String?? They used to and probably still do make tracking string that attaches bow to arrow. I have to believe it will affect arrow (bolt) flight. Has to.

Why not just make a good shot?

A sharp broadhead in the correct place and the animal is down in seconds. Sounds like you need practice and/or patience instead of a tracking crutch. Become proficient at 20 yards or less and take only broadside double lung shots at close deer.