I have this app but I'm still looking for the right knot to tie a boat to a float where another line has been strung between two cleats (to allow more small boa

I have this app but I'm still looking for the right knot to tie a boat to a float where another line has been strung between two cleats (to allow more small boats to tie up than would at two cleats). As I type this my boat is tied with what looked like a child's shoelace.