Excellent work!
My main comment is that you got the proportion correct, this is vitally important, especially as one often works with factories where they cheat and take the standard directly off your sketch when cutting patterns, meaning if your proportions are out, then the pattern will be out.
So, I have no criticism to make, only 'keep up the good work'.
If you want to see nice hand renders in marker, then I really recommend this blog,
http://naliniarora.com/blog/I don't do colour marker renders myself, if I'm going to use color then it's when I go to cad. Which is a shame really.