How to mark student reports more efficiently
I was in a very similar situation this past term. I was also tasked with grading engineering assignments that involved the use of software and a written component for which there were no solutions (mind you they were a little shorter than the ones you are grading). I am familiar with the software so it ended up taking me 5 days to mark.

Jigg has covered most of the bases, I just wanted to add another tip for efficiency:

In an effort to give them good feedback I wanted to write thorough comments. But after I got going on the marking I found that many students were making the same kinds of mistakes and I was writing the same comments over and over again. Eventually I just created an excel spreadsheet that really helped automate the marking that looked like this example:



The totals for points and points available are summation formulas so I didn't need to do any totalling myself. As I graded each assignment, I just compared it to the solutions I came up with and judged how well each of them met the criteria (nothing new). When I saw something wrong I just copied a detailed comment from the comments list and pasted it in the relevant cell. When I was finished with an assignment I just copied the everything in the thick black border into the online comments section, entered their grade, and moved on to the next one. If there was something new (and incorrect) that a student was doing I created a new comment and copied it into the list. Sometimes I also wrote something like "good work" next to the assignment total, just so that there were some positive comments.

Granted this worked really well when the students were all doing the exact same thing, but if your students are writing about different topics, it might not work as well.