Re: roller plotter or parallel rulers?
groggy, nav tools are like screwdrivers: You have a favorite for any particular use, even if another one can do the same job. You really need to try out both tools to see which will suit you better, the same way that some folks can't stand mechanical pencils and some folks can't stand pencil sharpeners.
There used to be an inexpensive "Rolling Ruler" sold in the craft and hobby stores, stationery stores, and I think even Martha Stewart's scrapbooking line has something similar. So check the local craft stores, you may be able to get something under ten bucks that is plenty good enough for practicing with. Then if you want the better one...you leave one at home, one on the boat, it isn't a total waste to have two tools for this job.