Poor Man's Proportional Dividers, by tomse3 (Tom)
When planking a hull, the planks are layed out in bands of strakes, with the strakes all the same width at any given frame. Planking bands are usually marked out using battens, to make sure they flow fair with the curve of the hull. Once a band is defined, you need to determine the plank width at each frame or bulkhead. For example, if the band consists of six strakes, you need to divide the band's width at a give frame by six to determine how wide the planks should be as they cross that frame.
 So much for theory. In practice, I've seen lots of references to a nifty tool called a proportional divider (see this example from Micro Mark, along with instructions). It's basically a double-ended compass with a movable pivot point. For a six-strake band, you'd set the pivot point so that the distance between the pair of points at one end of the dividers is one sixth the distance of the opposite pair. The tool is engineered with appropriate markings for you to set the pivot point to the desired proportions. When I first read about this tool I thought, "Cool! I want one!" I did some Googling, and harsh reality set in ... all the proportional dividers I found cost over $100. So ... back to the drawing board. Or, more accurately, back to the drawing program.

 I fired up Visio and drew a series of lines that all shared one endpoint and had the opposite endpoint the same vertical distance apart. Any vertical line through this figure would be divided into equal parts. I printed this out and glued the ends together to form a loop. I slipped it over a flat scrap of wood with one edge sanded to form a "ruler edge".

 I could then slide the loop around the wood until the width of the figure at the "ruler edge" matched the distance I wanted to be divided. Using this simple tool, I could work along the planking band and mark the correct divisions along each frame or bulkhead.
 Working the other way, I could mark a plank where it crossed each bulkhead, use the paper divider to transfer each desired plank width from the bulkheads to the plank, and then play "connect the dots" to define the desired plank edge.
 Granted, this solution is lacking in engineering precision, but if you cut your planks a bit wide, you can test-fit and sand them until they're right. (You do that anyway, don't you?) I also figured out that you can dispense with the wood scrap if you're careful to fold the paper loop straight across the image and not at an off angle. The paper does tend to get worn, but you can print out several on a sheet and use them up as fast as you want, for just pennies a sheet instead of a hundred bucks. The next idea I had was to create a loop for each band and mark lines across the loop indicating each bulkhead's reference location. This way I wouldn't have to re-measure from the model each time a strake crossed a bulkhead.
 You can either draw your own image or download (copy) and print out the one I've attached above. Give it a try and let me know how it works for you. Maybe one day I'll be able to justify buying a proportional divider, but until then, I'll just keep on planking away.
Tom