at day 10, one was looks like it went bad between setting and day 10 the other one stopped developing at about day 10 all were from my own mob and no more than 24hrs old when set they were from at most 6 different hens and from both of my cockerels trouble is that all but one of the eggs had the pencil rubbed off. what I need to know is what type of maker can I safely use that will stay the distance but be safe on the eggs

the hatch rate all through low doesn't bother me to much as at least this hatch was successful the first one of the year was a complete failure due to a useless broody who ate all the good eggs and sat on 2 bad ones both this one and the useless one are first timers