When working with non-squared and oddly shaped things – like boats, you generally need starting and reference points. The most convenient usually being from the “centers” of something. One general approach is to ‘work from the centers out’, which seems to work well overall too most times.
For a simple example, suppose you must install a seat in a boat (or, a seat bottom in a chair). You may know the seat width or depth, but ends are not squared to that. Working from centerlines is the handiest and most accurate way to layout your stock, and will best insure your end result is symmetrical. See image #1:
Image 1 – Finding centerline with try square and intersecting arcs
It is usually easy to find the center in a least one direction – simply use your ruler – or divide your dimension graphically as in my post on dividing into equal parts: (http://lumberjocks.com/GnarlyErik/blog/33282)
A try square may be used to draw 45 degree angles from the intersections (Image #1). Or, for relatively small dimensions, you may use a compass to swing equal arcs from the endpoints of a common line, which will intersect at your center line. (Image #1).
And here is a little “Gee Whiz” multi-tool you can make yourself in a half hour or so, to find the centers of round stock. You can buy a manufactured tool similar to this called a combination square with a centering head if money is no object, and there are various cheap plastic ones available too. Machinists and lathe using folks use centering heads. (Image #2, #3 & #4).
Image #2 – Layout of Gee Whiz
And, here is how it is used:
Finding center for round stock
Finding center for round stock – mark from two or more locations on end of stock and where lines intersect is the center.
But, this shop made multi-use version can also find and mark the center on long stock like planks (NOT a tapered planks!). In use, this Gee Whiz is slid along the plank with the pins held in contact with either edge of your plank, and a pencil inserted in the centering hole will mark the centerline down the length of the plank, except very near the ends – a straight edge can extend those.
The pictures should explain how this Gee Whiz is made.
Image #5 – Design of plank center finding side of Gee Whiz
If you make one, be very precise to locate your center hole EXACTLY CENTERED between the two pins since any error will be multiplied by two. The pins themselves are screws with the heads cut off and the ends filed for safety, and small bolts are even better. Nails will work too, except they tend to quickly get wobbly in use. You will notice a bit of a countersink at each hole in the pictures. That is so a tapered pencil point can be easily placed in the hole. Just be careful the countersink stops about an 1/8” from the marking side so the hole doesn’t wallow out, and in use, be sure to hold your pencil as nearly vertical as possible when marking. If you paint this ‘bright’ it can be easily found on your workbench or shop wall. Once you try one, you may use it more than you think! See image #6:
Image #5 – Finding & Marking center of length of plank
The overall shape of this tool is unimportant as long as the geometry is followed. Build one to suit your fancy!
You may also notice this Gee Whiz has two extra pencil holes, which I will try to explain the reason and use for, and where they should be placed in another post. Most wooden boatbuilders will already know the reason for those.
(and, here is the reason posted in a follow-up (part two) of this topic: http://lumberjocks.com/GnarlyErik/blog/33328)
I hope you find this helpful!
-- "Never let your dogma get run over by your karma!"