the depth a rototiller digs is dependent on how rocky the soil is and the size of the machine. A small Mantis, Stih Yardboss or similar wont really dig a trench. No rototiller will dig a trench but it could loosen the ground that it could then be shoveled more easily. They will cut a pretty wide path.
Perhaps just hire out the trench work? It doesn't make sense to buy a trencher for one project and if you don't want to rent (and get it done in one day) then you have limited options. Maybe even find some of those guys that sometimes hang out in front of the home center and hire them for a few hours and buy some $10 shovels. That may be cheaper than renting a machine actually. Don't run for political office if you do that and they are illegal immigrants tho. I don't support hiring illegals but I do support hiring day laborers who need the work.
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