If you've got Excel on your computer, or a similar spreadsheet program, then it's not too hard to set up a crude bit of design software. It's very limited, and it won't turn out anything pretty like SSW or quickmech does, but it's good enough, and for me, it beats using paper and pencils, as when I'm designing, I constantly adjust things like speed and overall tonnage to try and get the best fit for the role I want.
I tend to run a spreadsheet with a checklist of things that go into such a vehicle (Structure, engine, armour, control systems, weapons, ammunition, power amplifiers) off to one side so I don't forget anything, and then use the algorithms in the program to sum them all into a single figure. Because you can set certain values to auto-calculate, such as the internal structure weight, then if you decide to adjust the overall tonnage of your design, it adjusts most of the relevant factors for you.
I still haven't figured out a way to work out engine weights, so you have to put these in manually. The engine rating, however, is easily auto-calculated. (There was an old thread on the CBT forums over at Sarna.net that had a reasonable approximation, but that used sixth-degree indices, and I have no inclination to crunch numbers if I can just use the books).
Of course, naval vessels tend to be a bit confusing and lead to fairly large spreadsheets, what with all the individual components you can add, especially for 100+ ton vessels, but I then just take everything and copy-paste it into notepad for when I feel like typing it up properly.